[llvm] 3a503a4 - [x86] fix miscompile from wrongly identified fneg

[Sanjay Patel via llvm-commits]

Author: Sanjay Patel
Date: 2022-06-01T09:56:33-04:00
New Revision: 3a503a4a9c1fa04020a8c64524234ea0269f80bd

URL: https://github.com/llvm/llvm-project/commit/3a503a4a9c1fa04020a8c64524234ea0269f80bd
DIFF: https://github.com/llvm/llvm-project/commit/3a503a4a9c1fa04020a8c64524234ea0269f80bd.diff

LOG: [x86] fix miscompile from wrongly identified fneg

We may need to peek through a bitcast when identifying an fneg idiom
via its pool constant, but we can't allow a different-sized constant
in that match.

This is noted in issue #55758 with an example that needs fast-math,
but as the test here shows, this has potential to miscompile more
generally (no fast-math required).

Differential Revision: https://reviews.llvm.org/D126775

Added: 
    

Modified: 
    llvm/lib/Target/X86/X86ISelLowering.cpp
    llvm/test/CodeGen/X86/neg_fp.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index c2c93cf033f9..aa21b4533be1 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -50129,10 +50129,14 @@ static SDValue isFNEG(SelectionDAG &DAG, SDNode *N, unsigned Depth = 0) {
         if (!UndefElts[I] && !EltBits[I].isSignMask())
           return SDValue();
 
-      return peekThroughBitcasts(Op0);
+      // Only allow bitcast from correctly-sized constant.
+      Op0 = peekThroughBitcasts(Op0);
+      if (Op0.getScalarValueSizeInBits() == ScalarSize)
+        return Op0;
     }
-  }
-  }
+    break;
+  } // case
+  } // switch
 
   return SDValue();
 }

diff  --git a/llvm/test/CodeGen/X86/neg_fp.ll b/llvm/test/CodeGen/X86/neg_fp.ll
index 1d7260a8e78e..96af17367aa3 100644
--- a/llvm/test/CodeGen/X86/neg_fp.ll
+++ b/llvm/test/CodeGen/X86/neg_fp.ll
@@ -82,11 +82,13 @@ define float @fdiv_extra_use_changes_cost(float %a0, float %a1, float %a2) nounw
   ret float %div5
 }
 
-; FIXME: PR55758 - this is not -(-X)
+; PR55758 - this is not -(-X)
 
 define <2 x i64> @fneg_mismatched_sizes(<4 x float> %x) {
 ; CHECK-LABEL: fneg_mismatched_sizes:
 ; CHECK:       # %bb.0:
+; CHECK-NEXT:    xorps {{\.?LCPI[0-9]+_[0-9]+}}, %xmm0
+; CHECK-NEXT:    xorps {{\.?LCPI[0-9]+_[0-9]+}}, %xmm0
 ; CHECK-NEXT:    retl
   %n = fneg <4 x float> %x
   %b = bitcast <4 x float> %n to <2 x i64>