[InstCombine] Infer exact for lshr by cttz #136696
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@llvm/pr-subscribers-llvm-transforms Author: 黃國庭 (houngkoungting) ChangesFix #131444 Full diff: https://github.com/llvm/llvm-project/pull/136696.diff 1 Files Affected:
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 90cd279e8a457..c39b7dae434e0 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -994,6 +994,18 @@ static bool setShiftFlags(BinaryOperator &I, const SimplifyQuery &Q) {
I.setIsExact();
return true;
}
+ //Fix #131444
+ if (auto *Cttz = dyn_cast<IntrinsicInst>(I.getOperand(1))) {
+ if (Cttz->getIntrinsicID() == Intrinsic::cttz &&
+ Cttz->getOperand(0) == I.getOperand(0)) {
+ if (auto *Const = dyn_cast<ConstantInt>(Cttz->getOperand(1))) {
+ if (Const->isOne()) {
+ I.setIsExact();
+ return true;
+ }
+ }
+ }
+ }
}
// Compute what we know about shift count.
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Could you provide the testcase for this change |
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Note that an existing InstCombine test is failing as well, please update it. |
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✅ With the latest revision this PR passed the C/C++ code formatter. |
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Please add some tests to demonstrate the change. See also https://llvm.org/docs/InstCombineContributorGuide.html#tests
It is okay to fix them in this PR. |
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For the proofs, please prefer linking to alive2.llvm.org instead of directly embedding the proof in the PR, e.g. like this: https://alive2.llvm.org/ce/z/CQR2PG
Note that this also shows that you do not need the second argument to be true. The reason false works is that if the input is zero, cttz will return the bit width and shifting by the bit width returns poison. Adding an extra exact flag doesn't affect this.
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HI @nikic , I’m not sure if it should be like the cttz-shift-exact.ll I updated.Please take a look when you have time. Thanks! |
| %sh = ashr i32 %x, %cttz | ||
| ret i32 %sh | ||
| } | ||
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Please also add one negative test where the cttz operand is an unrelated value.
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HI @nikic ,I've added the negative test case where the cttz operand is unrelated.Please take a look when you have time. Thanks! |
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Infer the 'exact' flag on an 'lshr' or 'ashr' instruction when the shift amount is computed via a 'cttz' intrinsic on the same operand. Proof: https://alive2.llvm.org/ce/z/CQR2PG Fixes llvm#131444.
Infer the 'exact' flag on an 'lshr' or 'ashr' instruction when the shift amount is computed via a 'cttz' intrinsic on the same operand. Proof: https://alive2.llvm.org/ce/z/CQR2PG Fixes llvm#131444.
Infer the 'exact' flag on an 'lshr' or 'ashr' instruction when the shift amount is computed via a 'cttz' intrinsic on the same operand. Proof: https://alive2.llvm.org/ce/z/CQR2PG Fixes llvm#131444.
Infer the 'exact' flag on an 'lshr' or 'ashr' instruction when the shift amount is computed via a 'cttz' intrinsic on the same operand. Proof: https://alive2.llvm.org/ce/z/CQR2PG Fixes llvm#131444.
Infer the 'exact' flag on an 'lshr' or 'ashr' instruction when the shift amount is computed via a 'cttz' intrinsic on the same operand. Proof: https://alive2.llvm.org/ce/z/CQR2PG Fixes llvm#131444.
Infer the 'exact' flag on an 'lshr' or 'ashr' instruction when the shift amount is computed via a 'cttz' intrinsic on the same operand.
Proof: https://alive2.llvm.org/ce/z/CQR2PG
Fixes #131444.