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[mlir][Transforms] --remove-dead-values: keep values used in public function calls #83249

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[mlir][Transforms] --remove-dead-values: keep values used in public function calls #83249

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[mlir][Transforms] --remove-dead-values: keep values used in public f…
agentcooper Feb 28, 2024
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agentcooper Feb 28, 2024
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11 changes: 10 additions & 1 deletion mlir/lib/Transforms/RemoveDeadValues.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -169,7 +169,10 @@ static SmallVector<OpOperand *> operandsToOpOperands(OperandRange operands) {
/// Here, cleaning means:
/// (1) Dropping all its uses, AND
/// (2) Erasing it
/// iff it has no memory effects and none of its results are live.
/// iff:
/// (1) It has no memory effects
/// (2) None of its results are live
/// (3) It is not used as an argument in a public function call
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Why does it matter whether the function is private or public?

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@agentcooper agentcooper Feb 28, 2024 •
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I've posted a detailed analysis here: #82788 (comment). Please also see a test case. The short answer is that if the function is public, its unused parameters will not be removed, but the argument is erased, resulting in a NULL value error afterwards.

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You can't touch a function API when you don't see all the calls, and by definition a public function has external calls you don't see.

You may clone the function to specialize it though, but that's a different tradeoff to make

///
/// It is assumed that `op` is simple. Here, a simple op is one which isn't a
/// symbol op, a symbol-user op, a region branch op, a branch op, a region
Expand All @@ -178,6 +181,12 @@ static void cleanSimpleOp(Operation *op, RunLivenessAnalysis &la) {
if (!isMemoryEffectFree(op) || hasLive(op->getResults(), la))
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I never looked into the liveness analysis, but shouldn't a value be considered "alive" it is passed as a function call operand?

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From what I understand, if the parameter is not used inside the function, the value (argument) will not be considered "alive".

return;

for (OpOperand &use : op->getUses())
if (auto callOp = dyn_cast<CallOpInterface>(use.getOwner()))
if (auto funcOp = dyn_cast<FunctionOpInterface>(callOp.resolveCallable()))
if (funcOp.isPublic())
return;
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@joker-eph joker-eph Feb 28, 2024 •
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How does this work transitively though?

To me it seems like we can still kill the op here, we could replace the SSA value with "undef" since it is dead

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I see llvm.mlir.undef and spirv.Undef. Is there a generic "undef" value that I can use here?

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@joker-eph ping


op->dropAllUses();
op->erase();
}
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27 changes: 27 additions & 0 deletions mlir/test/Transforms/remove-dead-values.mlir
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Expand Up @@ -357,3 +357,30 @@ func.func @kernel(%arg0: memref<18xf32>) {
// CHECK: gpu.launch blocks
// CHECK: memref.store
// CHECK-NEXT: gpu.terminator

// -----

// Even though %arg0 is not used in func0,
// we can't remove the parameter, as the function is public,
// so `%true = arith.constant true` needs to be kept in func1 as well.
//
module {
// CHECK: func.func @func0(%arg0: i1, %arg1: i1) -> (i1, i1) {
// CHECK-NEXT: %true = arith.constant true
// CHECK-NEXT: return %arg1, %true : i1, i1
// CHECK-NEXT: }
func.func @func0(%arg0: i1, %arg1: i1) -> (i1, i1) {
%true = arith.constant true
return %arg1, %true : i1, i1
}
// CHECK: func.func @func1(%arg0: i1) -> (i1, i1) {
// CHECK-NEXT: %true = arith.constant true
// CHECK-NEXT: %0:2 = call @func0(%true, %arg0) : (i1, i1) -> (i1, i1)
// CHECK-NEXT: return %0#0, %0#1 : i1, i1
// CHECK-NEXT: }
func.func @func1(%arg0: i1) -> (i1, i1) {
%true = arith.constant true
%0:2 = call @func0(%true, %arg0) : (i1, i1) -> (i1, i1)
return %0#0, %0#1 : i1, i1
}
}