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EffectAnalyzer: Do not reorder mayNotReturn with traps or global writes #9211

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Merged
tlively merged 1 commit into from
Oct 5, 2026
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EffectAnalyzer: Do not reorder mayNotReturn with traps or global writes #9211

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8 changes: 8 additions & 0 deletions src/ir/effects.h
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Original file line number Diff line number Diff line change
Expand Up @@ -443,6 +443,12 @@ class EffectAnalyzer {
return true;
}
}
// Cannot reorder code that may not return with traps or global state
// changes.
if ((mayNotReturn && (other.trap || other.writesGlobalState())) ||
(other.mayNotReturn && (trap || writesGlobalState()))) {

@kripken kripken Oct 5, 2026

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Why handle traps and not throws etc?

@tlively tlively Oct 5, 2026

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The throws case and other control flow cases are already handled here: https://github.com/WebAssembly/binaryen/pull/9211/changes#diff-963609105fd78eff33b9674f5f72d45b5b8dbb8d8e4c866085fdd8c994b51a84R341-R345. mayNotReturn is already considered a side effect, so it is not reordered with respect to control flow.

return true;
}
return false;
}

Expand Down Expand Up @@ -833,6 +839,8 @@ class EffectAnalyzer {
parent.readOrder = parent.writeOrder = MemoryOrder::SeqCst;
// Traps on unaligned accesses.
parent.implicitTrap = true;
// If the timeout is negative and no-one wakes us.
parent.mayNotReturn = true;
}
void visitAtomicNotify(AtomicNotify* curr) {
// Notifies on unshared memories just return 0 or trap on unaligned
Expand Down
7 changes: 7 additions & 0 deletions src/passes/LoopInvariantCodeMotion.cpp
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Expand Up @@ -73,6 +73,13 @@ struct LoopInvariantCodeMotion
EffectAnalyzer loopEffects(getPassOptions(), *getModule(), loop);
loopEffects.localsRead.clear();
loopEffects.localsWritten.clear();
// We can ignore the loop's mayNotReturn effect because any hoisted
// instruction already executes on the first iteration before the loop can
// branch back (or before any inner loop after it; inner loops before it are
// tracked in effectsSoFar), and since the hoisted instruction is
// loop-invariant, it cannot trap on later iterations without trapping on
// the first.
loopEffects.mayNotReturn = false;
// Note all the sets in each loop, and how many per index. Currently
// EffectAnalyzer can't do that, and we need it to know if we
// can move a set out of the loop (if there is another set
Expand Down
11 changes: 11 additions & 0 deletions src/tools/fuzzing/fuzzing.cpp
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Expand Up @@ -1887,6 +1887,17 @@ void TranslateToFuzzReader::modFunction(Function* func) {

void TranslateToFuzzReader::addHangLimitChecks(Function* func) {
// loop limit
// TODO: To detect bugs where optimizations erroneously reorder or eliminate
// infinite loops relative to traps or global effects (e.g. #9182), we
// could selectively omit hang limit instrumentation on pure,
// state-invariant loops. If an innermost loop has mayNotReturn but no
// traps, control flow transfers, global writes, allocations, or updates
// to locals read within the loop, its termination behavior is invariant
// across iterations (it either exits on iteration 1 or loops forever).
// Omitting instrumentation on such loops and relying on an interpreter-
// level iteration limit (with a distinct HangLimit result that skips
// native JS VM comparison) allows detecting mayNotReturn reordering
// bugs without false positives from altered iteration counts.
for (auto* loop : FindAll<Loop>(func->body).list) {
loop->body =
builder.makeSequence(makeHangLimitCheck(), loop->body, loop->type);
Expand Down
90 changes: 90 additions & 0 deletions test/gtest/effects.cpp
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Expand Up @@ -110,4 +110,94 @@ TEST_F(EffectAnalyzerTest, UnknownCall) {
EXPECT_FALSE(effectsWithoutStackSwitch.suspends);
}

