[LoopVectorize] Fix zext/sext cost calculations when types are shrunk

[david-arm]

Summary

In getInstructionCost if we know a zext/sext is going to be shrunk
we should only be changing the destination type, and leave the
source type unchanged. For example, we may change a zext from

zext <16 x i8> %a to <16 x i32>

to

zext <16 x i8> %a to <16 x i16>

However, we were previously calculating the cost for doing

zext <16 x i16> %a to <16 x i16>

which is incorrect.

[kmclaughlin]

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

7372

Maybe not as part of this patch, but should we also leave SrcVecTy unchanged for truncates too?

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

case Instruction::FPTrunc: {
     if (canTruncateToMinimalBitwidth(I, VF)) {
       // This cast is going to be shrunk. This may remove the cast or it might
       // turn it into slightly different cast. For example, if MinBW == 16,
       // "zext i8 %1 to i32" becomes "zext i8 %1 to i16".
       //
       // Calculate the modified src and dest types.
       Type *MinVecTy = VectorTy;
       if (Opcode == Instruction::Trunc) {
         SrcVecTy = smallestIntegerVectorType(SrcVecTy, MinVecTy);
         VectorTy =
             largestIntegerVectorType(ToVectorTy(I->getType(), VF), MinVecTy);
       } else if (Opcode == Instruction::ZExt || Opcode == Instruction::SExt) {
-        SrcVecTy = largestIntegerVectorType(SrcVecTy, MinVecTy);
+        // Leave SrcVecTy unchanged - we only shrink the destination element
+        // type.
         VectorTy =
             smallestIntegerVectorType(ToVectorTy(I->getType(), VF), MinVecTy);
       }
     }
 
     return TTI.getCastInstrCost(Opcode, VectorTy, SrcVecTy, CCH, CostKind, I);
   }
   case Instruction::Call: {

llvm/test/Transforms/LoopVectorize/AArch64/type-shrinkage-zext-costs.ll

 ; CHECK-COST: LV: Found an estimated cost of 0 for VF 1 For instruction:   %conv = zext i8 %0 to i32
 ; CHECK-COST: LV: Found an estimated cost of 1 for VF 2 For instruction:   %conv = zext i8 %0 to i32
 ; CHECK-COST: LV: Found an estimated cost of 1 for VF 4 For instruction:   %conv = zext i8 %0 to i32
 ; CHECK-COST: LV: Found an estimated cost of 1 for VF 8 For instruction:   %conv = zext i8 %0 to i32
 ; CHECK-COST: LV: Found an estimated cost of 2 for VF 16 For instruction:   %conv = zext i8 %0 to i32
 ; CHECK-COST: LV: Found an estimated cost of 1 for VF vscale x 1 For instruction:   %conv = zext i8 %0 to i32
 ; CHECK-COST: LV: Found an estimated cost of 1 for VF vscale x 2 For instruction:   %conv = zext i8 %0 to i32
 ; CHECK-COST: LV: Found an estimated cost of 1 for VF vscale x 4 For instruction:   %conv = zext i8 %0 to i32
-; CHECK-COST: LV: Found an estimated cost of 1 for VF vscale x 8 For instruction:   %conv = zext i8 %0 to i32
+; CHECK-COST: LV: Found an estimated cost of 0 for VF vscale x 8 For instruction:   %conv = zext i8 %0 to i32
 
 ; CHECK-LABEL: define void @zext_i8_i16
 ; CHECK-SAME: (ptr noalias nocapture readonly [[P:%.*]], ptr noalias nocapture [[Q:%.*]], i32 [[LEN:%.*]]) #[[ATTR0:[0-9]+]] {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[CMP8:%.*]] = icmp sgt i32 [[LEN]], 0
 ; CHECK-NEXT:    br i1 [[CMP8]], label [[FOR_BODY_PREHEADER:%.*]], label [[FOR_COND_CLEANUP:%.*]]
 ; CHECK:       for.body.preheader:
 ; CHECK-NEXT:    [[TMP0:%.*]] = add i32 [[LEN]], -1

 ; CHECK-COST: LV: Found an estimated cost of 0 for VF 1 For instruction:   %conv = sext i8 %0 to i32
 ; CHECK-COST: LV: Found an estimated cost of 1 for VF 2 For instruction:   %conv = sext i8 %0 to i32
 ; CHECK-COST: LV: Found an estimated cost of 1 for VF 4 For instruction:   %conv = sext i8 %0 to i32
 ; CHECK-COST: LV: Found an estimated cost of 1 for VF 8 For instruction:   %conv = sext i8 %0 to i32
 ; CHECK-COST: LV: Found an estimated cost of 2 for VF 16 For instruction:   %conv = sext i8 %0 to i32
 ; CHECK-COST: LV: Found an estimated cost of 1 for VF vscale x 1 For instruction:   %conv = sext i8 %0 to i32
 ; CHECK-COST: LV: Found an estimated cost of 1 for VF vscale x 2 For instruction:   %conv = sext i8 %0 to i32
 ; CHECK-COST: LV: Found an estimated cost of 1 for VF vscale x 4 For instruction:   %conv = sext i8 %0 to i32
-; CHECK-COST: LV: Found an estimated cost of 1 for VF vscale x 8 For instruction:   %conv = sext i8 %0 to i32
+; CHECK-COST: LV: Found an estimated cost of 0 for VF vscale x 8 For instruction:   %conv = sext i8 %0 to i32
 
 ; CHECK-LABEL: define void @sext_i8_i16
 ; CHECK-SAME: (ptr noalias nocapture readonly [[P:%.*]], ptr noalias nocapture [[Q:%.*]], i32 [[LEN:%.*]]) #[[ATTR0]] {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[CMP8:%.*]] = icmp sgt i32 [[LEN]], 0
 ; CHECK-NEXT:    br i1 [[CMP8]], label [[FOR_BODY_PREHEADER:%.*]], label [[FOR_COND_CLEANUP:%.*]]
 ; CHECK:       for.body.preheader:
 ; CHECK-NEXT:    [[TMP0:%.*]] = add i32 [[LEN]], -1