[Mlir-commits] [mlir] [mlir][gpu] Fix crash in RemoveDeadValues pass with gpu.launch ops (PR #182711)
Fedor Nikolaev
llvmlistbot at llvm.org
Sat Feb 21 15:10:03 PST 2026
https://github.com/felichita created https://github.com/llvm/llvm-project/pull/182711
The RemoveDeadValues pass was crashing with an assertion failure when processing IR containing gpu.launch operations. The root cause was that gpu.launch was missing an explicit Write memory effect, causing wouldOpBeTriviallyDead to incorrectly consider it dead via recursive analysis of its region.
Fixes #182263
>From 9140ab32c68c12b82b266dff000ce8e30b26667c Mon Sep 17 00:00:00 2001
From: Fedor Nikolaev <fridrixnm at gmail.com>
Date: Sun, 22 Feb 2026 00:01:54 +0100
Subject: [PATCH] [mlir][gpu] Fix crash in RemoveDeadValues pass with
gpu.launch ops
The RemoveDeadValues pass was crashing with an assertion failure when
processing IR containing gpu.launch operations. The root cause was that
gpu.launch was missing an explicit Write memory effect, causing
wouldOpBeTriviallyDead to incorrectly consider it dead via recursive
analysis of its region.
Fixes #182263
---
mlir/include/mlir/Dialect/GPU/IR/GPUOps.td | 1 +
.../Analysis/DataFlow/LivenessAnalysis.cpp | 7 +++
mlir/lib/Dialect/GPU/IR/GPUDialect.cpp | 6 +++
mlir/lib/Transforms/RemoveDeadValues.cpp | 51 +++++++++++++++----
4 files changed, 55 insertions(+), 10 deletions(-)
diff --git a/mlir/include/mlir/Dialect/GPU/IR/GPUOps.td b/mlir/include/mlir/Dialect/GPU/IR/GPUOps.td
index 48de1a8bf118e..3515da360e129 100644
--- a/mlir/include/mlir/Dialect/GPU/IR/GPUOps.td
+++ b/mlir/include/mlir/Dialect/GPU/IR/GPUOps.td
@@ -808,6 +808,7 @@ def GPU_LaunchFuncOp :GPU_Op<"launch_func", [
def GPU_LaunchOp : GPU_Op<"launch", [
AffineScope, AutomaticAllocationScope, AttrSizedOperandSegments,
DeclareOpInterfaceMethods<InferIntRangeInterface, ["inferResultRanges"]>,
+ DeclareOpInterfaceMethods<MemoryEffectsOpInterface>,
GPU_AsyncOpInterface, RecursiveMemoryEffects]>,
Arguments<(ins Variadic<GPU_AsyncToken>:$asyncDependencies,
Index:$gridSizeX, Index:$gridSizeY, Index:$gridSizeZ,
diff --git a/mlir/lib/Analysis/DataFlow/LivenessAnalysis.cpp b/mlir/lib/Analysis/DataFlow/LivenessAnalysis.cpp
index 4afc35d23fafa..ec9827b38be39 100644
--- a/mlir/lib/Analysis/DataFlow/LivenessAnalysis.cpp
+++ b/mlir/lib/Analysis/DataFlow/LivenessAnalysis.cpp
@@ -17,6 +17,7 @@
#include <mlir/IR/Operation.h>
#include <mlir/IR/Value.h>
#include <mlir/Interfaces/CallInterfaces.h>
+#include <mlir/Interfaces/FunctionInterfaces.h>
#include <mlir/Interfaces/SideEffectInterfaces.h>
#include <mlir/Support/LLVM.h>
@@ -237,6 +238,12 @@ RunLivenessAnalysis::RunLivenessAnalysis(Operation *op) {
for (auto blockArg : llvm::enumerate(block.getArguments())) {
if (getLiveness(blockArg.value()))
continue;
+ // Skip block args of ops with regions that are not
+ // RegionBranchOpInterface or FunctionOpInterface
+ // (e.g. gpu.launch) - solver doesn't analyze their regions
+ if (!isa<RegionBranchOpInterface>(op) &&
+ !isa<FunctionOpInterface>(op))
+ continue;
LDBG() << "Block argument: " << blockArg.index() << " of "
<< OpWithFlags(op, OpPrintingFlags().skipRegions())
<< " has no liveness info, mark dead";
diff --git a/mlir/lib/Dialect/GPU/IR/GPUDialect.cpp b/mlir/lib/Dialect/GPU/IR/GPUDialect.cpp
index a66a83b7e3ca1..9822fa29ffe8a 100644
--- a/mlir/lib/Dialect/GPU/IR/GPUDialect.cpp
+++ b/mlir/lib/Dialect/GPU/IR/GPUDialect.cpp
@@ -942,6 +942,12 @@ static void printSizeAssignment(OpAsmPrinter &p, KernelDim3 size,
p << size.z << " = " << operands.z << ')';
}
+void LaunchOp::getEffects(
+ SmallVectorImpl<SideEffects::EffectInstance<MemoryEffects::Effect>>
+ &effects) {
+ effects.emplace_back(MemoryEffects::Write::get());
+}
+
void LaunchOp::print(OpAsmPrinter &p) {
if (getAsyncToken()) {
p << " async";
diff --git a/mlir/lib/Transforms/RemoveDeadValues.cpp b/mlir/lib/Transforms/RemoveDeadValues.cpp
index 12a47ba2fb65a..7874a92012ac1 100644
--- a/mlir/lib/Transforms/RemoveDeadValues.cpp
+++ b/mlir/lib/Transforms/RemoveDeadValues.cpp
@@ -235,6 +235,15 @@ static void processSimpleOp(Operation *op, RunLivenessAnalysis &la,
// "dead" if it had a side-effecting user that is reachable.
