[llvm] [RISCV][TTI] Fix a costing mistake for truncate/fp_round with LMUL>m1 (PR #101051)

[Shih-Po Hung via llvm-commits]

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@@ -1108,60 +1108,60 @@ define void @trunc() {
 ; RV32-NEXT:  Cost Model: Found an estimated cost of 2 for instruction: %nxv1i64_nxv1i1 = trunc <vscale x 1 x i64> undef to <vscale x 1 x i1>
 ; RV32-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %nxv2i16_nxv2i8 = trunc <vscale x 2 x i16> undef to <vscale x 2 x i8>
 ; RV32-NEXT:  Cost Model: Found an estimated cost of 2 for instruction: %nxv2i32_nxv2i8 = trunc <vscale x 2 x i32> undef to <vscale x 2 x i8>
-; RV32-NEXT:  Cost Model: Found an estimated cost of 3 for instruction: %nxv2i64_nxv2i8 = trunc <vscale x 2 x i64> undef to <vscale x 2 x i8>
+; RV32-NEXT:  Cost Model: Found an estimated cost of 4 for instruction: %nxv2i64_nxv2i8 = trunc <vscale x 2 x i64> undef to <vscale x 2 x i8>
 ; RV32-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %nxv2i32_nxv2i16 = trunc <vscale x 2 x i32> undef to <vscale x 2 x i16>
-; RV32-NEXT:  Cost Model: Found an estimated cost of 2 for instruction: %nxv2i64_nxv2i16 = trunc <vscale x 2 x i64> undef to <vscale x 2 x i16>
-; RV32-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %nxv2i64_nxv2i32 = trunc <vscale x 2 x i64> undef to <vscale x 2 x i32>
+; RV32-NEXT:  Cost Model: Found an estimated cost of 3 for instruction: %nxv2i64_nxv2i16 = trunc <vscale x 2 x i64> undef to <vscale x 2 x i16>
+; RV32-NEXT:  Cost Model: Found an estimated cost of 2 for instruction: %nxv2i64_nxv2i32 = trunc <vscale x 2 x i64> undef to <vscale x 2 x i32>
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arcbbb wrote:

In this case, (LMUL2, e64) is truncated to (LMUL1, e32).
`Cost Model: Found an estimated cost of 1 for instruction: %nxv2i64_nxv2i32 = trunc <vscale x 2 x i64> undef to <vscale x 2 x i32>`
I believe the cost is implementation-defined. It can be executed either as a single uop reading two registers from `vs2` or as two uops reading different parts from `vs2`. In the former case, the cost is 1, and in the latter case, it is 2.
The implementation-related decision would be encapsulated in `getRISCVInstructionCost(RISCV::VNSRL_WI)`, so we should always pass the destination type to `getRISCVInstructionCost`.

https://github.com/llvm/llvm-project/pull/101051