[MLIR][SCF] pipelining pass use of uninitialized value

[Hyffer]

Here in the code:

llvm-project/mlir/lib/Dialect/SCF/Transforms/LoopPipelining.cpp

Line 127 in 61529d9

if (numIteration > maxStage) {

llvm-project/mlir/lib/Dialect/SCF/Transforms/LoopPipelining.cpp

Lines 148 to 153 in 61529d9

	for (auto &opSchedule : schedule) {
	maxStage = std::max(maxStage, opSchedule.second);
	stages[opSchedule.first] = opSchedule.second;
	opOrder.push_back(opSchedule.first);
	}

maxStage is used before initialized.

When it checks numIteration > maxStage at line-127, maxStage is always 0 as default. So it will mistakenly decide if the loop trip count is actually greater than stage number, and thus pipelining a loop that should not have done.

---

At older llvm version(17), it was correct. The problem is introduced in this commit: ef11283

[llvmbot]

@llvm/issue-subscribers-mlir

Author: Hu Yufan (Hyffer)

Here in the code:

llvm-project/mlir/lib/Dialect/SCF/Transforms/LoopPipelining.cpp

Line 127 in 61529d9

if (numIteration > maxStage) {

llvm-project/mlir/lib/Dialect/SCF/Transforms/LoopPipelining.cpp

Lines 148 to 153 in 61529d9

	for (auto &opSchedule : schedule) {
	maxStage = std::max(maxStage, opSchedule.second);
	stages[opSchedule.first] = opSchedule.second;
	opOrder.push_back(opSchedule.first);
	}

maxStage is used before initialized.

When it checks numIteration > maxStage at line-127, maxStage is always 0 as default. So it will mistakenly decide if the loop trip count is actually greater than stage number, and thus pipelining a loop that should not have done.

---

At older llvm version(17), it was correct. The problem is introduced in this commit: ef11283