diff --git a/CHANGELOG b/CHANGELOG
index 472faa05b75..38e91fc3e98 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -10,6 +10,7 @@ v 8.11.0 (unreleased)
   - The Repository class is now instrumented
   - Cache the commit author in RequestStore to avoid extra lookups in PostReceive
   - Expand commit message width in repo view (ClemMakesApps)
+  - Cache highlighted diff lines for merge requests
   - Fix of 'Commits being passed to custom hooks are already reachable when using the UI'
   - Add support for using RequestStore within Sidekiq tasks via SIDEKIQ_REQUEST_STORE env variable
   - Optimize maximum user access level lookup in loading of notes
diff --git a/app/controllers/concerns/diff_for_path.rb b/app/controllers/concerns/diff_for_path.rb
index 026d8b2e1e0..aeec3009f15 100644
--- a/app/controllers/concerns/diff_for_path.rb
+++ b/app/controllers/concerns/diff_for_path.rb
@@ -1,8 +1,8 @@
 module DiffForPath
   extend ActiveSupport::Concern
 
-  def render_diff_for_path(diffs, diff_refs, project)
-    diff_file = safe_diff_files(diffs, diff_refs: diff_refs, repository: project.repository).find do |diff|
+  def render_diff_for_path(diffs)
+    diff_file = diffs.diff_files.find do |diff|
       diff.old_path == params[:old_path] && diff.new_path == params[:new_path]
     end
 
@@ -14,7 +14,7 @@ module DiffForPath
     locals = {
       diff_file: diff_file,
       diff_commit: diff_commit,
-      diff_refs: diff_refs,
+      diff_refs: diffs.diff_refs,
       blob: blob,
       project: project
     }
diff --git a/app/controllers/projects/commit_controller.rb b/app/controllers/projects/commit_controller.rb
index 7ae034f9398..fdfe7c65b7b 100644
--- a/app/controllers/projects/commit_controller.rb
+++ b/app/controllers/projects/commit_controller.rb
@@ -28,7 +28,7 @@ class Projects::CommitController < Projects::ApplicationController
   end
 
   def diff_for_path
-    render_diff_for_path(@diffs, @commit.diff_refs, @project)
+    render_diff_for_path(@commit.diffs(diff_options))
   end
 
   def builds
diff --git a/app/controllers/projects/compare_controller.rb b/app/controllers/projects/compare_controller.rb
index 8c004724f02..bee3d56076c 100644
--- a/app/controllers/projects/compare_controller.rb
+++ b/app/controllers/projects/compare_controller.rb
@@ -21,7 +21,7 @@ class Projects::CompareController < Projects::ApplicationController
   def diff_for_path
     return render_404 unless @compare
 
-    render_diff_for_path(@diffs, @diff_refs, @project)
+    render_diff_for_path(@compare.diffs(diff_options))
   end
 
   def create
@@ -40,18 +40,12 @@ class Projects::CompareController < Projects::ApplicationController
     @compare = CompareService.new.execute(@project, @head_ref, @project, @start_ref)
 
     if @compare
-      @commits = Commit.decorate(@compare.commits, @project)
-
-      @start_commit = @project.commit(@start_ref)
-      @commit = @project.commit(@head_ref)
-      @base_commit = @project.merge_base_commit(@start_ref, @head_ref)
+      @commits = @compare.commits
+      @start_commit = @compare.start_commit
+      @commit = @compare.commit
+      @base_commit = @compare.base_commit
 
       @diffs = @compare.diffs(diff_options)
-      @diff_refs = Gitlab::Diff::DiffRefs.new(
-        base_sha: @base_commit.try(:sha),
-        start_sha: @start_commit.try(:sha),
-        head_sha: @commit.try(:sha)
-      )
 
       @diff_notes_disabled = true
       @grouped_diff_discussions = {}
diff --git a/app/controllers/projects/merge_requests_controller.rb b/app/controllers/projects/merge_requests_controller.rb
index 116e7904a4e..2cf6a2dd1b3 100644
--- a/app/controllers/projects/merge_requests_controller.rb
+++ b/app/controllers/projects/merge_requests_controller.rb
@@ -85,7 +85,11 @@ class Projects::MergeRequestsController < Projects::ApplicationController
 
     respond_to do |format|
       format.html { define_discussion_vars }
-      format.json { render json: { html: view_to_html_string("projects/merge_requests/show/_diffs") } }
+      format.json do
+        @diffs = @merge_request.diffs(diff_options)
+
+        render json: { html: view_to_html_string("projects/merge_requests/show/_diffs") }
+      end
     end
   end
 
@@ -103,9 +107,8 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     end
 
     define_commit_vars
-    diffs = @merge_request.diffs(diff_options)
 
-    render_diff_for_path(diffs, @merge_request.diff_refs, @merge_request.project)
+    render_diff_for_path(@merge_request.diffs(diff_options))
   end
 
   def commits
@@ -153,7 +156,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     @commits = @merge_request.compare_commits.reverse
     @commit = @merge_request.diff_head_commit
     @base_commit = @merge_request.diff_base_commit
-    @diffs = @merge_request.compare.diffs(diff_options) if @merge_request.compare
+    @diffs = @merge_request.diffs(diff_options) if @merge_request.compare
     @diff_notes_disabled = true
 
     @pipeline = @merge_request.pipeline
diff --git a/app/helpers/commits_helper.rb b/app/helpers/commits_helper.rb
index f497626e21a..7a02d0b10d9 100644
--- a/app/helpers/commits_helper.rb
+++ b/app/helpers/commits_helper.rb
@@ -206,10 +206,10 @@ module CommitsHelper
     end
   end
 
-  def view_file_btn(commit_sha, diff, project)
+  def view_file_btn(commit_sha, diff_new_path, project)
     link_to(
       namespace_project_blob_path(project.namespace, project,
-                                  tree_join(commit_sha, diff.new_path)),
+                                  tree_join(commit_sha, diff_new_path)),
       class: 'btn view-file js-view-file btn-file-option'
     ) do
       raw('View file @') + content_tag(:span, commit_sha[0..6],
diff --git a/app/helpers/diff_helper.rb b/app/helpers/diff_helper.rb
index f35e2f6ddcd..cc7121b1163 100644
--- a/app/helpers/diff_helper.rb
+++ b/app/helpers/diff_helper.rb
@@ -30,11 +30,7 @@ module DiffHelper
       options[:paths] = params.values_at(:old_path, :new_path)
     end
 
-    Commit.max_diff_options.merge(options)
-  end
-
-  def safe_diff_files(diffs, diff_refs: nil, repository: nil)
-    diffs.decorate! { |diff| Gitlab::Diff::File.new(diff, diff_refs: diff_refs, repository: repository) }
+    options
   end
 
   def unfold_bottom_class(bottom)
diff --git a/app/models/commit.rb b/app/models/commit.rb
index c52b4a051c2..d58c2fb8106 100644
--- a/app/models/commit.rb
+++ b/app/models/commit.rb
@@ -104,7 +104,7 @@ class Commit
   end
 
   def diff_line_count
-    @diff_line_count ||= Commit::diff_line_count(self.diffs)
+    @diff_line_count ||= Commit::diff_line_count(raw_diffs)
     @diff_line_count
   end
 
@@ -317,6 +317,14 @@ class Commit
     nil
   end
 
+  def raw_diffs(*args)
+    raw.diffs(*args)
+  end
+
+  def diffs(diff_options = nil)
+    Gitlab::Diff::FileCollection::Commit.new(self, diff_options: diff_options)
+  end
+
   private
 
   def find_author_by_any_email
@@ -326,7 +334,7 @@ class Commit
   def repo_changes
     changes = { added: [], modified: [], removed: [] }
 
