[ConstantFold] Avoid creating undesirable cast expressions

Similar to what we do for binops, for undesirable casts we should
call the constant folding API instead of the constant expr API,
to avoid indirect creation of undesirable cast ops.

Author: nikic

llvm/lib/IR/ConstantFold.cpp

@@ -295,6 +295,13 @@ static Constant *ExtractConstantBytes(Constant *C, unsigned ByteStart,
   }
 }
 
+static Constant *foldMaybeUndesirableCast(unsigned opc, Constant *V,
+                                          Type *DestTy) {
+  return ConstantExpr::isDesirableCastOp(opc)
+             ? ConstantExpr::getCast(opc, V, DestTy)
+             : ConstantFoldCastInstruction(opc, V, DestTy);
+}
+
 Constant *llvm::ConstantFoldCastInstruction(unsigned opc, Constant *V,
                                             Type *DestTy) {
   if (isa<PoisonValue>(V))
@@ -320,7 +327,7 @@ Constant *llvm::ConstantFoldCastInstruction(unsigned opc, Constant *V,
     if (CE->isCast()) {
       // Try hard to fold cast of cast because they are often eliminable.
       if (unsigned newOpc = foldConstantCastPair(opc, CE, DestTy))
-        return ConstantExpr::getCast(newOpc, CE->getOperand(0), DestTy);
+        return foldMaybeUndesirableCast(newOpc, CE->getOperand(0), DestTy);
     } else if (CE->getOpcode() == Instruction::GetElementPtr &&
                // Do not fold addrspacecast (gep 0, .., 0). It might make the
                // addrspacecast uncanonicalized.
@@ -357,18 +364,22 @@ Constant *llvm::ConstantFoldCastInstruction(unsigned opc, Constant *V,
     Type *DstEltTy = DestVecTy->getElementType();
     // Fast path for splatted constants.
     if (Constant *Splat = V->getSplatValue()) {
+      Constant *Res = foldMaybeUndesirableCast(opc, Splat, DstEltTy);
+      if (!Res)
+        return nullptr;
       return ConstantVector::getSplat(
-          cast<VectorType>(DestTy)->getElementCount(),
-          ConstantExpr::getCast(opc, Splat, DstEltTy));
+          cast<VectorType>(DestTy)->getElementCount(), Res);
     }
     SmallVector<Constant *, 16> res;
     Type *Ty = IntegerType::get(V->getContext(), 32);
     for (unsigned i = 0,
                   e = cast<FixedVectorType>(V->getType())->getNumElements();
          i != e; ++i) {
-      Constant *C =
-        ConstantExpr::getExtractElement(V, ConstantInt::get(Ty, i));
-      res.push_back(ConstantExpr::getCast(opc, C, DstEltTy));
+      Constant *C = ConstantExpr::getExtractElement(V, ConstantInt::get(Ty, i));
+      Constant *Casted = foldMaybeUndesirableCast(opc, C, DstEltTy);
+      if (!Casted)
+        return nullptr;
+      res.push_back(Casted);
     }
     return ConstantVector::get(res);
   }

llvm/test/CodeGen/AArch64/arm64-codegen-prepare-extload.ll

@@ -338,7 +338,8 @@ entry:
 ; OPTALL: [[LD:%[a-zA-Z_0-9-]+]] = load i16, ptr %addr
 ;
 ; OPT-NEXT: [[SEXT:%[a-zA-Z_0-9-]+]] = sext i16 [[LD]] to i32
-; OPT-NEXT: [[RES:%[a-zA-Z_0-9-]+]] = add nuw nsw i32 [[SEXT]], zext (i1 icmp ne (ptr getelementptr inbounds ([2 x i32], ptr @c, i64 0, i64 1), ptr @a) to i32)
+; OPT-NEXT: [[SEXT2:%[a-zA-Z_0-9-]+]] = sext i16 zext (i1 icmp ne (ptr getelementptr inbounds ([2 x i32], ptr @c, i64 0, i64 1), ptr @a) to i16) to i32
+; OPT-NEXT: [[RES:%[a-zA-Z_0-9-]+]] = add nuw nsw i32 [[SEXT]], [[SEXT2]]
 ;
 ; DISABLE-NEXT: [[ADD:%[a-zA-Z_0-9-]+]] = add nuw nsw i16 [[LD]], zext (i1 icmp ne (ptr getelementptr inbounds ([2 x i32], ptr @c, i64 0, i64 1), ptr @a) to i16)
 ; DISABLE-NEXT: [[RES:%[a-zA-Z_0-9-]+]] = sext i16 [[ADD]] to i32

llvm/test/CodeGen/X86/codegen-prepare-extload.ll

@@ -546,7 +546,8 @@ define i32 @promotionOfArgEndsUpInValue(ptr %addr) {
 ; OPT-NEXT:  entry:
 ; OPT-NEXT:    [[VAL:%.*]] = load i16, ptr [[ADDR]], align 2
 ; OPT-NEXT:    [[CONV3:%.*]] = sext i16 [[VAL]] to i32
-; OPT-NEXT:    [[ADD:%.*]] = add nuw nsw i32 [[CONV3]], zext (i1 icmp ne (ptr getelementptr inbounds ([2 x i32], ptr @c, i64 0, i64 1), ptr @a) to i32)
+; OPT-NEXT:    [[PROMOTED:%.*]] = sext i16 zext (i1 icmp ne (ptr getelementptr inbounds ([2 x i32], ptr @c, i64 0, i64 1), ptr @a) to i16) to i32
+; OPT-NEXT:    [[ADD:%.*]] = add nuw nsw i32 [[CONV3]], [[PROMOTED]]
 ; OPT-NEXT:    ret i32 [[ADD]]
 ;
 ; DISABLE-LABEL: define i32 @promotionOfArgEndsUpInValue(