bool hasDeadOperand =
markLives(op->getOperands(), nonLiveSet, la).flip().any();
+
+ if (!isMemoryEffectFree(op) || hasLive(op->getResults(), nonLiveSet, la)) {
+ LDBG() << "Simple op is not memory effect free or has live results, "
+ "preserving it: "
+ << OpWithFlags(op,
+ OpPrintingFlags().skipRegions().printGenericOpForm());
+ return;
+ }
+
if (hasDeadOperand) {
LDBG() << "Simple op has dead operands, so the op must be dead: "
<< OpWithFlags(op,
@@ -247,14 +256,6 @@ static void processSimpleOp(Operation *op, RunLivenessAnalysis &la,
return;
}
- if (!isMemoryEffectFree(op) || hasLive(op->getResults(), nonLiveSet, la)) {
- LDBG() << "Simple op is not memory effect free or has live results, "
- "preserving it: "
- << OpWithFlags(op,
- OpPrintingFlags().skipRegions().printGenericOpForm());
- return;
- }
-
LDBG()
<< "Simple op has all dead results and is memory effect free, scheduling "
"for removal: "
@@ -511,8 +512,10 @@ static void processBranchOp(BranchOpInterface branchOp, RunLivenessAnalysis &la,
// Do (2)
BitVector successorNonLive =
markLives(operandValues, nonLiveSet, la).flip();
- collectNonLiveValues(nonLiveSet, successorBlock->getArguments(),
- successorNonLive);
+ if (std::distance(successorBlock->pred_begin(),
+ successorBlock->pred_end()) <= 1)
+ collectNonLiveValues(nonLiveSet, successorBlock->getArguments(),
+ successorNonLive);
// Do (3)
cl.blocks.push_back({successorBlock, successorNonLive});
@@ -561,6 +564,7 @@ static void cleanUpDeadVals(MLIRContext *ctx, RDVFinalCleanupList &list) {
// 1. Blocks, We must remove the block arguments and successor operands before
// deleting the operation, as they may reside in the region operation.
LDBG() << "Cleaning up " << list.blocks.size() << " block argument lists";
+ DenseSet<Block *> processedBlocks;
for (auto &b : list.blocks) {
// blocks that are accessed via multiple codepaths processed once
if (b.b->getNumArguments() != b.nonLiveArgs.size())
@@ -573,6 +577,20 @@ static void cleanUpDeadVals(MLIRContext *ctx, RDVFinalCleanupList &list) {
<< OpWithFlags(b.b->getParent()->getParentOp(),
OpPrintingFlags().skipRegions().printGenericOpForm());
});
+ // Skip if already processed
+ if (processedBlocks.count(b.b))
+ continue;
+ // Check if any entry has this block with live args
+ bool hasLiveFromAnyPred = false;
+ for (auto &other : list.blocks) {
+ if (other.b == b.b && other.nonLiveArgs.none()) {
+ hasLiveFromAnyPred = true;
+ break;
+ }
+ }
+ if (hasLiveFromAnyPred)
+ continue;
+ processedBlocks.insert(b.b);
// Note: Iterate from the end to make sure that that indices of not yet
// processes arguments do not change.
for (int i = b.nonLiveArgs.size() - 1; i >= 0; --i) {
@@ -599,6 +617,19 @@ static void cleanUpDeadVals(MLIRContext *ctx, RDVFinalCleanupList &list) {
<< OpWithFlags(op.branch.getOperation(),
OpPrintingFlags().skipRegions().printGenericOpForm());
});
+
+ // Check if any other branch to same block has live operands
+ Block *succBlock = op.branch->getSuccessor(op.successorIndex);
+ bool otherLivePred = false;
+ for (auto &other : list.successorOperands) {
+ Block *otherSucc = other.branch->getSuccessor(other.successorIndex);
+ if (otherSucc == succBlock && other.nonLiveOperands.none()) {
+ otherLivePred = true;
+ break;
+ }
+ }
+ if (otherLivePred)
+ continue;
// it iterates backwards because erase invalidates all successor indexes
for (int i = successorOperands.size() - 1; i >= 0; --i) {
if (!op.nonLiveOperands[i])
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