[AArch64][TTI] Set the cost of XTN to 1 for 2xi64 (to 2xi32) and 8xi16 (to8xi8).

[mingmingl]

Summary

From [1], XTN is used for {v8i16->v8i8, v4i32->v4i16, v2i64->v2i32}.
Without this, an IR instruction like trunc <8 x i16> %v to <8 x i8> is considered free and might be sinked to other basic blocks. As a result, the sinked 'trunc' is in a different basic block with its (usually not-free) vector operand and misses the chance to be combined during instruction selection. (examples in [2])

[1] https://github.com/llvm/llvm-project/blob/813ae2871d71f32cce46768e63185cd64651f6e9/llvm/lib/Target/AArch64/AArch64InstrInfo.td#L4472
[2] examples

• trunc(umin(X, 255)) -> UQXTRN v8i8 (and other {u,s}x{min,max} pattern for v8i16 operands) from https://github.com/llvm/llvm-project/blob/813ae2871d71f32cce46768e63185cd64651f6e9/llvm/lib/Target/AArch64/AArch64InstrInfo.td#L4515-L4528
• trunc (AArch64vlshr v8i16, imm) -> SHRNv8i8 (same missed for SHRNv2i32) from https://github.com/llvm/llvm-project/blob/813ae2871d71f32cce46768e63185cd64651f6e9/llvm/lib/Target/AArch64/AArch64InstrInfo.td#L6743-L6748

[mingmingl]

a duplicate of another reviewed revision (that was created by a mistaken operation), so abandon this.

llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp

InstructionCost AArch64TTIImpl::getCastInstrCost(unsigned Opcode, Type *Dst,
   EVT DstTy = TLI->getValueType(DL, Dst);
 
   if (!SrcTy.isSimple() || !DstTy.isSimple())
     return AdjustCost(
         BaseT::getCastInstrCost(Opcode, Dst, Src, CCH, CostKind, I));
 
   static const TypeConversionCostTblEntry
   ConversionTbl[] = {
-    { ISD::TRUNCATE, MVT::v4i16, MVT::v4i32,  1 },
+    { ISD::TRUNCATE, MVT::v4i16, MVT::v4i32,  1 },  // xtn
+    { ISD::TRUNCATE, MVT::v8i8,  MVT::v8i16,  1 },  // xtn
+    { ISD::TRUNCATE, MVT::v2i32, MVT::v2i64,  1 },  // xtn
     { ISD::TRUNCATE, MVT::v4i32, MVT::v4i64,  0 },
     { ISD::TRUNCATE, MVT::v8i8,  MVT::v8i32,  3 },
     { ISD::TRUNCATE, MVT::v16i8, MVT::v16i32, 6 },
 
     // Truncations on nxvmiN
     { ISD::TRUNCATE, MVT::nxv2i1, MVT::nxv2i16, 1 },
     { ISD::TRUNCATE, MVT::nxv2i1, MVT::nxv2i32, 1 },
     { ISD::TRUNCATE, MVT::nxv2i1, MVT::nxv2i64, 1 },

llvm/test/Analysis/CostModel/AArch64/cast.ll

 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %r27 = trunc i64 undef to i8
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %r28 = trunc i64 undef to i16
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %r29 = trunc i64 undef to i32
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s2i8i16 = trunc <2 x i16> undef to <2 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s2i8i32 = trunc <2 x i32> undef to <2 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %s2i8i64 = trunc <2 x i64> undef to <2 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s2i16i32 = trunc <2 x i32> undef to <2 x i16>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s2i16i64 = trunc <2 x i64> undef to <2 x i16>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s2i32i64 = trunc <2 x i64> undef to <2 x i32>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %s2i32i64 = trunc <2 x i64> undef to <2 x i32>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s4i8i16 = trunc <4 x i16> undef to <4 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s4i8i32 = trunc <4 x i32> undef to <4 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s4i8i64 = trunc <4 x i64> undef to <4 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %s4i16i32 = trunc <4 x i32> undef to <4 x i16>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s4i16i64 = trunc <4 x i64> undef to <4 x i16>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s4i32i64 = trunc <4 x i64> undef to <4 x i32>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s8i8i16 = trunc <8 x i16> undef to <8 x i8>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %s8i8i16 = trunc <8 x i16> undef to <8 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 3 for instruction: %s8i8i32 = trunc <8 x i32> undef to <8 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s8i8i64 = trunc <8 x i64> undef to <8 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 3 for instruction: %s8i16i32 = trunc <8 x i32> undef to <8 x i16>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %s8i16i64 = trunc <8 x i64> undef to <8 x i16>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s8i32i64 = trunc <8 x i64> undef to <8 x i32>
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %s16i8i16 = trunc <16 x i16> undef to <16 x i8>
+; CHECK-NEXT:  Cost Model: Found an estimated cost of 3 for instruction: %s16i8i16 = trunc <16 x i16> undef to <16 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 6 for instruction: %s16i8i32 = trunc <16 x i32> undef to <16 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %s16i8i64 = trunc <16 x i64> undef to <16 x i8>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 6 for instruction: %s16i16i32 = trunc <16 x i32> undef to <16 x i16>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 2 for instruction: %s16i16i64 = trunc <16 x i64> undef to <16 x i16>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: %s16i32i64 = trunc <16 x i64> undef to <16 x i32>
 ; CHECK-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: ret void
 ;
   %r8 = trunc i8 undef to i1