[llvm] [AMDGPU] Add encoding/decoding support for non-result-returning ATOMIC_CSUB instructions (PR #68684)

[Jay Foad via llvm-commits]

================
@@ -4,35 +4,70 @@
 declare i32 @llvm.amdgcn.buffer.atomic.csub(i32, <4 x i32>, i32, i32, i1)
 declare i32 @llvm.amdgcn.global.atomic.csub(ptr addrspace(1), i32)
 
-; GCN-LABEL: {{^}}buffer_atomic_csub:
+; GCN-LABEL: {{^}}buffer_atomic_csub_rtn:
 ; GCN: buffer_atomic_csub v0, v1, s[0:3], 0 idxen glc
-define amdgpu_ps void @buffer_atomic_csub(<4 x i32> inreg %rsrc, i32 %data, i32 %vindex) {
+define amdgpu_ps void @buffer_atomic_csub_rtn(<4 x i32> inreg %rsrc, i32 %data, i32 %vindex) {
 main_body:
   %ret = call i32 @llvm.amdgcn.buffer.atomic.csub(i32 %data, <4 x i32> %rsrc, i32 %vindex, i32 0, i1 0)
   ret void
 }
 
-; GCN-LABEL: {{^}}buffer_atomic_csub_off4_slc:
+; GCN-LABEL: {{^}}buffer_atomic_csub_no_rtn:
+; GCN: buffer_atomic_csub v0, v1, s[0:3], 0 idxen
+define amdgpu_ps void @buffer_atomic_csub_no_rtn(<4 x i32> inreg %rsrc, i32 %data, i32 %vindex) #0 {
----------------
jayfoad wrote:

Personally I prefer to put the attributes inline here, rather than indirecting via `#0`.
```suggestion
define amdgpu_ps void @buffer_atomic_csub_no_rtn(<4 x i32> inreg %rsrc, i32 %data, i32 %vindex) "target-features"="+atomic-csub-no-rtn-insts" {
```

https://github.com/llvm/llvm-project/pull/68684