TEST_F(EffectAnalyzerTest, MayNotReturnOrdering) {
auto moduleText = R"wasm(
(module
(memory 1 1 shared)
(global $g (mut i32) (i32.const 0))
(func $callee)
(func $test (param $x i32) (param $y i32)
(loop $l1
(br_if $l1 (local.get $x))
)
(loop $l2
(br_if $l2 (local.get $y))
)
(drop (i32.div_s (i32.const 1) (local.get $y)))
(call $callee)
(global.set $g (i32.const 1))
(local.set $y (i32.const 2))
(drop (memory.atomic.wait32 (i32.const 0) (i32.const 0) (i64.const -1)))
)
)
)wasm";

auto parseResult = WATParser::parseModule(wasm, moduleText);
ASSERT_FALSE(parseResult.getErr());

auto* func = wasm.getFunction("test");
ASSERT_NE(func, nullptr);
auto* block = func->body->cast<Block>();
ASSERT_EQ(block->list.size(), 7u);

auto* loop1 = block->list[0];
auto* loop2 = block->list[1];
auto* trapExpr = block->list[2];
auto* callExpr = block->list[3];
auto* globalSetExpr = block->list[4];
auto* localSetExpr = block->list[5];
auto* waitExpr = block->list[6];

EffectAnalyzer loop1Effects(options, wasm, loop1);
EffectAnalyzer loop2Effects(options, wasm, loop2);
EffectAnalyzer trapEffects(options, wasm, trapExpr);
EffectAnalyzer callEffects(options, wasm, callExpr);
EffectAnalyzer globalSetEffects(options, wasm, globalSetExpr);
EffectAnalyzer localSetEffects(options, wasm, localSetExpr);
EffectAnalyzer waitEffects(options, wasm, waitExpr);

EXPECT_THAT(&loop1Effects, MayNotReturn());
EXPECT_FALSE(loop1Effects.transfersControlFlow());
EXPECT_THAT(&waitEffects, MayNotReturn());

// Cannot reorder mayNotReturn with traps, calls, or global writes.
EXPECT_TRUE(loop1Effects.orderedBefore(trapEffects));
EXPECT_TRUE(trapEffects.orderedBefore(loop1Effects));
EXPECT_FALSE(EffectAnalyzer::canReorder(options, wasm, loop1, trapExpr));
EXPECT_FALSE(EffectAnalyzer::canReorder(options, wasm, trapExpr, loop1));

// Even with trapsNeverHappen, mayNotReturn (including atomic.wait) cannot be
// reordered with a trap.
PassOptions tnhOptions = options;
tnhOptions.trapsNeverHappen = true;
EXPECT_FALSE(EffectAnalyzer::canReorder(tnhOptions, wasm, loop1, trapExpr));
EXPECT_FALSE(EffectAnalyzer::canReorder(tnhOptions, wasm, trapExpr, loop1));
EXPECT_FALSE(
EffectAnalyzer::canReorder(tnhOptions, wasm, waitExpr, trapExpr));
EXPECT_FALSE(
EffectAnalyzer::canReorder(tnhOptions, wasm, trapExpr, waitExpr));

EXPECT_TRUE(loop1Effects.orderedBefore(callEffects));
EXPECT_TRUE(callEffects.orderedBefore(loop1Effects));
EXPECT_FALSE(EffectAnalyzer::canReorder(options, wasm, loop1, callExpr));
EXPECT_FALSE(EffectAnalyzer::canReorder(options, wasm, callExpr, loop1));

EXPECT_TRUE(loop1Effects.orderedBefore(globalSetEffects));
EXPECT_TRUE(globalSetEffects.orderedBefore(loop1Effects));
EXPECT_FALSE(EffectAnalyzer::canReorder(options, wasm, loop1, globalSetExpr));
EXPECT_FALSE(EffectAnalyzer::canReorder(options, wasm, globalSetExpr, loop1));