-    diffs.each do |diff|
+    raw_diffs.each do |diff|
       if diff.deleted_file
         changes[:removed] << diff.old_path
       elsif diff.renamed_file || diff.new_file
diff --git a/app/models/compare.rb b/app/models/compare.rb
new file mode 100644
index 00000000000..4856510f526
--- /dev/null
+++ b/app/models/compare.rb
@@ -0,0 +1,66 @@
+class Compare
+  delegate :same, :head, :base, to: :@compare
+
+  attr_reader :project
+
+  def self.decorate(compare, project)
+    if compare.is_a?(Compare)
+      compare
+    else
+      self.new(compare, project)
+    end
+  end
+
+  def initialize(compare, project)
+    @compare = compare
+    @project = project
+  end
+
+  def commits
+    @commits ||= Commit.decorate(@compare.commits, project)
+  end
+
+  def start_commit
+    return @start_commit if defined?(@start_commit)
+
+    commit = @compare.base
+    @start_commit = commit ? ::Commit.new(commit, project) : nil
+  end
+
+  def head_commit
+    return @head_commit if defined?(@head_commit)
+
+    commit = @compare.head
+    @head_commit = commit ? ::Commit.new(commit, project) : nil
+  end
+  alias_method :commit, :head_commit
+
+  def base_commit
+    return @base_commit if defined?(@base_commit)
+
+    @base_commit = if start_commit && head_commit
+                     project.merge_base_commit(start_commit.id, head_commit.id)
+                   else
+                     nil
+                   end
+  end
+
+  def raw_diffs(*args)
+    @compare.diffs(*args)
+  end
+
+  def diffs(diff_options = nil)
+    Gitlab::Diff::FileCollection::Compare.new(self,
+      project: project,
+      diff_options: diff_options,
+      diff_refs: diff_refs)
+  end
+
+  def diff_refs
+    Gitlab::Diff::DiffRefs.new(
+      base_sha:  base_commit.try(:sha),
+      start_sha: start_commit.try(:sha),
+      head_sha: commit.try(:sha)
+    )
+  end
+end
diff --git a/app/models/legacy_diff_note.rb b/app/models/legacy_diff_note.rb
index 865712268a0..6ed66001513 100644
--- a/app/models/legacy_diff_note.rb
+++ b/app/models/legacy_diff_note.rb
@@ -85,7 +85,7 @@ class LegacyDiffNote < Note
     return nil unless noteable
     return @diff if defined?(@diff)
 
-    @diff = noteable.diffs(Commit.max_diff_options).find do |d|
+    @diff = noteable.raw_diffs(Commit.max_diff_options).find do |d|
       d.new_path && Digest::SHA1.hexdigest(d.new_path) == diff_file_hash
     end
   end
@@ -116,7 +116,7 @@ class LegacyDiffNote < Note
 
   # Find the diff on noteable that matches our own
   def find_noteable_diff
-    diffs = noteable.diffs(Commit.max_diff_options)
+    diffs = noteable.raw_diffs(Commit.max_diff_options)
     diffs.find { |d| d.new_path == self.diff.new_path }
   end
 end
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index a99c4ba52a4..c4761fac2fb 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -164,8 +164,16 @@ class MergeRequest < ActiveRecord::Base
     merge_request_diff ? merge_request_diff.first_commit : compare_commits.first
   end
 
-  def diffs(*args)
-    merge_request_diff ? merge_request_diff.diffs(*args) : compare.diffs(*args)
+  def raw_diffs(*args)
+    merge_request_diff ? merge_request_diff.diffs(*args) : compare.raw_diffs(*args)
+  end
+
+  def diffs(diff_options = nil)
+    if self.compare
+      self.compare.diffs(diff_options)
+    else
+      Gitlab::Diff::FileCollection::MergeRequest.new(self, diff_options: diff_options)
+    end
   end
 
   def diff_size
@@ -313,6 +321,8 @@ class MergeRequest < ActiveRecord::Base
 
     merge_request_diff.reload_content
 
+    MergeRequests::MergeRequestDiffCacheService.new.execute(self)
+
     new_diff_refs = self.diff_refs
 
     update_diff_notes_positions(
diff --git a/app/models/repository.rb b/app/models/repository.rb
index bac37483c47..3d95344a68f 100644
--- a/app/models/repository.rb
+++ b/app/models/repository.rb
@@ -372,7 +372,7 @@ class Repository
     # We don't want to flush the cache if the commit didn't actually make any
     # changes to any of the possible avatar files.
     if revision && commit = self.commit(revision)
-      return unless commit.diffs.
+      return unless commit.raw_diffs.
         any? { |diff| AVATAR_FILES.include?(diff.new_path) }
     end
 
diff --git a/app/services/compare_service.rb b/app/services/compare_service.rb
index 149822aa647..6d6075628af 100644
--- a/app/services/compare_service.rb
+++ b/app/services/compare_service.rb
@@ -20,10 +20,12 @@ class CompareService
       )
     end
 
-    Gitlab::Git::Compare.new(
+    raw_compare = Gitlab::Git::Compare.new(
       target_project.repository.raw_repository,
       target_branch,
-      source_sha,
+      source_sha
     )
+
+    Compare.new(raw_compare, target_project)
   end
 end
diff --git a/app/services/merge_requests/build_service.rb b/app/services/merge_requests/build_service.rb
index 7fe57747265..290742f1506 100644
--- a/app/services/merge_requests/build_service.rb
+++ b/app/services/merge_requests/build_service.rb
@@ -34,7 +34,7 @@ module MergeRequests
       # At this point we decide if merge request can be created
       # If we have at least one commit to merge -> creation allowed
       if commits.present?
-        merge_request.compare_commits = Commit.decorate(commits, merge_request.source_project)
+        merge_request.compare_commits = commits
         merge_request.can_be_created = true
         merge_request.compare = compare
       else
diff --git a/app/services/merge_requests/merge_request_diff_cache_service.rb b/app/services/merge_requests/merge_request_diff_cache_service.rb
new file mode 100644
index 00000000000..2945a7fd4e4
--- /dev/null
+++ b/app/services/merge_requests/merge_request_diff_cache_service.rb
@@ -0,0 +1,8 @@
+module MergeRequests
+  class MergeRequestDiffCacheService
+    def execute(merge_request)
+      # Executing the iteration we cache all the highlighted diff information
+      merge_request.diffs.diff_files.to_a
+    end
+  end
+end
diff --git a/app/views/projects/commit/_ci_menu.html.haml b/app/views/projects/commit/_ci_menu.html.haml
index ea33aa472a6..935433306ea 100644
--- a/app/views/projects/commit/_ci_menu.html.haml
+++ b/app/views/projects/commit/_ci_menu.html.haml
@@ -2,7 +2,7 @@
   = nav_link(path: 'commit#show') do
     = link_to namespace_project_commit_path(@project.namespace, @project, @commit.id) do
       Changes
-      %span.badge= @diffs.count
+      %span.badge= @diffs.size
   = nav_link(path: 'commit#builds') do
     = link_to builds_namespace_project_commit_path(@project.namespace, @project, @commit.id) do
       Builds
diff --git a/app/views/projects/commit/show.html.haml b/app/views/projects/commit/show.html.haml
index d0da2606587..ed44d86a687 100644
--- a/app/views/projects/commit/show.html.haml
+++ b/app/views/projects/commit/show.html.haml
@@ -7,7 +7,7 @@
   = render "ci_menu"
 - else
   %div.block-connector
-= render "projects/diffs/diffs", diffs: @diffs, project: @project, diff_refs: @commit.diff_refs
+= render "projects/diffs/diffs", diffs: @diffs
 = render "projects/notes/notes_with_form"
 - if can_collaborate_with_project?
   - %w(revert cherry-pick).each do |type|
diff --git a/app/views/projects/compare/show.html.haml b/app/views/projects/compare/show.html.haml
index 28a50e7031a..819e9bc15ae 100644
--- a/app/views/projects/compare/show.html.haml
+++ b/app/views/projects/compare/show.html.haml
@@ -8,7 +8,7 @@
 
