[llvm] [WIP][AMDGPU][SIInsertWaitcnts] Do not add s_waitcnt when the counters are known to be 0 already (PR #65735)

[Jay Foad via llvm-commits]

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@@ -1277,9 +1277,18 @@ def S_WAKEUP : SOPP_Pseudo <"s_wakeup", (ins) > {
   let mayStore = 1;
 }
 
-let hasSideEffects = 1 in
 def S_WAITCNT : SOPP_Pseudo <"s_waitcnt" , (ins SWaitCnt:$simm16), "$simm16",
     [(int_amdgcn_s_waitcnt timm:$simm16)]>;
+
+// "_soft" waitcnts are waitcnts whose wait can be relaxed or completely removed.
+// These are inserted by to resolve memory dependencies by the memory legalizer and later optimized by SIInsertWaitcnts
+// For example, a S_WAITCNT_soft 0 can be completely removed on a function that doesn't access memory.
+def S_WAITCNT_soft : SOPP_Pseudo <"s_soft_waitcnt" , (ins SWaitCnt:$simm16), "$simm16">;
+def S_WAITCNT_VSCNT_soft : SOPP_Pseudo<"s_soft_waitcnt_vscnt", (ins SReg_32:$sdst, s16imm:$simm16), "$sdst, $simm16"> {
+  let mayLoad = 1;
+  let mayStore = 1;
+  let has_sdst = 1;
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jayfoad wrote:

Why are these needed? The definition of S_WAITCNT above does not have them.

https://github.com/llvm/llvm-project/pull/65735