[llvm] AMDGPU: Remove dead code trying to constrain a physical register (PR #161790)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm created https://github.com/llvm/llvm-project/pull/161790

This constrainRegClass check would never pass for a physical
register.

>From 633c24464c84612bd7736b5f4ff7bbf2d3a25bfc Mon Sep 17 00:00:00 2001
From: Matt Arsenault <Matthew.Arsenault at amd.com>
Date: Fri, 3 Oct 2025 14:42:32 +0900
Subject: [PATCH] AMDGPU: Remove dead code trying to constrain a physical
 register

This constrainRegClass check would never pass for a physical
register.
---
 llvm/lib/Target/AMDGPU/SIInstrInfo.cpp | 23 ++++++++---------------
 1 file changed, 8 insertions(+), 15 deletions(-)

diff --git a/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp b/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp
index 56435a50c87ad..fe6b8b96cbd57 100644
--- a/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp
+++ b/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp
@@ -8117,21 +8117,14 @@ void SIInstrInfo::moveToVALUImpl(SIInstrWorklist &Worklist,
     // hope for the best.
     if (Inst.isCopy() && DstReg.isPhysical() &&
         RI.isVGPR(MRI, Inst.getOperand(1).getReg())) {
-      // TODO: Only works for 32 bit registers.
-      if (MRI.constrainRegClass(DstReg, &AMDGPU::SReg_32_XM0RegClass)) {
-        BuildMI(*Inst.getParent(), &Inst, Inst.getDebugLoc(),
-                get(AMDGPU::V_READFIRSTLANE_B32), DstReg)
-            .add(Inst.getOperand(1));
-      } else {
-        Register NewDst =
-            MRI.createVirtualRegister(&AMDGPU::SReg_32_XM0RegClass);
-        BuildMI(*Inst.getParent(), &Inst, Inst.getDebugLoc(),
-                get(AMDGPU::V_READFIRSTLANE_B32), NewDst)
-            .add(Inst.getOperand(1));
-        BuildMI(*Inst.getParent(), &Inst, Inst.getDebugLoc(), get(AMDGPU::COPY),
-                DstReg)
-            .addReg(NewDst);
-      }
+      Register NewDst = MRI.createVirtualRegister(&AMDGPU::SReg_32_XM0RegClass);
+      BuildMI(*Inst.getParent(), &Inst, Inst.getDebugLoc(),
+              get(AMDGPU::V_READFIRSTLANE_B32), NewDst)
+          .add(Inst.getOperand(1));
+      BuildMI(*Inst.getParent(), &Inst, Inst.getDebugLoc(), get(AMDGPU::COPY),
+              DstReg)
+          .addReg(NewDst);
+
       Inst.eraseFromParent();
       return;
     }