   - if @commits.present?
     = render "projects/commits/commit_list"
-    = render "projects/diffs/diffs", diffs: @diffs, project: @project, diff_refs: @diff_refs
+    = render "projects/diffs/diffs", diffs: @diffs
   - else
     .light-well
       .center
diff --git a/app/views/projects/diffs/_diffs.html.haml b/app/views/projects/diffs/_diffs.html.haml
index 4bf3ccace20..20dc280c3b2 100644
--- a/app/views/projects/diffs/_diffs.html.haml
+++ b/app/views/projects/diffs/_diffs.html.haml
@@ -1,20 +1,19 @@
 - show_whitespace_toggle = local_assigns.fetch(:show_whitespace_toggle, true)
+- diff_files = diffs.diff_files
 - if diff_view == 'parallel'
   - fluid_layout true
 
-- diff_files = safe_diff_files(diffs, diff_refs: diff_refs, repository: project.repository)
-
 .content-block.oneline-block.files-changed
   .inline-parallel-buttons
     - if !expand_all_diffs? && diff_files.any? { |diff_file| diff_file.collapsed? }
       = link_to 'Expand all', url_for(params.merge(expand_all_diffs: 1, format: nil)), class: 'btn btn-default'
     - if show_whitespace_toggle
       - if current_controller?(:commit)
-        = commit_diff_whitespace_link(@project, @commit, class: 'hidden-xs')
+        = commit_diff_whitespace_link(diffs.project, @commit, class: 'hidden-xs')
       - elsif current_controller?(:merge_requests)
-        = diff_merge_request_whitespace_link(@project, @merge_request, class: 'hidden-xs')
+        = diff_merge_request_whitespace_link(diffs.project, @merge_request, class: 'hidden-xs')
       - elsif current_controller?(:compare)
-        = diff_compare_whitespace_link(@project, params[:from], params[:to], class: 'hidden-xs')
+        = diff_compare_whitespace_link(diffs.project, params[:from], params[:to], class: 'hidden-xs')
     .btn-group
       = inline_diff_btn
       = parallel_diff_btn
@@ -23,12 +22,12 @@
 - if diff_files.overflow?
   = render 'projects/diffs/warning', diff_files: diff_files
 
-.files{data: {can_create_note: (!@diff_notes_disabled && can?(current_user, :create_note, @project))}}
+.files{data: {can_create_note: (!@diff_notes_disabled && can?(current_user, :create_note, diffs.project))}}
   - diff_files.each_with_index do |diff_file, index|
     - diff_commit = commit_for_diff(diff_file)
     - blob = diff_file.blob(diff_commit)
     - next unless blob
-    - blob.load_all_data!(project.repository) unless blob.only_display_raw?
+    - blob.load_all_data!(diffs.project.repository) unless blob.only_display_raw?
 
-    = render 'projects/diffs/file', i: index, project: project,
-      diff_file: diff_file, diff_commit: diff_commit, blob: blob, diff_refs: diff_refs
+    = render 'projects/diffs/file', i: index, project: diffs.project,
+      diff_file: diff_file, diff_commit: diff_commit, blob: blob
diff --git a/app/views/projects/diffs/_file.html.haml b/app/views/projects/diffs/_file.html.haml
index 1854c64cbd7..f914e13a1ec 100644
--- a/app/views/projects/diffs/_file.html.haml
+++ b/app/views/projects/diffs/_file.html.haml
@@ -15,6 +15,6 @@
                 from_merge_request_id: @merge_request.id,
                 skip_visible_check: true)
 
-        = view_file_btn(diff_commit.id, diff_file, project)
+        = view_file_btn(diff_commit.id, diff_file.new_path, project)
 
-  = render 'projects/diffs/content', diff_file: diff_file, diff_commit: diff_commit, diff_refs: diff_refs, blob: blob, project: project
+  = render 'projects/diffs/content', diff_file: diff_file, diff_commit: diff_commit, blob: blob, project: project
diff --git a/app/views/projects/merge_requests/_new_submit.html.haml b/app/views/projects/merge_requests/_new_submit.html.haml
index a5e67b95727..598bd743676 100644
--- a/app/views/projects/merge_requests/_new_submit.html.haml
+++ b/app/views/projects/merge_requests/_new_submit.html.haml
@@ -42,7 +42,7 @@
           %h4 This comparison includes more than #{MergeRequestDiff::COMMITS_SAFE_SIZE} commits.
           %p To preserve performance the line changes are not shown.
       - else
-        = render "projects/diffs/diffs", diffs: @diffs, project: @project, diff_refs: @merge_request.diff_refs, show_whitespace_toggle: false
+        = render "projects/diffs/diffs", diffs: @diffs, show_whitespace_toggle: false
     - if @pipeline
       #builds.builds.tab-pane
         = render "projects/merge_requests/show/builds"
diff --git a/app/views/projects/merge_requests/show/_diffs.html.haml b/app/views/projects/merge_requests/show/_diffs.html.haml
index 1b0bae86ad4..013b05628fa 100644
--- a/app/views/projects/merge_requests/show/_diffs.html.haml
+++ b/app/views/projects/merge_requests/show/_diffs.html.haml
@@ -1,6 +1,5 @@
 - if @merge_request_diff.collected?
-  = render "projects/diffs/diffs", diffs: @merge_request.diffs(diff_options),
-    project: @merge_request.project, diff_refs: @merge_request.diff_refs
+  = render "projects/diffs/diffs", diffs: @diffs
 - elsif @merge_request_diff.empty?
   .nothing-here-block Nothing to merge from #{@merge_request.source_branch} into #{@merge_request.target_branch}
 - else
diff --git a/app/workers/emails_on_push_worker.rb b/app/workers/emails_on_push_worker.rb
index 0b6a01a3200..c6a5af2809a 100644
--- a/app/workers/emails_on_push_worker.rb
+++ b/app/workers/emails_on_push_worker.rb
@@ -33,25 +33,14 @@ class EmailsOnPushWorker
     reverse_compare = false
 
     if action == :push
-      merge_base_sha = project.merge_base_commit(before_sha, after_sha).try(:sha)
-      compare = Gitlab::Git::Compare.new(project.repository.raw_repository, before_sha, after_sha)
-
-      diff_refs = Gitlab::Diff::DiffRefs.new(
-        base_sha: merge_base_sha,
-        start_sha: before_sha,
-        head_sha: after_sha
-      )
+      compare = CompareService.new.execute(project, before_sha, project, after_sha)
+      diff_refs = compare.diff_refs
 
       return false if compare.same
 
       if compare.commits.empty?
-        compare = Gitlab::Git::Compare.new(project.repository.raw_repository, after_sha, before_sha)
-
-        diff_refs = Gitlab::Diff::DiffRefs.new(
-          base_sha: merge_base_sha,
-          start_sha: after_sha,
-          head_sha: before_sha
-        )
+        compare = CompareService.new.execute(project, after_sha, project, before_sha)
+        diff_refs = compare.diff_refs
 
         reverse_compare = true
 
diff --git a/app/workers/irker_worker.rb b/app/workers/irker_worker.rb
index 605ec4f04e5..07cc7c1cbd7 100644
--- a/app/workers/irker_worker.rb
+++ b/app/workers/irker_worker.rb
@@ -141,8 +141,10 @@ class IrkerWorker
   end
 
   def files_count(commit)
-    files = "#{commit.diffs.real_size} file"
-    files += 's' if commit.diffs.count > 1
+    diffs = commit.raw_diffs
+
+    files = "#{diffs.real_size} file"
+    files += 's' if diffs.size > 1
     files
   end
 
diff --git a/lib/api/commits.rb b/lib/api/commits.rb
index 4a11c8e3620..b4eaf1813d4 100644
--- a/lib/api/commits.rb
+++ b/lib/api/commits.rb
@@ -54,7 +54,7 @@ module API
         sha = params[:sha]
         commit = user_project.commit(sha)
         not_found! "Commit" unless commit
-        commit.diffs.to_a
+        commit.raw_diffs.to_a
       end
 
