[llvm] [AMDGPU] Make use of composeSubRegIndices. NFCI. (PR #95548)

[Jay Foad via llvm-commits]

https://github.com/jayfoad created https://github.com/llvm/llvm-project/pull/95548

Simplify SIInstrInfo::buildExtractSubReg by building one COPY with a
composed subreg index instead of two COPYs.


>From 0beb10a3e5a4cb9ee9ab42df781d9d8910ea98e8 Mon Sep 17 00:00:00 2001
From: Jay Foad <jay.foad at amd.com>
Date: Fri, 14 Jun 2024 14:55:30 +0100
Subject: [PATCH] [AMDGPU] Make use of composeSubRegIndices. NFCI.

Simplify SIInstrInfo::buildExtractSubReg by building one COPY with a
composed subreg index instead of two COPYs.
---
 llvm/lib/Target/AMDGPU/SIInstrInfo.cpp | 19 ++-----------------
 1 file changed, 2 insertions(+), 17 deletions(-)

diff --git a/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp b/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp
index 0edcdb337b5af..30c27b6439fc0 100644
--- a/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp
+++ b/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp
@@ -5651,24 +5651,9 @@ unsigned SIInstrInfo::buildExtractSubReg(
   DebugLoc DL = MI->getDebugLoc();
   Register SubReg = MRI.createVirtualRegister(SubRC);
 
-  if (SuperReg.getSubReg() == AMDGPU::NoSubRegister) {
-    BuildMI(*MBB, MI, DL, get(TargetOpcode::COPY), SubReg)
-      .addReg(SuperReg.getReg(), 0, SubIdx);
-    return SubReg;
-  }
-
-  // Just in case the super register is itself a sub-register, copy it to a new
-  // value so we don't need to worry about merging its subreg index with the
-  // SubIdx passed to this function. The register coalescer should be able to
-  // eliminate this extra copy.
-  Register NewSuperReg = MRI.createVirtualRegister(SuperRC);
-
-  BuildMI(*MBB, MI, DL, get(TargetOpcode::COPY), NewSuperReg)
-    .addReg(SuperReg.getReg(), 0, SuperReg.getSubReg());
-
+  unsigned NewSubIdx = RI.composeSubRegIndices(SuperReg.getSubReg(), SubIdx);
   BuildMI(*MBB, MI, DL, get(TargetOpcode::COPY), SubReg)
-    .addReg(NewSuperReg, 0, SubIdx);
-
+      .addReg(SuperReg.getReg(), 0, NewSubIdx);
   return SubReg;
 }