[llvm-branch-commits] [llvm] 692808e - [InstCombine] avoid infinite loop in demanded bits for select
Tom Stellard via llvm-branch-commits
llvm-branch-commits at lists.llvm.org
Tue Mar 2 20:03:05 PST 2021
Author: Sanjay Patel
Date: 2021-03-02T20:02:46-08:00
New Revision: 692808e5af8338f5a109a64b5b9d75d05ec6f590
URL: https://github.com/llvm/llvm-project/commit/692808e5af8338f5a109a64b5b9d75d05ec6f590
DIFF: https://github.com/llvm/llvm-project/commit/692808e5af8338f5a109a64b5b9d75d05ec6f590.diff
LOG: [InstCombine] avoid infinite loop in demanded bits for select
https://llvm.org/PR49205
(cherry picked from commit 9502061bcc86982641772f45b7e7a0eb7437f054)
Added:
Modified:
llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
llvm/test/Transforms/InstCombine/select-imm-canon.ll
Removed:
################################################################################
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index c265516213aa..16efe863779a 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -345,10 +345,14 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
return false;
// Get the constant out of the ICmp, if there is one.
+ // Only try this when exactly 1 operand is a constant (if both operands
+ // are constant, the icmp should eventually simplify). Otherwise, we may
+ // invert the transform that reduces set bits and infinite-loop.
+ Value *X;
const APInt *CmpC;
ICmpInst::Predicate Pred;
- if (!match(I->getOperand(0), m_c_ICmp(Pred, m_APInt(CmpC), m_Value())) ||
- CmpC->getBitWidth() != SelC->getBitWidth())
+ if (!match(I->getOperand(0), m_ICmp(Pred, m_Value(X), m_APInt(CmpC))) ||
+ isa<Constant>(X) || CmpC->getBitWidth() != SelC->getBitWidth())
return ShrinkDemandedConstant(I, OpNo, DemandedMask);
// If the constant is already the same as the ICmp, leave it as-is.
diff --git a/llvm/test/Transforms/InstCombine/select-imm-canon.ll b/llvm/test/Transforms/InstCombine/select-imm-canon.ll
index e230b3b92777..fec6d693954a 100644
--- a/llvm/test/Transforms/InstCombine/select-imm-canon.ll
+++ b/llvm/test/Transforms/InstCombine/select-imm-canon.ll
@@ -87,3 +87,41 @@ define i8 @original_logical(i32 %A, i32 %B) {
%conv7 = trunc i32 %spec.select.i to i8
ret i8 %conv7
}
+
+; This would infinite loop because we have potentially opposing
+; constant transforms on degenerate (unsimplified) cmps.
+
+define i32 @PR49205(i32 %t0, i1 %b) {
+; CHECK-LABEL: @PR49205(
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br label [[FOR_COND:%.*]]
+; CHECK: for.cond:
+; CHECK-NEXT: br i1 [[B:%.*]], label [[FOR_BODY:%.*]], label [[FOR_END:%.*]]
+; CHECK: for.body:
+; CHECK-NEXT: br label [[FOR_COND]]
+; CHECK: for.end:
+; CHECK-NEXT: ret i32 1
+;
+entry:
+ br label %for.cond
+
+for.cond:
+ %s = phi i32 [ 7, %entry ], [ %add, %for.body ]
+ br i1 %b, label %for.body, label %for.end
+
+for.body:
+ %div = add i32 %t0, undef
+ %add = add nsw i32 %div, 1
+ br label %for.cond
+
+for.end:
+ %cmp6 = icmp ne i32 %s, 4
+ %conv = zext i1 %cmp6 to i32
+ %and7 = and i32 %s, %conv
+ %sub = sub i32 %s, %and7
+ %cmp9 = icmp ne i32 %sub, 4
+ %conv10 = zext i1 %cmp9 to i32
+ %sub11 = sub i32 %conv10, %sub
+ %and = and i32 %sub11, 1
+ ret i32 %and
+}
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