// Can reorder mayNotReturn with unrelated local writes or another pure
// mayNotReturn.
EXPECT_FALSE(loop1Effects.orderedBefore(localSetEffects));
EXPECT_FALSE(localSetEffects.orderedBefore(loop1Effects));
EXPECT_TRUE(EffectAnalyzer::canReorder(options, wasm, loop1, localSetExpr));
EXPECT_TRUE(EffectAnalyzer::canReorder(options, wasm, localSetExpr, loop1));

EXPECT_FALSE(loop1Effects.orderedBefore(loop2Effects));
EXPECT_FALSE(loop2Effects.orderedBefore(loop1Effects));
EXPECT_TRUE(EffectAnalyzer::canReorder(options, wasm, loop1, loop2));
EXPECT_TRUE(EffectAnalyzer::canReorder(options, wasm, loop2, loop1));
}

} // anonymous namespace
89 changes: 82 additions & 7 deletions test/lit/passes/licm.wast
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Expand Up @@ -5,9 +5,9 @@
(module
(memory 10 20)

;; CHECK: (type $0 (func (param i32)))
;; CHECK: (type $0 (func (param i32) (result i32)))

;; CHECK: (type $1 (func (param i32) (result i32)))
;; CHECK: (type $1 (func (param i32)))

;; CHECK: (type $2 (func))

Expand Down Expand Up @@ -37,7 +37,7 @@
)
)

;; CHECK: (func $unreachable-get-call (type $0) (param $p i32)
;; CHECK: (func $unreachable-get-call (type $1) (param $p i32)
;; CHECK-NEXT: (local $x i32)
;; CHECK-NEXT: (loop $loop
;; CHECK-NEXT: (unreachable)
Expand All @@ -58,7 +58,7 @@
)
)

;; CHECK: (func $unreachable-get-store (type $0) (param $p i32)
;; CHECK: (func $unreachable-get-store (type $1) (param $p i32)
;; CHECK-NEXT: (local $x i32)
;; CHECK-NEXT: (loop $loop
;; CHECK-NEXT: (unreachable)
Expand All @@ -81,7 +81,7 @@
)
)

;; CHECK: (func $pause (type $0) (param $p i32)
;; CHECK: (func $pause (type $1) (param $p i32)
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (i32.const 0)
;; CHECK-NEXT: )
Expand All @@ -107,7 +107,7 @@
)
)

;; CHECK: (func $bug-inversion (type $1) (param $z i32) (result i32)
;; CHECK: (func $bug-inversion (type $0) (param $z i32) (result i32)
;; CHECK-NEXT: (local $x i32)
;; CHECK-NEXT: (local $y i32)
;; CHECK-NEXT: (local.set $y
Expand Down Expand Up @@ -145,7 +145,7 @@
(local.get $x)
)

;; CHECK: (func $bug-cross-statement-dependency (type $1) (param $z i32) (result i32)
;; CHECK: (func $bug-cross-statement-dependency (type $0) (param $z i32) (result i32)
;; CHECK-NEXT: (local $x i32)
;; CHECK-NEXT: (local $y i32)
;; CHECK-NEXT: (local.set $x
Expand Down Expand Up @@ -182,4 +182,79 @@
)
(local.get $y)
)

;; CHECK: (func $hoist-trap-before-backedge (type $0) (param $x i32) (result i32)
;; CHECK-NEXT: (local $y i32)
;; CHECK-NEXT: (block
;; CHECK-NEXT: (local.set $y
;; CHECK-NEXT: (i32.load
;; CHECK-NEXT: (i32.const 0)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (loop $loop
;; CHECK-NEXT: (nop)
;; CHECK-NEXT: (br_if $loop
;; CHECK-NEXT: (local.get $x)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (local.get $y)
;; CHECK-NEXT: )
(func $hoist-trap-before-backedge (param $x i32) (result i32)
(local $y i32)
;; A trapping instruction before the loop's own backedge can still be
;; hoisted, because the first iteration always executes it.
(loop $loop
(local.set $y
(i32.load
(i32.const 0)
)
)
(br_if $loop
(local.get $x)
)
)
(local.get $y)
)

