[llvm] [AMDGPU] Select S_CSELECT_B32/64 for uniform extensions from i1 (PR #204238)

[Jay Foad via llvm-commits]

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@@ -8687,14 +8687,30 @@ void SIInstrInfo::lowerSelect(SIInstrWorklist &Worklist, MachineInstr &Inst,
   Register CondReg = Cond.getReg();
   bool IsSCC = (CondReg == AMDGPU::SCC);
 
-  // If this is a trivial select where the condition is effectively not SCC
-  // (CondReg is a source of copy to SCC), then the select is semantically
-  // equivalent to copying CondReg. Hence, there is no need to create
-  // V_CNDMASK, we can just use that and bail out.
+  // Remove S_CSELECT instructions that we previously inserted to feed the SCC
+  // condition output from S_CMP into the SGPR condition input of V_CNDMASK. If
+  // the S_CMP has been promoted to V_CMP then we can feed its SGPR condition
+  // output directly into the V_CNDMASK.
   if (!IsSCC && Src0.isImm() && (Src0.getImm() == -1) && Src1.isImm() &&
       (Src1.getImm() == 0)) {
-    MRI.replaceRegWith(Dest.getReg(), CondReg);
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jayfoad wrote:

The new uniform sext patterns can generate `S_CSELECT -1, 0` which were getting removed here wrongly. I had to tighten the checks so that we only remove them if they're feeding into the condition input of V_CNDMASK. @alex-t FYI.

https://github.com/llvm/llvm-project/pull/204238