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Iterate in assembly order for jump threading

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While reading through the jump threading implementation, I noticed
something strange: ApplyForwarding iterates through the block list in
reverse post-order, not in assembly order. Thus, the value prev_fallthru
might not refer to the previous block in assembly order. Obviously it
works fine this way or we would have noticed by now, but I think that
this step would be a little easier to read and reason about if the
iteration used assembly order instead.

I've added a test case to demonstrate the difference when using
assembly order: in a diamond where the right side starts with an empty
deferred block, the current implementation would fail to replace that
block with a nop. I doubt this case would have any real-world impact.

Change-Id: I28abe2043434debb54896871d15c540ad52c6368
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3039261
Commit-Queue: Seth Brenith <seth.brenith@microsoft.com>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76067}
This commit is contained in:
Seth Brenith 2021-08-03 09:38:52 -07:00 committed by V8 LUCI CQ
parent 80aed7e2cc
commit fcc81d9a0c
2 changed files with 25 additions and 2 deletions
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2
src/compiler/backend/jump-threading.cc
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@ -206,7 +206,7 @@ void JumpThreading::ApplyForwarding(Zone* local_zone,
// Skip empty blocks when the previous block doesn't fall through.
bool prev_fallthru = true;
for (auto const block : code->instruction_blocks()) {
for (auto const block : code->ao_blocks()) {
RpoNumber block_rpo = block->rpo_number();
int block_num = block_rpo.ToInt();
RpoNumber result_rpo = result[block_num];

25
test/cctest/compiler/test-jump-threading.cc
View File

@ -736,7 +736,7 @@ TEST(Rewire1_deferred) {
code.Defer();
int j3 = code.Jump(3);
// B3
code.End();
code.Return(0);
static int forward[] = {3, 3, 3, 3};
ApplyForwarding(&code, kBlockCount, forward);
@ -774,6 +774,29 @@ TEST(Rewire2_deferred) {
CheckAssemblyOrder(&code, kBlockCount, assembly);
}
TEST(Rewire_deferred_diamond) {
constexpr size_t kBlockCount = 4;
TestCode code(kBlockCount);
// B0
int b1 = code.Branch(1, 2);
// B1
code.Fallthru(); // To B3
// B2
code.Defer();
int j1 = code.Jump(3);
// B3
code.Return(0);
static int forward[] = {0, 3, 3, 3};
VerifyForwarding(&code, kBlockCount, forward);
ApplyForwarding(&code, kBlockCount, forward);
CheckBranch(&code, b1, 3, 3);
CheckNop(&code, j1);
static int assembly[] = {0, 1, 2, 1};
CheckAssemblyOrder(&code, kBlockCount, assembly);
}
TEST(Rewire_diamond) {
constexpr size_t kBlockCount = 5;