;; CHECK: (func $no-hoist-trap-past-inner-loop (type $0) (param $x i32) (result i32)
;; CHECK-NEXT: (local $y i32)
;; CHECK-NEXT: (loop $outer
;; CHECK-NEXT: (loop $inner
;; CHECK-NEXT: (br_if $inner
;; CHECK-NEXT: (local.get $x)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (local.set $y
;; CHECK-NEXT: (i32.load
;; CHECK-NEXT: (i32.const 0)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (br_if $outer
;; CHECK-NEXT: (local.get $x)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (local.get $y)
;; CHECK-NEXT: )
(func $no-hoist-trap-past-inner-loop (param $x i32) (result i32)
(local $y i32)
;; A trapping instruction after an inner loop cannot be hoisted before the
;; outer loop, because the inner loop might not terminate.
(loop $outer
(loop $inner
(br_if $inner
(local.get $x)
)
)
(local.set $y
(i32.load
(i32.const 0)
)
)
(br_if $outer
(local.get $x)
)
)
(local.get $y)
)
)
87 changes: 85 additions & 2 deletions test/lit/passes/merge-blocks.wast
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Expand Up @@ -186,7 +186,7 @@
)
)

;; CHECK: (func $if-condition (type $6) (result i32)
;; CHECK: (func $if-condition (type $7) (result i32)
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (i32.const 0)
;; CHECK-NEXT: )
Expand Down Expand Up @@ -424,7 +424,90 @@
)
)

;; CHECK: (func $helper (type $7) (param $x i32) (result i32)
;; CHECK: (func $subsequent-children-loop-call (type $6) (param $x i32) (param $y i32) (result i32)
;; CHECK-NEXT: (call $subsequent-children
;; CHECK-NEXT: (loop $loop (result i32)
;; CHECK-NEXT: (br_if $loop
;; CHECK-NEXT: (local.get $x)
;; CHECK-NEXT: )
;; CHECK-NEXT: (i32.const 1)
;; CHECK-NEXT: )
;; CHECK-NEXT: (i32.const 2)
;; CHECK-NEXT: (block (result i32)
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (call $helper
;; CHECK-NEXT: (i32.const 3)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (i32.const 4)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: )
(func $subsequent-children-loop-call (param $x i32) (param $y i32) (result i32)
;; A loop that may not return in an earlier child prevents moving a call out
;; of a later child past it.
(call $subsequent-children
(loop $loop (result i32)
(br_if $loop
(local.get $x)
)
(i32.const 1)
)
(i32.const 2)
(block (result i32)
(drop
(call $helper
(i32.const 3)
)
)
(i32.const 4)
)
)
)

;; CHECK: (func $subsequent-children-loop-trap (type $6) (param $x i32) (param $y i32) (result i32)
;; CHECK-NEXT: (i32.add
;; CHECK-NEXT: (loop $loop (result i32)
;; CHECK-NEXT: (br_if $loop
;; CHECK-NEXT: (local.get $x)
;; CHECK-NEXT: )
;; CHECK-NEXT: (i32.const 1)
;; CHECK-NEXT: )
;; CHECK-NEXT: (block (result i32)
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (i32.div_s
;; CHECK-NEXT: (i32.const 1)
;; CHECK-NEXT: (local.get $y)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (i32.const 2)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: )
(func $subsequent-children-loop-trap (param $x i32) (param $y i32) (result i32)
;; A loop that may not return in an earlier child prevents moving a trap out
;; of a later child past it.
(i32.add
(loop $loop (result i32)
(br_if $loop
(local.get $x)
)
(i32.const 1)
)
(block (result i32)
(drop
(i32.div_s
(i32.const 1)
(local.get $y)
)
)
(i32.const 2)
)
)
)


;; CHECK: (func $helper (type $8) (param $x i32) (result i32)
;; CHECK-NEXT: (unreachable)
;; CHECK-NEXT: )
(func $helper (param $x i32) (result i32)
Expand Down
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