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Fix optimize memory.copy: check size for side effect as well #3072

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Merged
merged 1 commit into from
Aug 23, 2020
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Fix optimize memory.copy: check size for side effect as well #3072

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2 changes: 2 additions & 0 deletions src/passes/OptimizeInstructions.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1404,6 +1404,8 @@ struct OptimizeInstructions
// memory.copy(x, x, sz) ==> nop
if (!EffectAnalyzer(getPassOptions(), features, memCopy->dest)
.hasSideEffects() &&
!EffectAnalyzer(getPassOptions(), features, memCopy->size)
.hasSideEffects() &&
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@kripken kripken Aug 23, 2020

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how about just checking for side effects on memCopy itself? that would be shorter and handle all the cases.

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@MaxGraey MaxGraey Aug 23, 2020 •
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In this case:

(memory.copy  ;; nop
  (local.get $dst)
  (local.get $dst)
  (local.get $sz)
)

will not reduced to nop. I don't know why

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@MaxGraey MaxGraey Aug 23, 2020 •
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I guess it's because MemoryCopy just don't properly handle by EffectAnalyzer? It just use this:

void visitMemoryCopy(MemoryCopy* curr) {
  readsMemory = true;
  writesMemory = true;
  if (!ignoreImplicitTraps) {
    implicitTrap = true;
  }
}

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@kripken kripken Aug 23, 2020

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Oh, yes, that makes sense. It doesn't know it doesn't read/write in this case.

ExpressionAnalyzer::equal(memCopy->dest, memCopy->source)) {
return ExpressionManipulator::nop(memCopy);
}
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7 changes: 7 additions & 0 deletions test/passes/optimize-instructions_all-features.txt
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Expand Up @@ -3793,6 +3793,13 @@
(local.get $src)
(local.get $sz)
)
(memory.copy
(i32.const 0)
(i32.const 0)
(i32.load
(i32.const 3)
)
)
)
)
(module
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10 changes: 9 additions & 1 deletion test/passes/optimize-instructions_all-features.wast
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Expand Up @@ -4294,7 +4294,7 @@
(i32.const 8)
)

(memory.copy
(memory.copy
(local.get $dst)
(local.get $src)
(i32.const 16)
Expand All @@ -4305,6 +4305,14 @@
(local.get $src)
(local.get $sz)
)

(memory.copy ;; skip
(i32.const 0)
(i32.const 0)
(i32.load
(i32.const 3) ;; side effect
)
)
)
)
(module
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