[InstCombine] don't negate constant expression with fsub (PR37605)

X + (-C) would be transformed back into X - C, so infinite loop:
https://bugs.llvm.org/show_bug.cgi?id=37605

llvm-svn: 333610

2 files changed, 17 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 1e4c38c4c84..97ad92d2aad 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1795,8 +1795,10 @@ Instruction *InstCombiner::visitFSub(BinaryOperator &I) {
         return NV;
 
   // X - C --> X + (-C)
+  // But don't transform constant expressions because there's an inverse fold
+  // for X + (-Y) --> X - Y.
   Constant *C;
-  if (match(Op1, m_Constant(C)))
+  if (match(Op1, m_Constant(C)) && !isa<ConstantExpr>(Op1))
     return BinaryOperator::CreateFAddFMF(Op0, ConstantExpr::getFNeg(C), &I);
   
   // X - (-Y) --> X + Y
diff --git a/llvm/test/Transforms/InstCombine/fsub.ll b/llvm/test/Transforms/InstCombine/fsub.ll
index 9ce73084f79..22295490cc6 100644
--- a/llvm/test/Transforms/InstCombine/fsub.ll
+++ b/llvm/test/Transforms/InstCombine/fsub.ll
@@ -242,3 +242,17 @@ define float @neg_trunc_op1_extra_uses(double %a, float %b) {
   ret float %t3
 }
 
+; Don't negate a constant expression to form fadd and induce infinite looping:
+; https://bugs.llvm.org/show_bug.cgi?id=37605
+
+@b = external global i16, align 1
+
+define float @PR37605(float %conv) {
+; CHECK-LABEL: @PR37605(
+; CHECK-NEXT:    [[SUB:%.*]] = fsub float [[CONV:%.*]], bitcast (i32 ptrtoint (i16* @b to i32) to float)
+; CHECK-NEXT:    ret float [[SUB]]
+;
+  %sub = fsub float %conv, bitcast (i32 ptrtoint (i16* @b to i32) to float)
+  ret float %sub
+}
+