[llvm] [InstCombine] Don't use dominating conditions to transform sub into xor. (PR #88566)
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Fri Apr 12 12:40:30 PDT 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-analysis
Author: Craig Topper (topperc)
<details>
<summary>Changes</summary>
Other passes are unable to reverse this transform if we use dominating
conditions.
Fixes #<!-- -->88239.
---
Full diff: https://github.com/llvm/llvm-project/pull/88566.diff
3 Files Affected:
- (modified) llvm/include/llvm/Analysis/SimplifyQuery.h (+6)
- (modified) llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp (+4-2)
- (modified) llvm/test/Transforms/InstCombine/sub-xor.ll (+25)
``````````diff
diff --git a/llvm/include/llvm/Analysis/SimplifyQuery.h b/llvm/include/llvm/Analysis/SimplifyQuery.h
index e5e6ae0d3d8e3e..a10c0dc49fa22e 100644
--- a/llvm/include/llvm/Analysis/SimplifyQuery.h
+++ b/llvm/include/llvm/Analysis/SimplifyQuery.h
@@ -113,6 +113,12 @@ struct SimplifyQuery {
using namespace PatternMatch;
return match(V, m_Undef());
}
+
+ SimplifyQuery getWithoutDomCondCache() const {
+ SimplifyQuery Copy(*this);
+ Copy.DC = nullptr;
+ return Copy;
+ }
};
} // end namespace llvm
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 07c50d866544b3..c8734bc00c4b59 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -2281,8 +2281,10 @@ Instruction *InstCombinerImpl::visitSub(BinaryOperator &I) {
if (match(Op0, m_APInt(Op0C))) {
if (Op0C->isMask()) {
// Turn this into a xor if LHS is 2^n-1 and the remaining bits are known
- // zero.
- KnownBits RHSKnown = computeKnownBits(Op1, 0, &I);
+ // zero. We don't use information from dominating conditions so this
+ // transform is easier to reverse if necessary.
+ KnownBits RHSKnown = llvm::computeKnownBits(
+ Op1, 0, SQ.getWithInstruction(&I).getWithoutDomCondCache());
if ((*Op0C | RHSKnown.Zero).isAllOnes())
return BinaryOperator::CreateXor(Op1, Op0);
}
diff --git a/llvm/test/Transforms/InstCombine/sub-xor.ll b/llvm/test/Transforms/InstCombine/sub-xor.ll
index 71da73d51ae37e..2976598e043fee 100644
--- a/llvm/test/Transforms/InstCombine/sub-xor.ll
+++ b/llvm/test/Transforms/InstCombine/sub-xor.ll
@@ -157,3 +157,28 @@ define <2 x i8> @xor_add_splat_undef(<2 x i8> %x) {
%add = add <2 x i8> %xor, <i8 42, i8 42>
ret <2 x i8> %add
}
+
+; Make sure we don't convert sub to xor using dominating condition. That makes
+; it hard for other passe to reverse.
+define i32 @xor_dominating_cond(i32 %x) {
+; CHECK-LABEL: @xor_dominating_cond(
+; CHECK-NEXT: entry:
+; CHECK-NEXT: [[COND:%.*]] = icmp ult i32 [[X:%.*]], 256
+; CHECK-NEXT: br i1 [[COND]], label [[IF_THEN:%.*]], label [[IF_END:%.*]]
+; CHECK: if.then:
+; CHECK-NEXT: [[A:%.*]] = sub nuw nsw i32 255, [[X]]
+; CHECK-NEXT: ret i32 [[A]]
+; CHECK: if.end:
+; CHECK-NEXT: ret i32 [[X]]
+;
+entry:
+ %cond = icmp ult i32 %x, 256
+ br i1 %cond, label %if.then, label %if.end
+
+if.then:
+ %a = sub i32 255, %x
+ ret i32 %a
+
+if.end:
+ ret i32 %x
+}
``````````
</details>
https://github.com/llvm/llvm-project/pull/88566
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