Index: llvm/lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- llvm/lib/Target/X86/X86ISelLowering.cpp
+++ llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -50129,10 +50129,14 @@
         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();
 }
Index: llvm/test/CodeGen/X86/neg_fp.ll
===================================================================
--- llvm/test/CodeGen/X86/neg_fp.ll
+++ llvm/test/CodeGen/X86/neg_fp.ll
@@ -82,11 +82,13 @@
   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>