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YJIT: adjust branch shape properly when target already exists

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The old code decides branch->shape based on the write position of the
native code block, which is unsound in case the block already exists
and no new code is written to the write position.

Make this decision with the start address of the target block instead.
Also handle when the branch becomes smaller after patching.
This commit is contained in:
Alan Wu 2021-04-05 17:59:25 -04:00
parent e56bd95ca0
commit 21a6345023
2 changed files with 17 additions and 16 deletions
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31
yjit_core.c
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@ -418,7 +418,7 @@ uint8_t* gen_entry_point(const rb_iseq_t *iseq, uint32_t insn_idx, rb_execution_
// Called by the generated code when a branch stub is executed
// Triggers compilation of branches and code patching
static uint8_t *
branch_stub_hit(uint32_t branch_idx, uint32_t target_idx, rb_execution_context_t* ec)
branch_stub_hit(const uint32_t branch_idx, const uint32_t target_idx, rb_execution_context_t* ec)
{
uint8_t* dst_addr;
@ -449,18 +449,6 @@ branch_stub_hit(uint32_t branch_idx, uint32_t target_idx, rb_execution_context_t
// may be out of sync in JITted code
ec->cfp->pc = iseq_pc_at_idx(target.iseq, target.idx);
// If either of the target blocks will be placed next
if (cb->write_pos == branch->end_pos)
{
//fprintf(stderr, "target idx %d will be placed next\n", target_idx);
branch->shape = (uint8_t)target_idx;
// Rewrite the branch with the new, potentially more compact shape
cb_set_pos(cb, branch->start_pos);
branch->gen_fn(cb, branch->dst_addrs[0], branch->dst_addrs[1], branch->shape);
RUBY_ASSERT(cb->write_pos <= branch->end_pos);
}
// Try to find an existing compiled version of this block
block_t* p_block = find_block_version(target, target_ctx);
@ -487,13 +475,26 @@ branch_stub_hit(uint32_t branch_idx, uint32_t target_idx, rb_execution_context_t
dst_addr = cb_get_ptr(cb, p_block->start_pos);
branch->dst_addrs[target_idx] = dst_addr;
// Adjust brach shape base on block placement relative to the branch
if (branch->end_pos == p_block->start_pos) {
branch->shape = (branch_shape_t)target_idx;
}
// Rewrite the branch with the new jump target address
RUBY_ASSERT(branch->dst_addrs[0] != NULL);
uint32_t cur_pos = cb->write_pos;
cb_set_pos(cb, branch->start_pos);
branch->gen_fn(cb, branch->dst_addrs[0], branch->dst_addrs[1], branch->shape);
RUBY_ASSERT(cb->write_pos <= branch->end_pos);
branch->end_pos = cb->write_pos;
RUBY_ASSERT(cb->write_pos <= branch->end_pos && "can't enlarge a branch");
// If the branch got smaller
if (cb->write_pos < branch->end_pos) {
// fill the difference with nops
uint32_t shrinkage = branch->end_pos - cb->write_pos;
nop(cb, shrinkage);
}
// Done patching the branch. Restore write position.
cb_set_pos(cb, cur_pos);
// Restore interpreter sp, since the code hitting the stub expects the original.

2
yjit_core.h
View file

@ -156,7 +156,7 @@ typedef void (*branchgen_fn)(codeblock_t* cb, uint8_t* target0, uint8_t* target1
Store info about an outgoing branch in a code segment
Note: care must be taken to minimize the size of branch_t objects
*/
typedef struct BranchEntry
typedef struct yjit_branch_entry
{
// Positions where the generated code starts and ends
uint32_t start_pos;