       # Get a commit's comments
@@ -96,7 +96,7 @@ module API
         }
 
         if params[:path] && params[:line] && params[:line_type]
-          commit.diffs(all_diffs: true).each do |diff|
+          commit.raw_diffs(all_diffs: true).each do |diff|
             next unless diff.new_path == params[:path]
             lines = Gitlab::Diff::Parser.new.parse(diff.diff.each_line)
 
diff --git a/lib/api/entities.rb b/lib/api/entities.rb
index 3e21b7a0b8a..e5b00dc45a5 100644
--- a/lib/api/entities.rb
+++ b/lib/api/entities.rb
@@ -224,7 +224,7 @@ module API
 
     class MergeRequestChanges < MergeRequest
       expose :diffs, as: :changes, using: Entities::RepoDiff do |compare, _|
-        compare.diffs(all_diffs: true).to_a
+        compare.raw_diffs(all_diffs: true).to_a
       end
     end
 
diff --git a/lib/gitlab/diff/file.rb b/lib/gitlab/diff/file.rb
index b09ca1fb8b0..e47df508ca2 100644
--- a/lib/gitlab/diff/file.rb
+++ b/lib/gitlab/diff/file.rb
@@ -63,15 +63,18 @@ module Gitlab
         diff_refs.try(:head_sha)
       end
 
+      attr_writer :highlighted_diff_lines
+
       # Array of Gitlab::Diff::Line objects
       def diff_lines
-        @lines ||= Gitlab::Diff::Parser.new.parse(raw_diff.each_line).to_a
+        @diff_lines ||= Gitlab::Diff::Parser.new.parse(raw_diff.each_line).to_a
       end
 
       def highlighted_diff_lines
         @highlighted_diff_lines ||= Gitlab::Diff::Highlight.new(self, repository: self.repository).highlight
       end
 
+      # Array[<Hash>] with right/left keys that contains Gitlab::Diff::Line objects which text is hightlighted
       def parallel_diff_lines
         @parallel_diff_lines ||= Gitlab::Diff::ParallelDiff.new(self).parallelize
       end
diff --git a/lib/gitlab/diff/file_collection/base.rb b/lib/gitlab/diff/file_collection/base.rb
new file mode 100644
index 00000000000..2b9fc65b985
--- /dev/null
+++ b/lib/gitlab/diff/file_collection/base.rb
@@ -0,0 +1,35 @@
+module Gitlab
+  module Diff
+    module FileCollection
+      class Base
+        attr_reader :project, :diff_options, :diff_view, :diff_refs
+
+        delegate :count, :size, :real_size, to: :diff_files
+
+        def self.default_options
+          ::Commit.max_diff_options.merge(ignore_whitespace_change: false, no_collapse: false)
+        end
+
+        def initialize(diffable, project:, diff_options: nil, diff_refs: nil)
+          diff_options = self.class.default_options.merge(diff_options || {})
+
+          @diffable     = diffable
+          @diffs        = diffable.raw_diffs(diff_options)
+          @project      = project
+          @diff_options = diff_options
+          @diff_refs    = diff_refs
+        end
+
+        def diff_files
+          @diff_files ||= @diffs.decorate! { |diff| decorate_diff!(diff) }
+        end
+
+        private
+
+        def decorate_diff!(diff)
+          Gitlab::Diff::File.new(diff, repository: project.repository, diff_refs: diff_refs)
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/diff/file_collection/commit.rb b/lib/gitlab/diff/file_collection/commit.rb
new file mode 100644
index 00000000000..4dc297ec036
--- /dev/null
+++ b/lib/gitlab/diff/file_collection/commit.rb
@@ -0,0 +1,14 @@
+module Gitlab
+  module Diff
+    module FileCollection
+      class Commit < Base
+        def initialize(commit, diff_options:)
+          super(commit,
+            project: commit.project,
+            diff_options: diff_options,
+            diff_refs: commit.diff_refs)
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/diff/file_collection/compare.rb b/lib/gitlab/diff/file_collection/compare.rb
new file mode 100644
index 00000000000..20d8f891cc3
--- /dev/null
+++ b/lib/gitlab/diff/file_collection/compare.rb
@@ -0,0 +1,14 @@
+module Gitlab
+  module Diff
+    module FileCollection
+      class Compare < Base
+        def initialize(compare, project:, diff_options:, diff_refs: nil)
+          super(compare,
+            project:      project,
+            diff_options: diff_options,
+            diff_refs:    diff_refs)
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/diff/file_collection/merge_request.rb b/lib/gitlab/diff/file_collection/merge_request.rb
new file mode 100644
index 00000000000..4f946908e2f
--- /dev/null
+++ b/lib/gitlab/diff/file_collection/merge_request.rb
@@ -0,0 +1,73 @@
+module Gitlab
+  module Diff
+    module FileCollection
+      class MergeRequest < Base
+        def initialize(merge_request, diff_options:)
+          @merge_request = merge_request
+
+          super(merge_request,
+            project: merge_request.project,
+            diff_options: diff_options,
+            diff_refs: merge_request.diff_refs)
+        end
+
+        def diff_files
+          super.tap { |_| store_highlight_cache }
+        end
+
+        private
+
+        # Extracted method to highlight in the same iteration to the diff_collection.
+        def decorate_diff!(diff)
+          diff_file = super
+          cache_highlight!(diff_file) if cacheable?
+          diff_file
+        end
+
+        def highlight_diff_file_from_cache!(diff_file, cache_diff_lines)
+          diff_file.highlighted_diff_lines = cache_diff_lines.map do |line|
+            Gitlab::Diff::Line.init_from_hash(line)
+          end
+        end
+
+        #
+        # If we find the highlighted diff files lines on the cache we replace existing diff_files lines (no highlighted)
+        # for the highlighted ones, so we just skip their execution.
+        # If the highlighted diff files lines are not cached we calculate and cache them.
+        #
+        # The content of the cache is a Hash where the key correspond to the file_path and the values are Arrays of
+        # hashes that represent serialized diff lines.
+        #
+        def cache_highlight!(diff_file)
+          file_path = diff_file.file_path
+
+          if highlight_cache[file_path]
+            highlight_diff_file_from_cache!(diff_file, highlight_cache[file_path])
+          else
+            highlight_cache[file_path] = diff_file.highlighted_diff_lines.map(&:to_hash)
+          end
+        end
+
+        def highlight_cache
+          return @highlight_cache if defined?(@highlight_cache)
+
+          @highlight_cache = Rails.cache.read(cache_key) || {}
+          @highlight_cache_was_empty = @highlight_cache.empty?
+          @highlight_cache
+        end
+
+        def store_highlight_cache
+          Rails.cache.write(cache_key, highlight_cache) if @highlight_cache_was_empty
+        end
+
+        def cacheable?
+          @merge_request.merge_request_diff.present?
+        end
+
+        def cache_key
+          [@merge_request.merge_request_diff, 'highlighted-diff-files', diff_options]
+        end
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/diff/highlight.rb b/lib/gitlab/diff/highlight.rb
index 649a265a02c..9ea976e18fa 100644
--- a/lib/gitlab/diff/highlight.rb
+++ b/lib/gitlab/diff/highlight.rb
@@ -40,8 +40,6 @@ module Gitlab
       def highlight_line(diff_line)
         return unless diff_file && diff_file.diff_refs
 
