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Merged
merged 3 commits into from
Jul 8, 2024
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StackIR: Optimize away a drop before an unreachable #6719

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1c47a58
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kripken Jul 8, 2024
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kripken Jul 8, 2024
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39 changes: 38 additions & 1 deletion src/wasm/wasm-stack-opts.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -44,8 +44,9 @@ void StackIROptimizer::run() {
vacuum();
}

// Remove unreachable code.
void StackIROptimizer::dce() {
// Remove code after an unreachable instruction: anything after it, up to the
// next control flow barrier, can simply be removed.
bool inUnreachableCode = false;
for (Index i = 0; i < insts.size(); i++) {
auto* inst = insts[i];
Expand All @@ -64,6 +65,42 @@ void StackIROptimizer::dce() {
inUnreachableCode = true;
}
}

// Remove code before an Unreachable. Consider this:
//
// (drop
// ..
// )
// (unreachable)
//
// The drop is not needed, as the unreachable puts the stack in the
// polymorphic state anyhow. Note that we don't need to optimize anything
// other than a drop here, as in general the Binaryen IR DCE pass will handle
// everything else. A drop followed by an unreachable is the only thing that
// pass cannot handle, as the structured form of Binaryen IR does not allow
// removing such a drop, and so we can only do it here in StackIR.
//
// TODO: We can look even further back, say if there is another drop of
// something before, then we can remove that drop as well. To do that
// we'd need to inspect the stack going backwards.
for (Index i = 1; i < insts.size(); i++) {
auto* inst = insts[i];
if (!inst || inst->op != StackInst::Basic ||
!inst->origin->is<Unreachable>()) {
continue;
}

// Look back past nulls.
Index j = i - 1;
while (j > 0 && !insts[j]) {
j--;
}

auto*& prev = insts[j];
if (prev && prev->op == StackInst::Basic && prev->origin->is<Drop>()) {
prev = nullptr;
}
}
}

// Remove obviously-unneeded code.
Expand Down
118 changes: 118 additions & 0 deletions test/lit/passes/stack-ir-dce.wast
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@@ -0,0 +1,118 @@
;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.

;; RUN: wasm-opt %s --generate-stack-ir --optimize-stack-ir -all --print-stack-ir | filecheck %s
;; Also verify we roundtrip the output here properly.
;; RUN: wasm-opt %s --generate-stack-ir --optimize-stack-ir -all --roundtrip --print | filecheck %s --check-prefix=ROUNDTRIP

(module
;; CHECK: (func $drop-unreachable (type $0) (result i32)
;; CHECK-NEXT: call $drop-unreachable
;; CHECK-NEXT: unreachable
;; CHECK-NEXT: )
;; ROUNDTRIP: (func $drop-unreachable (type $0) (result i32)
;; ROUNDTRIP-NEXT: (drop
;; ROUNDTRIP-NEXT: (call $drop-unreachable)
;; ROUNDTRIP-NEXT: )
;; ROUNDTRIP-NEXT: (unreachable)
;; ROUNDTRIP-NEXT: )
(func $drop-unreachable (result i32)
;; This drop can be removed.
(drop
(call $drop-unreachable)
)
(unreachable)
)

;; CHECK: (func $unreachable (type $0) (result i32)
;; CHECK-NEXT: unreachable
;; CHECK-NEXT: )
;; ROUNDTRIP: (func $unreachable (type $0) (result i32)
;; ROUNDTRIP-NEXT: (unreachable)
;; ROUNDTRIP-NEXT: )
(func $unreachable (result i32)
;; An unreachable with nothing before it. Check we do not error here.
(unreachable)
)

;; CHECK: (func $unreachable-non-drop (type $1)
;; CHECK-NEXT: call $unreachable-non-drop
;; CHECK-NEXT: unreachable
;; CHECK-NEXT: )
;; ROUNDTRIP: (func $unreachable-non-drop (type $1)
;; ROUNDTRIP-NEXT: (call $unreachable-non-drop)
;; ROUNDTRIP-NEXT: (unreachable)
;; ROUNDTRIP-NEXT: )
(func $unreachable-non-drop
;; An unreachable with something other than a drop before it. Check we do
;; not error here.
(call $unreachable-non-drop)
(unreachable)
)

;; CHECK: (func $many-drop-unreachable (type $0) (result i32)
;; CHECK-NEXT: i32.const 1
;; CHECK-NEXT: if (result i32)
;; CHECK-NEXT: call $drop-unreachable
;; CHECK-NEXT: unreachable
;; CHECK-NEXT: else
;; CHECK-NEXT: call $drop-unreachable
;; CHECK-NEXT: unreachable
;; CHECK-NEXT: end
;; CHECK-NEXT: drop
;; CHECK-NEXT: call $drop-unreachable
;; CHECK-NEXT: unreachable
;; CHECK-NEXT: )
;; ROUNDTRIP: (func $many-drop-unreachable (type $0) (result i32)
;; ROUNDTRIP-NEXT: (drop
;; ROUNDTRIP-NEXT: (if (result i32)
;; ROUNDTRIP-NEXT: (i32.const 1)
;; ROUNDTRIP-NEXT: (then
;; ROUNDTRIP-NEXT: (drop
;; ROUNDTRIP-NEXT: (call $drop-unreachable)
;; ROUNDTRIP-NEXT: )
;; ROUNDTRIP-NEXT: (unreachable)
;; ROUNDTRIP-NEXT: )
;; ROUNDTRIP-NEXT: (else
;; ROUNDTRIP-NEXT: (drop
;; ROUNDTRIP-NEXT: (call $drop-unreachable)
;; ROUNDTRIP-NEXT: )
;; ROUNDTRIP-NEXT: (unreachable)
;; ROUNDTRIP-NEXT: )
;; ROUNDTRIP-NEXT: )
;; ROUNDTRIP-NEXT: )
;; ROUNDTRIP-NEXT: (drop
;; ROUNDTRIP-NEXT: (call $drop-unreachable)
;; ROUNDTRIP-NEXT: )
;; ROUNDTRIP-NEXT: (unreachable)
;; ROUNDTRIP-NEXT: )
(func $many-drop-unreachable (result i32)
;; Two drop-unreachables in an if. The drop on the if can remain, but all
;; others are removable.
(drop
(if (result i32)
(i32.const 1)
(then
(drop
(call $drop-unreachable)
)
(unreachable)
)
(else
(drop
(call $drop-unreachable)
)
(unreachable)
)
)
)
;; Two more outside the if.
(drop
(call $drop-unreachable)
)
(unreachable)
(drop
(call $drop-unreachable)
)
(unreachable)
)
)