[ValueTracking] Bail out on x86_fp80 when computing fpclass with knownbits

Author: dtcxzyw

llvm/lib/Analysis/ValueTracking.cpp

@@ -6208,13 +6208,14 @@ void computeKnownFPClass(const Value *V, const APInt &DemandedElts,
     else if (Bits.isNegative())
       Known.signBitMustBeOne();
 
-    if (Ty->isIEEE()) {
+    if (Ty->isIEEELikeFPTy()) {
       // IEEE floats are NaN when all bits of the exponent plus at least one of
       // the fraction bits are 1. This means:
       //   - If we assume unknown bits are 0 and the value is NaN, it will
       //     always be NaN
       //   - If we assume unknown bits are 1 and the value is not NaN, it can
       //     never be NaN
+      // Note: They do not hold for x86_fp80 format.
       if (APFloat(Ty->getFltSemantics(), Bits.One).isNaN())
         Known.KnownFPClasses = fcNan;
       else if (!APFloat(Ty->getFltSemantics(), ~Bits.Zero).isNaN())

llvm/test/Transforms/InstSimplify/fcmp.ll

@@ -19,7 +19,12 @@ define i1 @poison2(float %x) {
 
 define i1 @pr130408(x86_fp80 %x) {
 ; CHECK-LABEL: @pr130408(
-; CHECK-NEXT:    ret i1 true
+; CHECK-NEXT:    [[BITS:%.*]] = bitcast x86_fp80 [[X:%.*]] to i80
+; CHECK-NEXT:    [[MASKED:%.*]] = and i80 [[BITS]], -604444463063240877801473
+; CHECK-NEXT:    [[OR:%.*]] = or i80 [[MASKED]], 302194561415509874573312
+; CHECK-NEXT:    [[FP:%.*]] = bitcast i80 [[OR]] to x86_fp80
+; CHECK-NEXT:    [[RES:%.*]] = fcmp uno x86_fp80 [[FP]], 0xK00000000000000000000
+; CHECK-NEXT:    ret i1 [[RES]]
 ;
   %bits = bitcast x86_fp80 %x to i80
   %masked = and i80 %bits, -604444463063240877801473