-        line_prefix = diff_line.text.match(/\A(.)/) ? $1 : ' '
-
         rich_line =
           if diff_line.unchanged? || diff_line.added?
             new_lines[diff_line.new_pos - 1]
@@ -51,7 +49,10 @@ module Gitlab
 
         # Only update text if line is found. This will prevent
         # issues with submodules given the line only exists in diff content.
-        "#{line_prefix}#{rich_line}".html_safe if rich_line
+        if rich_line
+          line_prefix = diff_line.text.match(/\A(.)/) ? $1 : ' '
+          "#{line_prefix}#{rich_line}".html_safe
+        end
       end
 
       def inline_diffs
diff --git a/lib/gitlab/diff/line.rb b/lib/gitlab/diff/line.rb
index c6189d660c2..cf097e0d0de 100644
--- a/lib/gitlab/diff/line.rb
+++ b/lib/gitlab/diff/line.rb
@@ -9,6 +9,20 @@ module Gitlab
         @old_pos, @new_pos = old_pos, new_pos
       end
 
+      def self.init_from_hash(hash)
+        new(hash[:text], hash[:type], hash[:index], hash[:old_pos], hash[:new_pos])
+      end
+
+      def serialize_keys
+        @serialize_keys ||= %i(text type index old_pos new_pos)
+      end
+
+      def to_hash
+        hash = {}
+        serialize_keys.each { |key| hash[key] = send(key) }
+        hash
+      end
+
       def old_line
         old_pos unless added? || meta?
       end
diff --git a/lib/gitlab/email/message/repository_push.rb b/lib/gitlab/email/message/repository_push.rb
index 97701b0cd42..0e3b65fceb4 100644
--- a/lib/gitlab/email/message/repository_push.rb
+++ b/lib/gitlab/email/message/repository_push.rb
@@ -35,21 +35,22 @@ module Gitlab
         def commits
           return unless compare
 
-          @commits ||= Commit.decorate(compare.commits, project)
+          @commits ||= compare.commits
         end
 
         def diffs
           return unless compare
-          
-          @diffs ||= safe_diff_files(compare.diffs(max_files: 30), diff_refs: diff_refs, repository: project.repository)
+
+          # This diff is more moderated in number of files and lines
+          @diffs ||= compare.diffs(max_files: 30, max_lines: 5000, no_collapse: true).diff_files
         end
 
         def diffs_count
-          diffs.count if diffs
+          diffs.size if diffs
         end
 
         def compare
-          @opts[:compare]
+          @opts[:compare] if @opts[:compare]
         end
 
         def diff_refs
@@ -97,16 +98,18 @@ module Gitlab
             if commits.length > 1
               namespace_project_compare_url(project_namespace,
                                             project,
-                                            from: Commit.new(compare.base, project),
-                                            to:   Commit.new(compare.head, project))
+                                            from: compare.start_commit,
+                                            to:   compare.head_commit)
             else
               namespace_project_commit_url(project_namespace,
-                                           project, commits.first)
+                                           project,
+                                           commits.first)
             end
           else
             unless @action == :delete
               namespace_project_tree_url(project_namespace,
-                                         project, ref_name)
+                                         project,
+                                         ref_name)
             end
           end
         end
diff --git a/spec/controllers/projects/commit_controller_spec.rb b/spec/controllers/projects/commit_controller_spec.rb
index df902da86f8..940019b708b 100644
--- a/spec/controllers/projects/commit_controller_spec.rb
+++ b/spec/controllers/projects/commit_controller_spec.rb
@@ -83,7 +83,7 @@ describe Projects::CommitController do
       let(:format) { :diff }
 
       it "should really only be a git diff" do
-        go(id: commit.id, format: format)
+        go(id: '66eceea0db202bb39c4e445e8ca28689645366c5', format: format)
 
         expect(response.body).to start_with("diff --git")
       end
@@ -92,8 +92,9 @@ describe Projects::CommitController do
         go(id: '66eceea0db202bb39c4e445e8ca28689645366c5', format: format, w: 1)
 
         expect(response.body).to start_with("diff --git")
-        # without whitespace option, there are more than 2 diff_splits
-        diff_splits = assigns(:diffs).first.diff.split("\n")
+
+        # without whitespace option, there are more than 2 diff_splits for other formats
+        diff_splits = assigns(:diffs).diff_files.first.diff.diff.split("\n")
         expect(diff_splits.length).to be <= 2
       end
     end
@@ -266,9 +267,9 @@ describe Projects::CommitController do
           end
 
           it 'only renders the diffs for the path given' do
-            expect(controller).to receive(:render_diff_for_path).and_wrap_original do |meth, diffs, diff_refs, project|
-              expect(diffs.map(&:new_path)).to contain_exactly(existing_path)
-              meth.call(diffs, diff_refs, project)
+            expect(controller).to receive(:render_diff_for_path).and_wrap_original do |meth, diffs|
+              expect(diffs.diff_files.map(&:new_path)).to contain_exactly(existing_path)
+              meth.call(diffs)
             end
 
             diff_for_path(id: commit.id, old_path: existing_path, new_path: existing_path)
diff --git a/spec/controllers/projects/compare_controller_spec.rb b/spec/controllers/projects/compare_controller_spec.rb
index 4058d5e2453..ed4cc36de58 100644
--- a/spec/controllers/projects/compare_controller_spec.rb
+++ b/spec/controllers/projects/compare_controller_spec.rb
@@ -19,7 +19,7 @@ describe Projects::CompareController do
         to: ref_to)
 
     expect(response).to be_success
-    expect(assigns(:diffs).first).not_to be_nil
+    expect(assigns(:diffs).diff_files.first).not_to be_nil
     expect(assigns(:commits).length).to be >= 1
   end
 
@@ -32,10 +32,11 @@ describe Projects::CompareController do
         w: 1)
 
     expect(response).to be_success
-    expect(assigns(:diffs).first).not_to be_nil
+    diff_file = assigns(:diffs).diff_files.first
+    expect(diff_file).not_to be_nil
     expect(assigns(:commits).length).to be >= 1
     # without whitespace option, there are more than 2 diff_splits
-    diff_splits = assigns(:diffs).first.diff.split("\n")
+    diff_splits = diff_file.diff.diff.split("\n")
     expect(diff_splits.length).to be <= 2
   end
 
@@ -48,7 +49,7 @@ describe Projects::CompareController do
           to: ref_to)
 
       expect(response).to be_success
-      expect(assigns(:diffs).to_a).to eq([])
+      expect(assigns(:diffs).diff_files.to_a).to eq([])
       expect(assigns(:commits)).to eq([])
     end
 
