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Reland "[wasm][liftoff] Optimize one-armed ifs"

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This is a reland of c2aaf0a6fa

Original change's description:
> [wasm][liftoff] Optimize one-armed ifs
> 
> Do not implement one-armed ifs by emulating an empty else branch. In
> Liftoff, we can generate better code and save compile time by handling
> this specially. If the merge point at the end of the if is not reached
> by the if-branch, we do not need to generate any merge code.
> 
> R=titzer@chromium.org
> 
> Bug: v8:6600, v8:8423
> Change-Id: Ie8ea69dd7491f225605a8e1b986d275d869aa90b
> Reviewed-on: https://chromium-review.googlesource.com/c/1356508
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Reviewed-by: Ben Titzer <titzer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#57968}

Bug: v8:6600, v8:8423
Change-Id: I6d5eea9f860486768779a33bf6bd7b87cbfc2af0
Reviewed-on: https://chromium-review.googlesource.com/c/1361040
Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58024}
This commit is contained in:
Clemens Hammacher 2018-12-04 14:59:58 +01:00 committed by Commit Bot
parent 1fba4b7cff
commit 7768b245d9
4 changed files with 61 additions and 16 deletions
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38
src/wasm/baseline/liftoff-compiler.cc
View File

@ -498,8 +498,10 @@ class LiftoffCompiler {
if (c->end_merge.reached) {
__ MergeFullStackWith(c->label_state);
} else if (c->is_onearmed_if()) {
c->label_state.InitMerge(*__ cache_state(), __ num_locals(),
c->br_merge()->arity);
// Init the merge point from the else state, then merge the if state into
// that.
DCHECK_EQ(0, c->end_merge.arity);
c->label_state.InitMerge(c->else_state->state, __ num_locals(), 0);
__ MergeFullStackWith(c->label_state);
} else {
c->label_state.Split(*__ cache_state());
@ -508,7 +510,22 @@ class LiftoffCompiler {
}
void PopControl(FullDecoder* decoder, Control* c) {
if (!c->is_loop() && c->end_merge.reached) {
if (c->is_onearmed_if()) {
if (c->end_merge.reached) {
// Generate the code to merge the else state into the end state.
// TODO(clemensh): Do this without switching to the else state first.
__ emit_jump(c->label.get());
__ bind(c->else_state->label.get());
__ cache_state()->Steal(c->else_state->state);
__ MergeFullStackWith(c->label_state);
__ cache_state()->Steal(c->label_state);
} else {
// There is no merge at the end of the if, so just continue with the
// else state.
__ bind(c->else_state->label.get());
__ cache_state()->Steal(c->else_state->state);
}
} else if (!c->is_loop() && c->end_merge.reached) {
__ cache_state()->Steal(c->label_state);
}
if (!c->label.get()->is_bound()) {
@ -1310,10 +1327,17 @@ class LiftoffCompiler {
DCHECK(!table_iterator.has_next());
}
void Else(FullDecoder* decoder, Control* if_block) {
if (if_block->reachable()) __ emit_jump(if_block->label.get());
__ bind(if_block->else_state->label.get());
__ cache_state()->Steal(if_block->else_state->state);
void Else(FullDecoder* decoder, Control* c) {
if (c->reachable()) {
if (!c->end_merge.reached) {
c->label_state.InitMerge(*__ cache_state(), __ num_locals(),
c->end_merge.arity);
}
__ MergeFullStackWith(c->label_state);
__ emit_jump(c->label.get());
}
__ bind(c->else_state->label.get());
__ cache_state()->Steal(c->else_state->state);
}
Label* AddOutOfLineTrap(WasmCodePosition position,

