[InstCombine] Rewrite the binary op shrinking in visitFPTrunc to avoid creating overly small ConstantFPs that we'll just need to extend again.

[craig.topper]

Summary

Instead of returning the smaller FP constant we now return the minimal Type the constant can fit into. We also return the Type of the input to any fp extends. The legality checks are then done on just the size of these Types. If we find something profitable we then emit FPTruncs in front of the smaller binop and assume those FPTruncs will be constant folded or combined with any ConstantFPs or fpextends.

[scanon]

Works for me.

lib/Transforms/InstCombine/InstCombineCasts.cpp

Instruction *InstCombiner::visitSExt(SExtInst &CI) {
   }
 
   return nullptr;
 }
 
 
 /// Return a Constant* for the specified floating-point constant if it fits
 /// in the specified FP type without changing its value.
-static Constant *fitsInFPType(ConstantFP *CFP, const fltSemantics &Sem) {
+static bool fitsInFPType(ConstantFP *CFP, const fltSemantics &Sem) {
   bool losesInfo;
   APFloat F = CFP->getValueAPF();
   (void)F.convert(Sem, APFloat::rmNearestTiesToEven, &losesInfo);
-  if (!losesInfo)
-    return ConstantFP::get(CFP->getContext(), F);
-  return nullptr;
+  return !losesInfo;
 }
 
-static Constant *shrinkFPConstant(ConstantFP *CFP) {
+static Type *shrinkFPConstant(ConstantFP *CFP) {
   if (CFP->getType() == Type::getPPC_FP128Ty(CFP->getContext()))
     return nullptr;  // No constant folding of this.
   // See if the value can be truncated to half and then reextended.
-  if (Constant *NewCFP = fitsInFPType(CFP, APFloat::IEEEhalf()))
-    return NewCFP;
+  if (fitsInFPType(CFP, APFloat::IEEEhalf()))
+    return Type::getHalfTy(CFP->getContext());
   // See if the value can be truncated to float and then reextended.
-  if (Constant *NewCFP = fitsInFPType(CFP, APFloat::IEEEsingle()))
-    return NewCFP;
+  if (fitsInFPType(CFP, APFloat::IEEEsingle()))
+    return Type::getFloatTy(CFP->getContext());
   if (CFP->getType()->isDoubleTy())
     return nullptr;  // Won't shrink.
-  if (Constant *NewCFP = fitsInFPType(CFP, APFloat::IEEEdouble()))
-    return NewCFP;
+  if (fitsInFPType(CFP, APFloat::IEEEdouble()))
+    return Type::getDoubleTy(CFP->getContext());
   // Don't try to shrink to various long double types.
   return nullptr;
 }
 
-/// Look through floating-point extensions until we get the source value.
-static Value *lookThroughFPExtensions(Value *V) {
-  while (auto *FPExt = dyn_cast<FPExtInst>(V))
-    V = FPExt->getOperand(0);
+/// 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
   // that can accurately represent it.  This allows us to turn
   // (float)((double)X+2.0) into x+2.0f.
   if (auto *CFP = dyn_cast<ConstantFP>(V))
-    if (Constant *NewCFP = shrinkFPConstant(CFP))
-      return NewCFP;
+    if (Type *T = shrinkFPConstant(CFP))
+      return T;
 