@@ -87,9 +88,9 @@ describe Projects::CompareController do
           end
 
           it 'only renders the diffs for the path given' do
-            expect(controller).to receive(:render_diff_for_path).and_wrap_original do |meth, diffs, diff_refs, project|
-              expect(diffs.map(&:new_path)).to contain_exactly(existing_path)
-              meth.call(diffs, diff_refs, project)
+            expect(controller).to receive(:render_diff_for_path).and_wrap_original do |meth, diffs|
+              expect(diffs.diff_files.map(&:new_path)).to contain_exactly(existing_path)
+              meth.call(diffs)
             end
 
             diff_for_path(from: ref_from, to: ref_to, old_path: existing_path, new_path: existing_path)
diff --git a/spec/controllers/projects/merge_requests_controller_spec.rb b/spec/controllers/projects/merge_requests_controller_spec.rb
index 210085e3b1a..1f6bc84dfe8 100644
--- a/spec/controllers/projects/merge_requests_controller_spec.rb
+++ b/spec/controllers/projects/merge_requests_controller_spec.rb
@@ -392,9 +392,9 @@ describe Projects::MergeRequestsController do
             end
 
             it 'only renders the diffs for the path given' do
-              expect(controller).to receive(:render_diff_for_path).and_wrap_original do |meth, diffs, diff_refs, project|
-                expect(diffs.map(&:new_path)).to contain_exactly(existing_path)
-                meth.call(diffs, diff_refs, project)
+              expect(controller).to receive(:render_diff_for_path).and_wrap_original do |meth, diffs|
+                expect(diffs.diff_files.map(&:new_path)).to contain_exactly(existing_path)
+                meth.call(diffs)
               end
 
               diff_for_path(id: merge_request.iid, old_path: existing_path, new_path: existing_path)
@@ -455,9 +455,9 @@ describe Projects::MergeRequestsController do
         end
 
         it 'only renders the diffs for the path given' do
-          expect(controller).to receive(:render_diff_for_path).and_wrap_original do |meth, diffs, diff_refs, project|
-            expect(diffs.map(&:new_path)).to contain_exactly(existing_path)
-            meth.call(diffs, diff_refs, project)
+          expect(controller).to receive(:render_diff_for_path).and_wrap_original do |meth, diffs|
+            expect(diffs.diff_files.map(&:new_path)).to contain_exactly(existing_path)
+            meth.call(diffs)
           end
 
           diff_for_path(old_path: existing_path, new_path: existing_path, merge_request: { source_branch: 'feature', target_branch: 'master' })
@@ -477,9 +477,9 @@ describe Projects::MergeRequestsController do
           end
 
           it 'only renders the diffs for the path given' do
-            expect(controller).to receive(:render_diff_for_path).and_wrap_original do |meth, diffs, diff_refs, project|
-              expect(diffs.map(&:new_path)).to contain_exactly(existing_path)
-              meth.call(diffs, diff_refs, project)
+            expect(controller).to receive(:render_diff_for_path).and_wrap_original do |meth, diffs|
+              expect(diffs.diff_files.map(&:new_path)).to contain_exactly(existing_path)
+              meth.call(diffs)
             end
 
             diff_for_path(old_path: existing_path, new_path: existing_path, merge_request: { source_project: other_project, source_branch: 'feature', target_branch: 'master' })
diff --git a/spec/helpers/diff_helper_spec.rb b/spec/helpers/diff_helper_spec.rb
index c2fd2c8a533..4949280d641 100644
--- a/spec/helpers/diff_helper_spec.rb
+++ b/spec/helpers/diff_helper_spec.rb
@@ -6,7 +6,7 @@ describe DiffHelper do
   let(:project) { create(:project) }
   let(:repository) { project.repository }
   let(:commit) { project.commit(sample_commit.id) }
-  let(:diffs) { commit.diffs }
+  let(:diffs) { commit.raw_diffs }
   let(:diff) { diffs.first }
   let(:diff_refs) { [commit.parent, commit] }
   let(:diff_file) { Gitlab::Diff::File.new(diff, diff_refs: diff_refs, repository: repository) }
@@ -32,16 +32,6 @@ describe DiffHelper do
   end
   
   describe 'diff_options' do
-    it 'should return hard limit for a diff if force diff is true' do
-      allow(controller).to receive(:params) { { force_show_diff: true } }
-      expect(diff_options).to include(Commit.max_diff_options)
-    end
-
-    it 'should return hard limit for a diff if expand_all_diffs is true' do
-      allow(controller).to receive(:params) { { expand_all_diffs: true } }
-      expect(diff_options).to include(Commit.max_diff_options)
-    end
-
     it 'should return no collapse false' do
       expect(diff_options).to include(no_collapse: false)
     end
@@ -55,6 +45,18 @@ describe DiffHelper do
       allow(controller).to receive(:action_name) { 'diff_for_path' }
       expect(diff_options).to include(no_collapse: true)
     end
+
+    it 'should return paths if action name diff_for_path and param old path' do
+      allow(controller).to receive(:params) { { old_path: 'lib/wadus.rb' } }
+      allow(controller).to receive(:action_name) { 'diff_for_path' }
+      expect(diff_options[:paths]).to include('lib/wadus.rb')
+    end
+
+    it 'should return paths if action name diff_for_path and param new path' do
+      allow(controller).to receive(:params) { { new_path: 'lib/wadus.rb' } }
+      allow(controller).to receive(:action_name) { 'diff_for_path' }
+      expect(diff_options[:paths]).to include('lib/wadus.rb')
+    end
   end
 
   describe 'unfold_bottom_class' do
diff --git a/spec/lib/gitlab/diff/file_spec.rb b/spec/lib/gitlab/diff/file_spec.rb
index e883a6eb9c2..0650cb291e5 100644
--- a/spec/lib/gitlab/diff/file_spec.rb
+++ b/spec/lib/gitlab/diff/file_spec.rb
@@ -5,7 +5,7 @@ describe Gitlab::Diff::File, lib: true do
 
   let(:project) { create(:project) }
   let(:commit) { project.commit(sample_commit.id) }
-  let(:diff) { commit.diffs.first }
+  let(:diff) { commit.raw_diffs.first }
   let(:diff_file) { Gitlab::Diff::File.new(diff, diff_refs: commit.diff_refs, repository: project.repository) }
 
   describe '#diff_lines' do
diff --git a/spec/lib/gitlab/diff/highlight_spec.rb b/spec/lib/gitlab/diff/highlight_spec.rb
index 88e4115c453..1c2ddeed692 100644
--- a/spec/lib/gitlab/diff/highlight_spec.rb
+++ b/spec/lib/gitlab/diff/highlight_spec.rb
@@ -5,7 +5,7 @@ describe Gitlab::Diff::Highlight, lib: true do
 
   let(:project) { create(:project) }
   let(:commit) { project.commit(sample_commit.id) }
-  let(:diff) { commit.diffs.first }
+  let(:diff) { commit.raw_diffs.first }
   let(:diff_file) { Gitlab::Diff::File.new(diff, diff_refs: commit.diff_refs, repository: project.repository) }
 
   describe '#highlight' do
diff --git a/spec/lib/gitlab/diff/line_mapper_spec.rb b/spec/lib/gitlab/diff/line_mapper_spec.rb
index 4e50e03bb7e..4b943fa382d 100644
--- a/spec/lib/gitlab/diff/line_mapper_spec.rb
+++ b/spec/lib/gitlab/diff/line_mapper_spec.rb
@@ -6,7 +6,7 @@ describe Gitlab::Diff::LineMapper, lib: true do
   let(:project) { create(:project) }
   let(:repository) { project.repository }
   let(:commit) { project.commit(sample_commit.id) }
-  let(:diffs) { commit.diffs }
+  let(:diffs) { commit.raw_diffs }
   let(:diff) { diffs.first }
   let(:diff_file) { Gitlab::Diff::File.new(diff, diff_refs: commit.diff_refs, repository: repository) }
   subject { described_class.new(diff_file) }
diff --git a/spec/lib/gitlab/diff/parallel_diff_spec.rb b/spec/lib/gitlab/diff/parallel_diff_spec.rb
index 2aa5ae44f54..af18d3c25a6 100644
--- a/spec/lib/gitlab/diff/parallel_diff_spec.rb
+++ b/spec/lib/gitlab/diff/parallel_diff_spec.rb
@@ -6,7 +6,7 @@ describe Gitlab::Diff::ParallelDiff, lib: true do
   let(:project) { create(:project) }
   let(:repository) { project.repository }
   let(:commit) { project.commit(sample_commit.id) }
-  let(:diffs) { commit.diffs }
+  let(:diffs) { commit.raw_diffs }
   let(:diff) { diffs.first }
   let(:diff_file) { Gitlab::Diff::File.new(diff, diff_refs: commit.diff_refs, repository: repository) }
   subject { described_class.new(diff_file) }
diff --git a/spec/lib/gitlab/diff/parser_spec.rb b/spec/lib/gitlab/diff/parser_spec.rb
index c3359627652..b983d73f8be 100644
--- a/spec/lib/gitlab/diff/parser_spec.rb
+++ b/spec/lib/gitlab/diff/parser_spec.rb
@@ -5,7 +5,7 @@ describe Gitlab::Diff::Parser, lib: true do
 
