[InstCombine] Add undef elements support for shrinkFPConstantVector

[Chenbing.Zheng]

Summary

This patch add support for undef elements vector in shrinkFPConstantVector
function.

[RKSimon]

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

1679

Not sure Undef or ConstantFP are the only cases (ConstantExpr)? - so continue for Undef but the return nullptr if not ConstantFP ?

[Chenbing.Zheng]

address comment

[spatel]

LGTM

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

static Type *shrinkFPConstant(ConstantFP *CFP) {
   if (fitsInFPType(CFP, APFloat::IEEEdouble()))
     return Type::getDoubleTy(CFP->getContext());
   // Don't try to shrink to various long double types.
   return nullptr;
 }
 
 // Determine if this is a vector of ConstantFPs and if so, return the minimal
 // type we can safely truncate all elements to.
-// TODO: Make these support undef elements.
 static Type *shrinkFPConstantVector(Value *V) {
   auto *CV = dyn_cast<Constant>(V);
   auto *CVVTy = dyn_cast<FixedVectorType>(V->getType());
   if (!CV || !CVVTy)
     return nullptr;
 
   Type *MinType = nullptr;
 
   unsigned NumElts = CVVTy->getNumElements();
 
   // For fixed-width vectors we find the minimal type by looking
   // through the constant values of the vector.
   for (unsigned i = 0; i != NumElts; ++i) {
+    if (isa<UndefValue>(CV->getAggregateElement(i)))
+      continue;
+
     auto *CFP = dyn_cast_or_null<ConstantFP>(CV->getAggregateElement(i));
     if (!CFP)
       return nullptr;
 
     Type *T = shrinkFPConstant(CFP);
     if (!T)
       return nullptr;
 
     // If we haven't found a type yet or this type has a larger mantissa than
     // our previous type, this is our new minimal type.
     if (!MinType || T->getFPMantissaWidth() > MinType->getFPMantissaWidth())
       MinType = T;
   }
 
   // Make a vector type from the minimal type.
-  return FixedVectorType::get(MinType, NumElts);
+  return MinType ? FixedVectorType::get(MinType, NumElts) : nullptr;
 }
 
 /// Find the minimum FP type we can safely truncate to.
 static Type *getMinimumFPType(Value *V) {
   if (auto *FPExt = dyn_cast<FPExtInst>(V))
     return FPExt->getOperand(0)->getType();
 
   // If this value is a constant, return the constant in the smallest FP type

llvm/test/Transforms/InstCombine/fpextend.ll

 ;
   %t1 = fpext <2 x float> %x to <2 x double>
   %t3 = fadd <2 x double> %t1, <double 0.000000e+00, double -0.000000e+00>
   %t34 = fptrunc <2 x double> %t3 to <2 x float>
   ret <2 x float> %t34
 }
 
 ; Test with an undef element
-; TODO: Support undef elements.
 define <2 x float> @test6_undef(<2 x float> %x) nounwind  {
 ; CHECK-LABEL: @test6_undef(
-; CHECK-NEXT:    [[T1:%.*]] = fpext <2 x float> [[X:%.*]] to <2 x double>
-; CHECK-NEXT:    [[T3:%.*]] = fadd <2 x double> [[T1]], <double 0.000000e+00, double undef>
-; CHECK-NEXT:    [[T34:%.*]] = fptrunc <2 x double> [[T3]] to <2 x float>
+; CHECK-NEXT:    [[T34:%.*]] = fadd <2 x float> [[X:%.*]], <float 0.000000e+00, float undef>
 ; CHECK-NEXT:    ret <2 x float> [[T34]]
 ;
   %t1 = fpext <2 x float> %x to <2 x double>
   %t3 = fadd <2 x double> %t1, <double 0.000000e+00, double undef>
   %t34 = fptrunc <2 x double> %t3 to <2 x float>
   ret <2 x float> %t34
 }