19
src/wasm/function-body-decoder-impl.h
View File

@ -1748,9 +1748,10 @@ class WasmFullDecoder : public WasmDecoder<validate> {
this->error(this->pc_, "else already present for if");
break;
}
FallThruTo(c);
if (!TypeCheckFallThru(c)) break;
c->kind = kControlIfElse;
CALL_INTERFACE_IF_PARENT_REACHABLE(Else, c);
if (c->reachable()) c->end_merge.reached = true;
PushMergeValues(c, &c->start_merge);
c->reachability = control_at(1)->innerReachability();
break;
@ -1758,7 +1759,7 @@ class WasmFullDecoder : public WasmDecoder<validate> {
case kExprEnd: {
if (!VALIDATE(!control_.empty())) {
this->error("end does not match any if, try, or block");
return;
break;
}
Control* c = &control_.back();
if (!VALIDATE(!c->is_incomplete_try())) {
@ -1766,12 +1767,12 @@ class WasmFullDecoder : public WasmDecoder<validate> {
break;
}
if (c->is_onearmed_if()) {
// Emulate empty else arm.
FallThruTo(c);
if (this->failed()) break;
CALL_INTERFACE_IF_PARENT_REACHABLE(Else, c);
PushMergeValues(c, &c->start_merge);
c->reachability = control_at(1)->innerReachability();
if (!VALIDATE(c->end_merge.arity == c->start_merge.arity)) {
this->error(
c->pc,
"start-arity and end-arity of one-armed if must match");
break;
}
}
if (c->is_try_catch()) {
// Emulate catch-all + re-throw.
@ -2301,7 +2302,7 @@ class WasmFullDecoder : public WasmDecoder<validate> {
void PopControl(Control* c) {
DCHECK_EQ(c, &control_.back());
CALL_INTERFACE_IF_PARENT_REACHABLE(PopControl, c);
bool reached = c->end_merge.reached;
bool reached = c->end_merge.reached || c->is_onearmed_if();
control_.pop_back();
// If the parent block was reachable before, but the popped control does not
// return to here, this block becomes indirectly unreachable.

9
src/wasm/graph-builder-interface.cc
View File

@ -200,6 +200,11 @@ class WasmGraphBuildingInterface {
}
void PopControl(FullDecoder* decoder, Control* block) {
if (block->is_onearmed_if()) {
// Merge the else branch into the end merge.
SetEnv(block->false_env);
MergeValuesInto(decoder, block, &block->end_merge);
}
if (!block->is_loop()) SetEnv(block->end_env);
}
@ -338,6 +343,10 @@ class WasmGraphBuildingInterface {
}
void Else(FullDecoder* decoder, Control* if_block) {
if (if_block->reachable()) {
// Merge the if branch into the end merge.
MergeValuesInto(decoder, if_block, &if_block->end_merge);
}
SetEnv(if_block->false_env);
}

11
test/unittests/wasm/function-body-decoder-unittest.cc
View File

@ -18,6 +18,7 @@
#include "test/common/wasm/flag-utils.h"
#include "test/common/wasm/test-signatures.h"
#include "test/common/wasm/wasm-macro-gen.h"
#include "testing/gmock-support.h"
namespace v8 {
namespace internal {
@ -146,6 +147,9 @@ class FunctionBodyDecoderTest : public TestWithZone {
str << "Verification successed, expected failure; pc = +" << pc;
}
EXPECT_EQ(result.ok(), expected_success) << str.str();
if (!expected_success && message) {
EXPECT_THAT(result.error_msg(), ::testing::HasSubstr(message));
}
}
void TestBinop(WasmOpcode opcode, FunctionSig* success) {
@ -786,6 +790,13 @@ TEST_F(FunctionBodyDecoderTest, IfElseUnreachable2) {
}
}
TEST_F(FunctionBodyDecoderTest, OneArmedIfWithArity) {
static const byte code[] = {WASM_ZERO, kExprIf, kLocalI32, WASM_ONE,
kExprEnd};
EXPECT_FAILURE_C(i_v, code,
"start-arity and end-arity of one-armed if must match");
}
TEST_F(FunctionBodyDecoderTest, IfBreak) {
EXPECT_VERIFIES(v_i, WASM_IF(WASM_GET_LOCAL(0), WASM_BR(0)));
EXPECT_VERIFIES(v_i, WASM_IF(WASM_GET_LOCAL(0), WASM_BR(1)));