   let(:project) { create(:project) }
   let(:commit) { project.commit(sample_commit.id) }
-  let(:diff) { commit.diffs.first }
+  let(:diff) { commit.raw_diffs.first }
   let(:parser) { Gitlab::Diff::Parser.new }
 
   describe '#parse' do
diff --git a/spec/lib/gitlab/email/message/repository_push_spec.rb b/spec/lib/gitlab/email/message/repository_push_spec.rb
index c19f33e2224..5b966bddb6a 100644
--- a/spec/lib/gitlab/email/message/repository_push_spec.rb
+++ b/spec/lib/gitlab/email/message/repository_push_spec.rb
@@ -16,9 +16,12 @@ describe Gitlab::Email::Message::RepositoryPush do
       { author_id: author.id, ref: 'master', action: :push, compare: compare,
         send_from_committer_email: true }
     end
-    let(:compare) do
+    let(:raw_compare) do
       Gitlab::Git::Compare.new(project.repository.raw_repository,
-                               sample_image_commit.id, sample_commit.id)
+        sample_image_commit.id, sample_commit.id)
+    end
+    let(:compare) do
+      Compare.decorate(raw_compare, project)
     end
 
     describe '#project' do
@@ -62,17 +65,17 @@ describe Gitlab::Email::Message::RepositoryPush do
 
     describe '#diffs_count' do
       subject { message.diffs_count }
-      it { is_expected.to eq compare.diffs.count }
+      it { is_expected.to eq raw_compare.diffs.size }
     end
 
     describe '#compare' do
       subject { message.compare }
-      it { is_expected.to be_an_instance_of Gitlab::Git::Compare }
+      it { is_expected.to be_an_instance_of Compare }
     end
 
     describe '#compare_timeout' do
       subject { message.compare_timeout }
-      it { is_expected.to eq compare.diffs.overflow? }
+      it { is_expected.to eq raw_compare.diffs.overflow? }
     end
 
     describe '#reverse_compare?' do
diff --git a/spec/mailers/notify_spec.rb b/spec/mailers/notify_spec.rb
index 3685b2b17b5..e2866ef160c 100644
--- a/spec/mailers/notify_spec.rb
+++ b/spec/mailers/notify_spec.rb
@@ -944,8 +944,9 @@ describe Notify do
   describe 'email on push with multiple commits' do
     let(:example_site_path) { root_path }
     let(:user) { create(:user) }
-    let(:compare) { Gitlab::Git::Compare.new(project.repository.raw_repository, sample_image_commit.id, sample_commit.id) }
-    let(:commits) { Commit.decorate(compare.commits, nil) }
+    let(:raw_compare) { Gitlab::Git::Compare.new(project.repository.raw_repository, sample_image_commit.id, sample_commit.id) }
+    let(:compare) { Compare.decorate(raw_compare, project) }
+    let(:commits) { compare.commits }
     let(:diff_path) { namespace_project_compare_path(project.namespace, project, from: Commit.new(compare.base, project), to: Commit.new(compare.head, project)) }
     let(:send_from_committer_email) { false }
     let(:diff_refs) { Gitlab::Diff::DiffRefs.new(base_sha: project.merge_base_commit(sample_image_commit.id, sample_commit.id).id, head_sha: sample_commit.id) }
@@ -1046,8 +1047,9 @@ describe Notify do
   describe 'email on push with a single commit' do
     let(:example_site_path) { root_path }
     let(:user) { create(:user) }
-    let(:compare) { Gitlab::Git::Compare.new(project.repository.raw_repository, sample_commit.parent_id, sample_commit.id) }
-    let(:commits) { Commit.decorate(compare.commits, nil) }
+    let(:raw_compare) { Gitlab::Git::Compare.new(project.repository.raw_repository, sample_commit.parent_id, sample_commit.id) }
+    let(:compare) { Compare.decorate(raw_compare, project) }
+    let(:commits) { compare.commits }
     let(:diff_path) { namespace_project_commit_path(project.namespace, project, commits.first) }
     let(:diff_refs) { Gitlab::Diff::DiffRefs.new(base_sha: project.merge_base_commit(sample_image_commit.id, sample_commit.id).id, head_sha: sample_commit.id) }
 
diff --git a/spec/models/compare_spec.rb b/spec/models/compare_spec.rb
new file mode 100644
index 00000000000..49ab3c4b6e9
--- /dev/null
+++ b/spec/models/compare_spec.rb
@@ -0,0 +1,77 @@
+require 'spec_helper'
+
+describe Compare, models: true do
+  include RepoHelpers
+
+  let(:project) { create(:project, :public) }
+  let(:commit)  { project.commit }
+
+  let(:start_commit) { sample_image_commit }
+  let(:head_commit) { sample_commit }
+
+  let(:raw_compare) { Gitlab::Git::Compare.new(project.repository.raw_repository, start_commit.id, head_commit.id) }
+
+  subject { described_class.new(raw_compare, project) }
+
+  describe '#start_commit' do
+    it 'returns raw compare base commit' do
+      expect(subject.start_commit.id).to eq(start_commit.id)
+    end
+
+    it 'returns nil if compare base commit is nil' do
+      expect(raw_compare).to receive(:base).and_return(nil)
+
+      expect(subject.start_commit).to eq(nil)
+    end
+  end
+
+  describe '#commit' do
+    it 'returns raw compare head commit' do
+      expect(subject.commit.id).to eq(head_commit.id)
+    end
+
+    it 'returns nil if compare head commit is nil' do
+      expect(raw_compare).to receive(:head).and_return(nil)
+
+      expect(subject.commit).to eq(nil)
+    end
+  end
+
+  describe '#base_commit' do
+    let(:base_commit) { Commit.new(another_sample_commit, project) }
+
+    it 'returns project merge base commit' do
+      expect(project).to receive(:merge_base_commit).with(start_commit.id, head_commit.id).and_return(base_commit)
+
+      expect(subject.base_commit).to eq(base_commit)
+    end
+
+    it 'returns nil if there is no start_commit' do
+      expect(subject).to receive(:start_commit).and_return(nil)
+
+      expect(subject.base_commit).to eq(nil)
+    end
+
+    it 'returns nil if there is no head commit' do
+      expect(subject).to receive(:head_commit).and_return(nil)
+
+      expect(subject.base_commit).to eq(nil)
+    end
+  end
+
+  describe '#diff_refs' do
+    it 'uses base_commit sha as base_sha' do
+      expect(subject).to receive(:base_commit).at_least(:once).and_call_original
+
+      expect(subject.diff_refs.base_sha).to eq(subject.base_commit.id)
+    end
+
+    it 'uses start_commit sha as start_sha' do
+      expect(subject.diff_refs.start_sha).to eq(start_commit.id)
+    end
+
+    it 'uses commit sha as head sha' do
+      expect(subject.diff_refs.head_sha).to eq(head_commit.id)
+    end
+  end
+end
diff --git a/spec/models/legacy_diff_note_spec.rb b/spec/models/legacy_diff_note_spec.rb
index d23fc06c3ad..c8ee656fe3b 100644
--- a/spec/models/legacy_diff_note_spec.rb
+++ b/spec/models/legacy_diff_note_spec.rb
@@ -58,7 +58,7 @@ describe LegacyDiffNote, models: true do
 
