From 738160db0a7fb32d609133fb68fbdff02ff315f2 Mon Sep 17 00:00:00 2001
From: Maxime Chevalier-Boisvert <maxime.chevalierboisvert@shopify.com>
Date: Fri, 22 Jan 2021 14:57:44 -0500
Subject: [PATCH] Write ctx_diff() function to compate ctx_t objects

---
 ujit_core.c | 48 +++++++++++++++++++++++++++++++++++++++++++-----
 ujit_core.h |  1 +
 2 files changed, 44 insertions(+), 5 deletions(-)

diff --git a/ujit_core.c b/ujit_core.c
index 2143ab3923..8e7ce31330 100644
--- a/ujit_core.c
+++ b/ujit_core.c
@@ -103,6 +103,42 @@ ctx_get_top_type(ctx_t* ctx)
     return ctx->temp_types[ctx->stack_size - 1];
 }
 
+/**
+Compute a difference score for two context objects
+Returns 0 if the two contexts are the same
+Returns -1 if incompatible
+Returns > 0 if different but compatible
+*/
+int ctx_diff(const ctx_t* src, const ctx_t* dst)
+{
+    if (dst->stack_size != src->stack_size)
+        return -1;
+
+    if (dst->self_is_object != src->self_is_object)
+        return -1;
+
+    // Difference sum
+    int diff = 0;
+
+    // For each temporary variable
+    for (size_t i = 0; i < MAX_TEMP_TYPES; ++i)
+    {
+        int t_src = src->temp_types[i];
+        int t_dst = dst->temp_types[i];
+
+        if (t_dst != t_src)
+        {
+            // It's OK to lose some type information
+            if (t_dst == T_NONE)
+                diff += 1;
+            else
+                return -1;
+        }
+    }
+
+    return diff;
+}
+
 // Add an incoming branch for a given block version
 static void add_incoming(block_t* p_block, uint32_t branch_idx)
 {
@@ -118,16 +154,18 @@ static void add_incoming(block_t* p_block, uint32_t branch_idx)
 block_t* find_block_version(blockid_t blockid, const ctx_t* ctx)
 {
     // If there exists a version for this block id
-    st_data_t st_version;
-    if (rb_st_lookup(version_tbl, (st_data_t)&blockid, &st_version)) {
-        return (block_t*)st_version;
-    }
+    block_t* first_version;
+    if (!rb_st_lookup(version_tbl, (st_data_t)&blockid, (st_data_t*)&first_version))
+        return NULL;
 
     //
     // TODO: use the ctx parameter to search existing versions for a match
     //
 
-    return NULL;
+    // Check that the version found is actually compatible
+    RUBY_ASSERT(ctx_diff(ctx, &first_version->ctx) >= 0);
+
+    return first_version;
 }
 // Compile a new block version immediately
 block_t* gen_block_version(blockid_t blockid, const ctx_t* start_ctx)
diff --git a/ujit_core.h b/ujit_core.h
index ff73890e4d..7b230d779d 100644
--- a/ujit_core.h
+++ b/ujit_core.h
@@ -130,6 +130,7 @@ x86opnd_t ctx_stack_push(ctx_t* ctx, int type);
 x86opnd_t ctx_stack_pop(ctx_t* ctx, size_t n);
 x86opnd_t ctx_stack_opnd(ctx_t* ctx, int32_t idx);
 int ctx_get_top_type(ctx_t* ctx);
+int ctx_diff(const ctx_t* src, const ctx_t* dst);
 
 block_t* find_block_version(blockid_t blockid, const ctx_t* ctx);
 block_t* gen_block_version(blockid_t blockid, const ctx_t* ctx);