[RISCV] Set CopyCost on register classes [NFC]

[preames]

This change sets up CopyCost to reflect the cost of copying a vector register group, a vector segment, or a register pair.

This is NFC because the only actual usage of this field in tree is simply checking that cost is not negative. That heuristic is around ABI copy elimination during instruction emission + scheduling, and doesn't immediately seem like it should apply for us.

[llvmbot]

@llvm/pr-subscribers-backend-risc-v

Author: Philip Reames (preames)

Changes

This change sets up CopyCost to reflect the cost of copying a vector register group, a vector segment, or a register pair.

This is NFC because the only actual usage of this field in tree is simply checking that cost is not negative. That heuristic is around ABI copy elimination during instruction emission + scheduling, and doesn't immediately seem like it should apply for us.

---

Full diff: https://github.com/llvm/llvm-project/pull/131185.diff

1 Files Affected:

• (modified) llvm/lib/Target/RISCV/RISCVRegisterInfo.td (+3-2)

diff --git a/llvm/lib/Target/RISCV/RISCVRegisterInfo.td b/llvm/lib/Target/RISCV/RISCVRegisterInfo.td
index a5dfb5ba1a2fc..bbb1e82fbadaa 100644
--- a/llvm/lib/Target/RISCV/RISCVRegisterInfo.td
+++ b/llvm/lib/Target/RISCV/RISCVRegisterInfo.td
@@ -226,6 +226,7 @@ class RISCVRegisterClass<list<ValueType> regTypes, int align, dag regList>
   int NF = 1;
 
   let Size = !if(IsVRegClass, !mul(VLMul, NF, 64), 0);
+  let CopyCost = !if(IsVRegClass, !mul(VLMul, NF), 1);
 
   let TSFlags{0} = IsVRegClass;
   let TSFlags{3-1} = !logtwo(VLMul);
@@ -343,7 +344,7 @@ let RegAltNameIndices = [ABIRegAltName] in {
   }
 }
 
-let RegInfos = XLenPairRI,
+let RegInfos = XLenPairRI, CopyCost = 2,
     DecoderMethod = "DecodeGPRPairRegisterClass" in {
 def GPRPair : RISCVRegisterClass<[XLenPairVT, XLenPairFVT], 64, (add
     X10_X11, X12_X13, X14_X15, X16_X17,
@@ -357,7 +358,7 @@ def GPRPair : RISCVRegisterClass<[XLenPairVT, XLenPairFVT], 64, (add
 def GPRPairNoX0 : RISCVRegisterClass<[XLenPairVT, XLenPairFVT], 64, (sub GPRPair, X0_Pair)>;
 } // let RegInfos = XLenPairRI, DecoderMethod = "DecodeGPRPairRegisterClass"
 
-let RegInfos = XLenPairRI in
+let RegInfos = XLenPairRI, CopyCost = 2 in
 def GPRPairC : RISCVRegisterClass<[XLenPairVT, XLenPairFVT], 64, (add
   X10_X11, X12_X13, X14_X15, X8_X9
 )>;

[topperc]

LGTM.