[mlir][Transforms] --remove-dead-values: keep values used in public function calls #83249
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…unction calls Fixes llvm#82788
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@llvm/pr-subscribers-mlir @llvm/pr-subscribers-mlir-core Author: Artem Tyurin (agentcooper) ChangesFixes #82788 Full diff: https://github.com/llvm/llvm-project/pull/83249.diff 2 Files Affected:
diff --git a/mlir/lib/Transforms/RemoveDeadValues.cpp b/mlir/lib/Transforms/RemoveDeadValues.cpp
index 055256903a1522..4fad27e5adabda 100644
--- a/mlir/lib/Transforms/RemoveDeadValues.cpp
+++ b/mlir/lib/Transforms/RemoveDeadValues.cpp
@@ -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
///
/// 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
@@ -178,6 +181,17 @@ static void cleanSimpleOp(Operation *op, RunLivenessAnalysis &la) {
if (!isMemoryEffectFree(op) || hasLive(op->getResults(), la))
return;
+ for (auto &use : op->getUses()) {
+ if (auto callOp = dyn_cast<CallOpInterface>(use.getOwner())) {
+ if (auto funcOp =
+ dyn_cast<FunctionOpInterface>(callOp.resolveCallable())) {
+ if (funcOp.isPublic()) {
+ return;
+ }
+ }
+ }
+ }
+
op->dropAllUses();
op->erase();
}
diff --git a/mlir/test/Transforms/remove-dead-values.mlir b/mlir/test/Transforms/remove-dead-values.mlir
index 69426fdb620832..de83e80acc3ca6 100644
--- a/mlir/test/Transforms/remove-dead-values.mlir
+++ b/mlir/test/Transforms/remove-dead-values.mlir
@@ -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
+ }
+}
|
| @@ -178,6 +181,17 @@ static void cleanSimpleOp(Operation *op, RunLivenessAnalysis &la) { | |||
| if (!isMemoryEffectFree(op) || hasLive(op->getResults(), la)) | |||
| return; | |||
|
|
|||
| for (auto &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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LLVM style is to remove braces if there is only a trivial single statement. So I'd remove all these braces here
| /// 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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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
| @@ -178,6 +181,17 @@ 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".
| if (auto callOp = dyn_cast<CallOpInterface>(use.getOwner())) | ||
| if (auto funcOp = dyn_cast<FunctionOpInterface>(callOp.resolveCallable())) | ||
| if (funcOp.isPublic()) | ||
| return; |
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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?
Fixes #82788