[llvm] [RISCV][TTI] Extract subvector at index zero is free (PR #81751)
Philip Reames via llvm-commits
llvm-commits at lists.llvm.org
Wed Feb 14 10:09:17 PST 2024
================
@@ -432,12 +432,20 @@ InstructionCost RISCVTTIImpl::getShuffleCost(TTI::ShuffleKind Kind,
// must be implemented here.
break;
case TTI::SK_ExtractSubvector:
+ // Extract at zero is always a subregister extract
+ if (Index == 0)
+ return TTI::TCC_Free;
+
// Example sequence:
// vsetivli zero, 4, e8, mf2, tu, ma (ignored)
// vslidedown.vi v8, v9, 2
return LT.first *
getRISCVInstructionCost(RISCV::VSLIDEDOWN_VI, LT.second, CostKind);
case TTI::SK_InsertSubvector:
+ // Insert at zero is always a subregister insert
+ if (Index == 0)
+ return TTI::TCC_Free;
----------------
preames wrote:
I reframed this review to be specifically about the extract case. Once this lands, I'll take another attempt at the insert case. That ones slightly annoying since I don't see an easy way in the current API to figure out what the passthru is. I may have to resort to inspecting the operands of the instruction directly.
https://github.com/llvm/llvm-project/pull/81751
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