[AMDGPU] Enable saving SHARED_BASE to VCC

[carlobertolli]

This is observed when exhausting scalar registers in kernels with high register usage.

[llvmbot]

@llvm/pr-subscribers-backend-amdgpu

Author: None (carlobertolli)

Changes

This is observed when exhausting scalar registers in kernels with high register usage.

---

Full diff: https://github.com/llvm/llvm-project/pull/163244.diff

2 Files Affected:

• (modified) llvm/lib/Target/AMDGPU/SIInstrInfo.cpp (+2-1)
• (modified) llvm/test/CodeGen/AMDGPU/sgpr-phys-copy.mir (+9)

diff --git a/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp b/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp
index ec5c5bb349ac4..86a60e5a6242b 100644
--- a/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp
+++ b/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp
@@ -899,7 +899,8 @@ void SIInstrInfo::copyPhysReg(MachineBasicBlock &MBB,
     }
 
     if (DestReg == AMDGPU::VCC) {
-      if (AMDGPU::SReg_64RegClass.contains(SrcReg)) {
+      if (AMDGPU::SReg_64RegClass.contains(SrcReg) ||
+      AMDGPU::SReg_64_EncodableRegClass.contains(SrcReg)) {
         BuildMI(MBB, MI, DL, get(AMDGPU::S_MOV_B64), AMDGPU::VCC)
           .addReg(SrcReg, getKillRegState(KillSrc));
       } else {
diff --git a/llvm/test/CodeGen/AMDGPU/sgpr-phys-copy.mir b/llvm/test/CodeGen/AMDGPU/sgpr-phys-copy.mir
index 9553fcc1c51c8..f11fe4aa6e00e 100644
--- a/llvm/test/CodeGen/AMDGPU/sgpr-phys-copy.mir
+++ b/llvm/test/CodeGen/AMDGPU/sgpr-phys-copy.mir
@@ -58,6 +58,15 @@ body:             |
     $sgpr0_sgpr1 = COPY $src_shared_base
 ...
 
+---
+name: src_shared_base_to_vcc
+body:             |
+  bb.0:
+    ; GFX9-LABEL: name: src_shared_base_to_vcc
+    ; GFX9: $vcc = S_MOV_B64 $src_shared_base
+    $vcc = COPY $src_shared_base
+...
+
 ---
 name: sgpr96_aligned_src_dst
 body:             |

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[rampitec]

LGTM

[arsenm]

Also test the inverse case?

[arsenm]

ping. We do not structurally prevent writing to constant registers and it is encodable

[carlobertolli]

@arsenm I added a test to copy vcc to shared_base but it fails

name: vcc_to_src_shared_base
body: |
bb.0:
; GFX9-LABEL: name: vcc_to_src_shared_base
; GFX9: $src_shared_base = S_MOV_B64 $vcc
$src_shared_base = COPY $vcc
...

I';ll have to fix the copy to enable this, unless I misunderstood you?

[carlobertolli]

like this?
#164138