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StackIR: Optimize away a drop before an unreachable (#6719)
Anything else right before an unreachable is removed by the main DCE pass anyhow, but because of the structured form of BinaryenIR we can't remove a drop. That is, this is the difference between (i32.eqz (i32.const 42) (unreachable) ) and (drop (call $foo) ) (unreachable) In both cases the unreachable is preceded by something we don't need, but in the latter case it must remain in BinaryenIR for validation. To optimize this, add a rule in StackIR. Fixes #6715
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‎src/wasm/wasm-stack-opts.cpp‎

Lines changed: 38 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -44,8 +44,9 @@ void StackIROptimizer::run() {
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vacuum();
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}
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// Remove unreachable code.
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void StackIROptimizer::dce() {
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// Remove code after an unreachable instruction: anything after it, up to the
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// next control flow barrier, can simply be removed.
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bool inUnreachableCode = false;
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for (Index i = 0; i < insts.size(); i++) {
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auto* inst = insts[i];
@@ -64,6 +65,42 @@ void StackIROptimizer::dce() {
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inUnreachableCode = true;
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}
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}
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// Remove code before an Unreachable. Consider this:
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//
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// (drop
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// ..
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// )
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// (unreachable)
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//
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// The drop is not needed, as the unreachable puts the stack in the
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// polymorphic state anyhow. Note that we don't need to optimize anything
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// other than a drop here, as in general the Binaryen IR DCE pass will handle
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// everything else. A drop followed by an unreachable is the only thing that
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// pass cannot handle, as the structured form of Binaryen IR does not allow
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// removing such a drop, and so we can only do it here in StackIR.
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//
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// TODO: We can look even further back, say if there is another drop of
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// something before, then we can remove that drop as well. To do that
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// we'd need to inspect the stack going backwards.
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for (Index i = 1; i < insts.size(); i++) {
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auto* inst = insts[i];
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if (!inst || inst->op != StackInst::Basic ||
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!inst->origin->is<Unreachable>()) {
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continue;
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}
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// Look back past nulls.
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Index j = i - 1;
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while (j > 0 && !insts[j]) {
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j--;
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}
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auto*& prev = insts[j];
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if (prev && prev->op == StackInst::Basic && prev->origin->is<Drop>()) {
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prev = nullptr;
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}
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}
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}
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// Remove obviously-unneeded code.

‎test/lit/passes/stack-ir-dce.wast‎

Lines changed: 118 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,118 @@
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;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
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;; RUN: wasm-opt %s --generate-stack-ir --optimize-stack-ir -all --print-stack-ir | filecheck %s
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;; Also verify we roundtrip the output here properly.
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;; RUN: wasm-opt %s --generate-stack-ir --optimize-stack-ir -all --roundtrip --print | filecheck %s --check-prefix=ROUNDTRIP
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(module
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;; CHECK: (func $drop-unreachable (type $0) (result i32)
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;; CHECK-NEXT: call $drop-unreachable
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;; CHECK-NEXT: unreachable
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;; CHECK-NEXT: )
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;; ROUNDTRIP: (func $drop-unreachable (type $0) (result i32)
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;; ROUNDTRIP-NEXT: (drop
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;; ROUNDTRIP-NEXT: (call $drop-unreachable)
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;; ROUNDTRIP-NEXT: )
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;; ROUNDTRIP-NEXT: (unreachable)
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;; ROUNDTRIP-NEXT: )
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(func $drop-unreachable (result i32)
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;; This drop can be removed.
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(drop
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(call $drop-unreachable)
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)
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(unreachable)
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)
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;; CHECK: (func $unreachable (type $0) (result i32)
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;; CHECK-NEXT: unreachable
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;; CHECK-NEXT: )
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;; ROUNDTRIP: (func $unreachable (type $0) (result i32)
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;; ROUNDTRIP-NEXT: (unreachable)
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;; ROUNDTRIP-NEXT: )
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(func $unreachable (result i32)
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;; An unreachable with nothing before it. Check we do not error here.
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(unreachable)
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)
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;; CHECK: (func $unreachable-non-drop (type $1)
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;; CHECK-NEXT: call $unreachable-non-drop
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;; CHECK-NEXT: unreachable
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;; CHECK-NEXT: )
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;; ROUNDTRIP: (func $unreachable-non-drop (type $1)
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;; ROUNDTRIP-NEXT: (call $unreachable-non-drop)
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;; ROUNDTRIP-NEXT: (unreachable)
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;; ROUNDTRIP-NEXT: )
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(func $unreachable-non-drop
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;; An unreachable with something other than a drop before it. Check we do
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;; not error here.
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(call $unreachable-non-drop)
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(unreachable)
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)
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;; CHECK: (func $many-drop-unreachable (type $0) (result i32)
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;; CHECK-NEXT: i32.const 1
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;; CHECK-NEXT: if (result i32)
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;; CHECK-NEXT: call $drop-unreachable
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;; CHECK-NEXT: unreachable
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;; CHECK-NEXT: else
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;; CHECK-NEXT: call $drop-unreachable
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;; CHECK-NEXT: unreachable
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;; CHECK-NEXT: end
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;; CHECK-NEXT: drop
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;; CHECK-NEXT: call $drop-unreachable
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;; CHECK-NEXT: unreachable
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;; CHECK-NEXT: )
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;; ROUNDTRIP: (func $many-drop-unreachable (type $0) (result i32)
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;; ROUNDTRIP-NEXT: (drop
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;; ROUNDTRIP-NEXT: (if (result i32)
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;; ROUNDTRIP-NEXT: (i32.const 1)
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;; ROUNDTRIP-NEXT: (then
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;; ROUNDTRIP-NEXT: (drop
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;; ROUNDTRIP-NEXT: (call $drop-unreachable)
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;; ROUNDTRIP-NEXT: )
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;; ROUNDTRIP-NEXT: (unreachable)
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;; ROUNDTRIP-NEXT: )
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;; ROUNDTRIP-NEXT: (else
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;; ROUNDTRIP-NEXT: (drop
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;; ROUNDTRIP-NEXT: (call $drop-unreachable)
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;; ROUNDTRIP-NEXT: )
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;; ROUNDTRIP-NEXT: (unreachable)
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;; ROUNDTRIP-NEXT: )
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;; ROUNDTRIP-NEXT: )
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;; ROUNDTRIP-NEXT: )
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;; ROUNDTRIP-NEXT: (drop
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;; ROUNDTRIP-NEXT: (call $drop-unreachable)
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;; ROUNDTRIP-NEXT: )
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;; ROUNDTRIP-NEXT: (unreachable)
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;; ROUNDTRIP-NEXT: )
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(func $many-drop-unreachable (result i32)
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;; Two drop-unreachables in an if. The drop on the if can remain, but all
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;; others are removable.
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(drop
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(if (result i32)
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(i32.const 1)
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(then
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(drop
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(call $drop-unreachable)
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)
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(unreachable)
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)
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(else
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(drop
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(call $drop-unreachable)
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)
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(unreachable)
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)
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)
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)
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;; Two more outside the if.
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(drop
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(call $drop-unreachable)
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)
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(unreachable)
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(drop
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(call $drop-unreachable)
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)
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(unreachable)
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)
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)

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