[llvm-branch-commits] [llvm] [AMDGPU] always emit a soft wait even if it is trivially ~0 (PR #147257)

[Sameer Sahasrabuddhe via llvm-branch-commits]

================
@@ -669,6 +679,7 @@ define amdgpu_kernel void @global_volatile_store_1(
 ; GFX12-WGP-NEXT:    s_wait_kmcnt 0x0
 ; GFX12-WGP-NEXT:    s_wait_storecnt 0x0
 ; GFX12-WGP-NEXT:    global_store_b32 v0, v1, s[0:1] scope:SCOPE_SYS
+; GFX12-WGP-NEXT:    s_wait_loadcnt 0x3f
----------------
ssahasra wrote:

Not directly related to this discussion, but this line does exist:
```
   1390       // Merge consecutive waitcnt of the same type by erasing multiples.
   1391       if (WaitcntInstr || (!Wait.hasWaitExceptStoreCnt() && TrySimplify)) {
```
It is meant to preserver S_WAITCNT_soft even if there is no actual wait required. @jayfoad , you had introduced `TrySimplify` ... do you think it is okay to relax its uses?

```
   1373       if (TrySimplify **|| (Opcode != II.getOpcode() && OldWait.hasValuesSetToMax()**)
   1374         ScoreBrackets.simplifyWaitcnt(OldWait);
```
Here, `hasValuesSetToMax()` is a hypothetical function that checks the encoding of each count separately to have all bits set to 1, and not just a ~0 in the data structure.

https://github.com/llvm/llvm-project/pull/147257