[llvm] [AMDGPU][SIInsertWaitcnts] Do not add s_waitcnt when the counters are known to be 0 already (PR #72830)

[Pierre van Houtryve via llvm-commits]

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@@ -870,6 +875,15 @@ static bool updateOperandIfDifferent(MachineInstr &MI, uint16_t OpName,
   return true;
 }
 
+bool SIInsertWaitcnts::updateWaitcntIfSoft(MachineInstr *Waitcnt) const {
+  unsigned Opcode = Waitcnt->getOpcode();
+  if (!SIInstrInfo::isSoftWaitcnt(Opcode))
+    return false;
+
+  Waitcnt->setDesc(TII->get(SIInstrInfo::getNonSoftWaitcntOpcode(Opcode)));
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Pierre-vh wrote:

> Why do we need this here as well as in pseudoToMCOpcode?

So soft waitcnts can be lowered into MCInsts
When I picked up this patch, IIRC, I tried to remove the one in `pseudoToMCOpcode` (and make soft waitcnts illegal to lower to MCInst) but it didn't work. I can try again if you want

> Note that SIInsertWaitcnts is iterative, so it may process the same block several times. If that happens then by the time the it runs the second time on a block, all remaining soft waitcnts will have been converted to hard ones by the first pass. Is that desirable for some reason?

Not sure, I'll think about it

https://github.com/llvm/llvm-project/pull/72830