-  return V;
+  return V->getType();
 }
 
 Instruction *InstCombiner::visitFPTrunc(FPTruncInst &CI) {
   if (Instruction *I = commonCastTransforms(CI))
     return I;
   // If we have fptrunc(OpI (fpextend x), (fpextend y)), we would like to
   // simplify this expression to avoid one or more of the trunc/extend
   // operations if we can do so without changing the numerical results.
   //
   // The exact manner in which the widths of the operands interact to limit
   // what we can and cannot do safely varies from operation to operation, and
   // is explained below in the various case statements.
   BinaryOperator *OpI = dyn_cast<BinaryOperator>(CI.getOperand(0));
   if (OpI && OpI->hasOneUse()) {
-    Value *LHSOrig = lookThroughFPExtensions(OpI->getOperand(0));
-    Value *RHSOrig = lookThroughFPExtensions(OpI->getOperand(1));
+    Type *LHSMinType = getMinimumFPType(OpI->getOperand(0));
+    Type *RHSMinType = getMinimumFPType(OpI->getOperand(1));
     unsigned OpWidth = OpI->getType()->getFPMantissaWidth();
-    unsigned LHSWidth = LHSOrig->getType()->getFPMantissaWidth();
-    unsigned RHSWidth = RHSOrig->getType()->getFPMantissaWidth();
+    unsigned LHSWidth = LHSMinType->getFPMantissaWidth();
+    unsigned RHSWidth = RHSMinType->getFPMantissaWidth();
     unsigned SrcWidth = std::max(LHSWidth, RHSWidth);
     unsigned DstWidth = CI.getType()->getFPMantissaWidth();
     switch (OpI->getOpcode()) {
       default: break;
       case Instruction::FAdd:
       case Instruction::FSub:
         // For addition and subtraction, the infinitely precise result can
         // essentially be arbitrarily wide; proving that double rounding
switch (OpI->getOpcode()) {
         // "A Rigorous Framework for Fully Supporting the IEEE Standard ..."
         // for proof of this fact).
         //
         // Note: Figueroa does not consider the case where DstFormat !=
         // SrcFormat.  It's possible (likely even!) that this analysis
         // could be tightened for those cases, but they are rare (the main
         // case of interest here is (float)((double)float + float)).
         if (OpWidth >= 2*DstWidth+1 && DstWidth >= SrcWidth) {
-          if (LHSOrig->getType() != CI.getType())
-            LHSOrig = Builder.CreateFPExt(LHSOrig, CI.getType());
-          if (RHSOrig->getType() != CI.getType())
-            RHSOrig = Builder.CreateFPExt(RHSOrig, CI.getType());
+          Value *LHS = Builder.CreateFPTrunc(OpI->getOperand(0), CI.getType());
+          Value *RHS = Builder.CreateFPTrunc(OpI->getOperand(1), CI.getType());
           Instruction *RI =
-            BinaryOperator::Create(OpI->getOpcode(), LHSOrig, RHSOrig);
+            BinaryOperator::Create(OpI->getOpcode(), LHS, RHS);
           RI->copyFastMathFlags(OpI);
           return RI;
         }
         break;
       case Instruction::FMul:
         // For multiplication, the infinitely precise result has at most
         // LHSWidth + RHSWidth significant bits; if OpWidth is sufficient
         // that such a value can be exactly represented, then no double
         // rounding can possibly occur; we can safely perform the operation
         // in the destination format if it can represent both sources.
         if (OpWidth >= LHSWidth + RHSWidth && DstWidth >= SrcWidth) {
-          if (LHSOrig->getType() != CI.getType())
-            LHSOrig = Builder.CreateFPExt(LHSOrig, CI.getType());
-          if (RHSOrig->getType() != CI.getType())
-            RHSOrig = Builder.CreateFPExt(RHSOrig, CI.getType());
+          Value *LHS = Builder.CreateFPTrunc(OpI->getOperand(0), CI.getType());
+          Value *RHS = Builder.CreateFPTrunc(OpI->getOperand(1), CI.getType());
           Instruction *RI =
-            BinaryOperator::CreateFMul(LHSOrig, RHSOrig);
+            BinaryOperator::CreateFMul(LHS, RHS);
           RI->copyFastMathFlags(OpI);
           return RI;
         }
         break;
       case Instruction::FDiv:
         // For division, we use again use the bound from Figueroa's
         // dissertation.  I am entirely certain that this bound can be
         // tightened in the unbalanced operand case by an analysis based on
         // the diophantine rational approximation bound, but the well-known
         // condition used here is a good conservative first pass.
         // TODO: Tighten bound via rigorous analysis of the unbalanced case.
         if (OpWidth >= 2*DstWidth && DstWidth >= SrcWidth) {
-          if (LHSOrig->getType() != CI.getType())
-            LHSOrig = Builder.CreateFPExt(LHSOrig, CI.getType());
-          if (RHSOrig->getType() != CI.getType())
-            RHSOrig = Builder.CreateFPExt(RHSOrig, CI.getType());
+          Value *LHS = Builder.CreateFPTrunc(OpI->getOperand(0), CI.getType());
+          Value *RHS = Builder.CreateFPTrunc(OpI->getOperand(1), CI.getType());
           Instruction *RI =
-            BinaryOperator::CreateFDiv(LHSOrig, RHSOrig);
+            BinaryOperator::CreateFDiv(LHS, RHS);
           RI->copyFastMathFlags(OpI);
           return RI;
         }
         break;
-      case Instruction::FRem:
+      case Instruction::FRem: {
         // Remainder is straightforward.  Remainder is always exact, so the
         // type of OpI doesn't enter into things at all.  We simply evaluate
         // in whichever source type is larger, then convert to the
         // destination type.
         if (SrcWidth == OpWidth)
           break;
-        if (LHSWidth < SrcWidth)
-          LHSOrig = Builder.CreateFPExt(LHSOrig, RHSOrig->getType());
-        else if (RHSWidth <= SrcWidth)
-          RHSOrig = Builder.CreateFPExt(RHSOrig, LHSOrig->getType());
-        if (LHSOrig != OpI->getOperand(0) || RHSOrig != OpI->getOperand(1)) {
-          Value *ExactResult = Builder.CreateFRem(LHSOrig, RHSOrig);
-          if (Instruction *RI = dyn_cast<Instruction>(ExactResult))
-            RI->copyFastMathFlags(OpI);
-          return CastInst::CreateFPCast(ExactResult, CI.getType());
-        }
+        Value *LHS, *RHS;
+        if (LHSWidth == SrcWidth) {
+           LHS = Builder.CreateFPTrunc(OpI->getOperand(0), LHSMinType);
+           RHS = Builder.CreateFPTrunc(OpI->getOperand(1), LHSMinType);
+        } else {
+           LHS = Builder.CreateFPTrunc(OpI->getOperand(0), RHSMinType);
+           RHS = Builder.CreateFPTrunc(OpI->getOperand(1), RHSMinType);
+        }
+
+        Value *ExactResult = Builder.CreateFRem(LHS, RHS);
     }
 
     // (fptrunc (fneg x)) -> (fneg (fptrunc x))
     if (BinaryOperator::isFNeg(OpI)) {
       Value *InnerTrunc = Builder.CreateFPTrunc(OpI->getOperand(1),
                                                 CI.getType());
       Instruction *RI = BinaryOperator::CreateFNeg(InnerTrunc);
       RI->copyFastMathFlags(OpI);