[CostModel][AArch64] Remove amortization factor for some of the vector select instructions

[sbaranga]

Summary

We are not scalarizing the wide selects in codegen for i16 and i32 and
therefore we can remove the amortization factor. We still have issues
with i64 vectors in codegen though.

[sbaranga]

Tested with spec2k, spec2k6 and lnt on cortex-a57, and didn't get any significant changes.

[mcrosier]

This looks pretty reasonable.

[sbaranga]

Thanks, r247156!

lib/Target/AArch64/AArch64TargetTransformInfo.cpp

int AArch64TTIImpl::getAddressComputationCost(Type *Ty, bool IsComplex) {
   // addressing mode.
   return 1;
 }
 
 int AArch64TTIImpl::getCmpSelInstrCost(unsigned Opcode, Type *ValTy,
                                        Type *CondTy) {
 
   int ISD = TLI->InstructionOpcodeToISD(Opcode);
-  // We don't lower vector selects well that are wider than the register width.
+  // We don't lower some vector selects well that are wider than the register
+  // width.
   if (ValTy->isVectorTy() && ISD == ISD::SELECT) {
     // We would need this many instructions to hide the scalarization happening.
     const int AmortizationCost = 20;
     static const TypeConversionCostTblEntry<MVT::SimpleValueType>
     VectorSelectTbl[] = {
-      { ISD::SELECT, MVT::v16i1, MVT::v16i16, 16 * AmortizationCost },
-      { ISD::SELECT, MVT::v8i1, MVT::v8i32, 8 * AmortizationCost },
-      { ISD::SELECT, MVT::v16i1, MVT::v16i32, 16 * AmortizationCost },
+      { ISD::SELECT, MVT::v16i1, MVT::v16i16, 16 },
+      { ISD::SELECT, MVT::v8i1, MVT::v8i32, 8 },
+      { ISD::SELECT, MVT::v16i1, MVT::v16i32, 16 },
       { ISD::SELECT, MVT::v4i1, MVT::v4i64, 4 * AmortizationCost },
       { ISD::SELECT, MVT::v8i1, MVT::v8i64, 8 * AmortizationCost },
       { ISD::SELECT, MVT::v16i1, MVT::v16i64, 16 * AmortizationCost }
     };
 
     EVT SelCondTy = TLI->getValueType(DL, CondTy);
     EVT SelValTy = TLI->getValueType(DL, ValTy);
     if (SelCondTy.isSimple() && SelValTy.isSimple()) {

test/Analysis/CostModel/AArch64/select.ll

define void @select() {
   %v3 = select i1 undef, i32 undef, i32 undef
   ; CHECK: cost of 1 {{.*}} select
   %v4 = select i1 undef, i64 undef, i64 undef
   ; CHECK: cost of 1 {{.*}} select
   %v5 = select i1 undef, float undef, float undef
   ; CHECK: cost of 1 {{.*}} select
   %v6 = select i1 undef, double undef, double undef
 
-  ; Vector values - check for vectors that have a high cost because they end up
-  ; scalarized.
-  ; CHECK: cost of 320 {{.*}} select
+  ; CHECK: cost of 16 {{.*}} select
   %v13b = select <16 x i1>  undef, <16 x i16> undef, <16 x i16> undef
 
-  ; CHECK: cost of 160 {{.*}} select
+  ; CHECK: cost of 8 {{.*}} select
   %v15b = select <8 x i1>  undef, <8 x i32> undef, <8 x i32> undef
-  ; CHECK: cost of 320 {{.*}} select
+  ; CHECK: cost of 16 {{.*}} select
   %v15c = select <16 x i1>  undef, <16 x i32> undef, <16 x i32> undef
 
+  ; Vector values - check for vectors of i64s that have a high cost because
+  ; they end up scalarized.
   ; CHECK: cost of 80 {{.*}} select
   %v16a = select <4 x i1> undef, <4 x i64> undef, <4 x i64> undef
   ; CHECK: cost of 160 {{.*}} select
   %v16b = select <8 x i1> undef, <8 x i64> undef, <8 x i64> undef
   ; CHECK: cost of 320 {{.*}} select
   %v16c = select <16 x i1> undef, <16 x i64> undef, <16 x i64> undef
 
-    ret void
 }