[llvm] [AMDGPU][GlobalISel] Fix issue with copy_scc_vcc on gfx7 (PR #165355)

[Matt Arsenault via llvm-commits]

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@@ -221,12 +221,22 @@ bool AMDGPUInstructionSelector::selectCOPY(MachineInstr &I) const {
 bool AMDGPUInstructionSelector::selectCOPY_SCC_VCC(MachineInstr &I) const {
   const DebugLoc &DL = I.getDebugLoc();
   MachineBasicBlock *BB = I.getParent();
+  Register VCCReg = I.getOperand(1).getReg();
+  MachineInstr *Cmp;
+
+  if (STI.getGeneration() >= AMDGPUSubtarget::VOLCANIC_ISLANDS) {
+    unsigned CmpOpc =
+        STI.isWave64() ? AMDGPU::S_CMP_LG_U64 : AMDGPU::S_CMP_LG_U32;
+    Cmp = BuildMI(*BB, &I, DL, TII.get(CmpOpc)).addReg(VCCReg).addImm(0);
+  } else {
+    // For gfx7 and earlier, S_CMP_LG_U64 doesn't exist, so we use S_OR_B64
+    // which sets SCC as a side effect.
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arsenm wrote:

Both of these paths use sac 

https://github.com/llvm/llvm-project/pull/165355