[llvm] AMDGPU/GFX12: Do not wait unnecessarily before barriers (PR #154970)

[Jay Foad via llvm-commits]

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@@ -2010,11 +2010,19 @@ bool SIInsertWaitcnts::generateWaitcntInstBefore(MachineInstr &MI,
     }
   }
 
-  // The subtarget may have an implicit S_WAITCNT 0 before barriers. If it does
-  // not, we need to ensure the subtarget is capable of backing off barrier
-  // instructions in case there are any outstanding memory operations that may
-  // cause an exception. Otherwise, insert an explicit S_WAITCNT 0 here.
-  if (TII->isBarrierStart(MI.getOpcode()) &&
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jayfoad wrote:

I think this was the only substantive use of isBarrierStart.

https://github.com/llvm/llvm-project/pull/154970