[llvm] Improve selection of conditional branch on amdgcn.ballot!=0 condition in SelectionDAG. (PR #68714)

[Matt Arsenault via llvm-commits]

================
@@ -2272,11 +2296,44 @@ void AMDGPUDAGToDAGISel::SelectBRCOND(SDNode *N) {
   const SIRegisterInfo *TRI = ST->getRegisterInfo();
 
   bool UseSCCBr = isCBranchSCC(N) && isUniformBr(N);
-  unsigned BrOp = UseSCCBr ? AMDGPU::S_CBRANCH_SCC1 : AMDGPU::S_CBRANCH_VCCNZ;
+  bool AndExec = !UseSCCBr;
+  bool Negate = false;
+
+  if (Cond.getOpcode() == ISD::SETCC &&
+      Cond->getOperand(0)->getOpcode() == AMDGPUISD::SETCC) {
+    auto CC = cast<CondCodeSDNode>(Cond->getOperand(2))->get();
+    auto *CRHS = dyn_cast<ConstantSDNode>(Cond->getOperand(1));
+    if ((CC == ISD::SETEQ || CC == ISD::SETNE) && CRHS && CRHS->isZero()) {
+      // %VCMP = i(WaveSize) AMDGPUISD::SETCC ...
+      // %C = i1 ISD::SETCC %VCMP, 0, setne/seteq
----------------
arsenm wrote:

I think it's slightly confusing to not include the root brcond in the pseudocode 

https://github.com/llvm/llvm-project/pull/68714