         # Generate a real line_code value so we know it will match. We use a
         # random line from a random diff just for funsies.
-        diff = merge.diffs.to_a.sample
+        diff = merge.raw_diffs.to_a.sample
         line = Gitlab::Diff::Parser.new.parse(diff.diff.each_line).to_a.sample
         code = Gitlab::Diff::LineCode.generate(diff.new_path, line.new_pos, line.old_pos)
 
diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index 21d22c776e9..e43008c1a4d 100644
--- a/spec/models/merge_request_spec.rb
+++ b/spec/models/merge_request_spec.rb
@@ -128,7 +128,7 @@ describe MergeRequest, models: true do
     end
   end
 
-  describe '#diffs' do
+  describe '#raw_diffs' do
     let(:merge_request) { build(:merge_request) }
     let(:options) { { paths: ['a/b', 'b/a', 'c/*'] } }
 
@@ -138,6 +138,31 @@ describe MergeRequest, models: true do
 
         expect(merge_request.merge_request_diff).to receive(:diffs).with(options)
 
+        merge_request.raw_diffs(options)
+      end
+    end
+
+    context 'when there are no MR diffs' do
+      it 'delegates to the compare object' do
+        merge_request.compare = double(:compare)
+
+        expect(merge_request.compare).to receive(:raw_diffs).with(options)
+
+        merge_request.raw_diffs(options)
+      end
+    end
+  end
+
+  describe '#diffs' do
+    let(:merge_request) { build(:merge_request) }
+    let(:options) { { paths: ['a/b', 'b/a', 'c/*'] } }
+
+    context 'when there are MR diffs' do
+      it 'delegates to the MR diffs' do
+        merge_request.merge_request_diff = MergeRequestDiff.new
+
+        expect(merge_request.merge_request_diff).to receive(:diffs).with(hash_including(options))
+
         merge_request.diffs(options)
       end
     end
@@ -660,6 +685,12 @@ describe MergeRequest, models: true do
       subject.reload_diff
     end
 
+    it "executs diff cache service" do
+      expect_any_instance_of(MergeRequests::MergeRequestDiffCacheService).to receive(:execute).with(subject)
+
+      subject.reload_diff
+    end
+
     it "updates diff note positions" do
       old_diff_refs = subject.diff_refs
 
diff --git a/spec/models/repository_spec.rb b/spec/models/repository_spec.rb
index cce15538b93..2a053b1804f 100644
--- a/spec/models/repository_spec.rb
+++ b/spec/models/repository_spec.rb
@@ -1154,7 +1154,7 @@ describe Repository, models: true do
       it 'does not flush the cache if the commit does not change any logos' do
         diff = double(:diff, new_path: 'test.txt')
 
-        expect(commit).to receive(:diffs).and_return([diff])
+        expect(commit).to receive(:raw_diffs).and_return([diff])
         expect(cache).not_to receive(:expire)
 
         repository.expire_avatar_cache(repository.root_ref, '123')
@@ -1163,7 +1163,7 @@ describe Repository, models: true do
       it 'flushes the cache if the commit changes any of the logos' do
         diff = double(:diff, new_path: Repository::AVATAR_FILES[0])
 
-        expect(commit).to receive(:diffs).and_return([diff])
+        expect(commit).to receive(:raw_diffs).and_return([diff])
         expect(cache).to receive(:expire).with(:avatar)
 
         repository.expire_avatar_cache(repository.root_ref, '123')
diff --git a/spec/requests/api/commits_spec.rb b/spec/requests/api/commits_spec.rb
index e4ea8506598..51ee2167d47 100644
--- a/spec/requests/api/commits_spec.rb
+++ b/spec/requests/api/commits_spec.rb
@@ -173,10 +173,10 @@ describe API::API, api: true  do
       end
 
       it 'should return the inline comment' do
-        post api("/projects/#{project.id}/repository/commits/#{project.repository.commit.id}/comments", user), note: 'My comment', path: project.repository.commit.diffs.first.new_path, line: 7, line_type: 'new'
+        post api("/projects/#{project.id}/repository/commits/#{project.repository.commit.id}/comments", user), note: 'My comment', path: project.repository.commit.raw_diffs.first.new_path, line: 7, line_type: 'new'
         expect(response).to have_http_status(201)
         expect(json_response['note']).to eq('My comment')
-        expect(json_response['path']).to eq(project.repository.commit.diffs.first.new_path)
+        expect(json_response['path']).to eq(project.repository.commit.raw_diffs.first.new_path)
         expect(json_response['line']).to eq(7)
         expect(json_response['line_type']).to eq('new')
       end
diff --git a/spec/services/merge_requests/build_service_spec.rb b/spec/services/merge_requests/build_service_spec.rb
index 782d74ec5ec..232508cda23 100644
--- a/spec/services/merge_requests/build_service_spec.rb
+++ b/spec/services/merge_requests/build_service_spec.rb
@@ -61,7 +61,7 @@ describe MergeRequests::BuildService, services: true do
     end
 
     context 'one commit in the diff' do
-      let(:commits) { [commit_1] }
+      let(:commits) { Commit.decorate([commit_1], project) }
 
       it 'allows the merge request to be created' do
         expect(merge_request.can_be_created).to eq(true)
@@ -84,7 +84,7 @@ describe MergeRequests::BuildService, services: true do
       end
 
       context 'commit has no description' do
-        let(:commits) { [commit_2] }
+        let(:commits) { Commit.decorate([commit_2], project) }
 
         it 'uses the title of the commit as the title of the merge request' do
           expect(merge_request.title).to eq(commit_2.safe_message)
@@ -111,7 +111,7 @@ describe MergeRequests::BuildService, services: true do
         end
 
         context 'commit has no description' do
-          let(:commits) { [commit_2] }
+          let(:commits) { Commit.decorate([commit_2], project) }
 
           it 'sets the description to "Closes #$issue-iid"' do
             expect(merge_request.description).to eq("Closes ##{issue.iid}")
@@ -121,7 +121,7 @@ describe MergeRequests::BuildService, services: true do
     end
 
     context 'more than one commit in the diff' do
-      let(:commits) { [commit_1, commit_2] }
+      let(:commits) { Commit.decorate([commit_1, commit_2], project) }
 
       it 'allows the merge request to be created' do
         expect(merge_request.can_be_created).to eq(true)
diff --git a/spec/services/merge_requests/merge_request_diff_cache_service_spec.rb b/spec/services/merge_requests/merge_request_diff_cache_service_spec.rb
new file mode 100644
index 00000000000..8f71d71b0f0
--- /dev/null
+++ b/spec/services/merge_requests/merge_request_diff_cache_service_spec.rb
@@ -0,0 +1,18 @@
+require 'spec_helper'
+
+describe MergeRequests::MergeRequestDiffCacheService do
+
+  let(:subject) { MergeRequests::MergeRequestDiffCacheService.new }
+
+  describe '#execute' do
+    it 'retrieves the diff files to cache the highlighted result' do
+      merge_request = create(:merge_request)
+      cache_key = [merge_request.merge_request_diff, 'highlighted-diff-files', Gitlab::Diff::FileCollection::MergeRequest.default_options]
+
+      expect(Rails.cache).to receive(:read).with(cache_key).and_return({})
+      expect(Rails.cache).to receive(:write).with(cache_key, anything)
+
+      subject.execute(merge_request)
+    end
+  end
+end