[AMDGPU] SCC live interval must be recomputed after CF lowering

[perlfu]

Most operations added or removed during CF lowering interact with SCC. As with EXEC the live interval of SCC must be recomputed. Tidy up RegUnit recomputation requests to end of pass if any work was done.

[llvmbot]

@llvm/pr-subscribers-backend-amdgpu

Author: Carl Ritson (perlfu)

Changes

Most operations added or removed during CF lowering interact with SCC. As with EXEC the live interval of SCC must be recomputed. Tidy up RegUnit recomputation requests to end of pass if any work was done.

---

Full diff: https://github.com/llvm/llvm-project/pull/81817.diff

1 Files Affected:

• (modified) llvm/lib/Target/AMDGPU/SILowerControlFlow.cpp (+5-6)

diff --git a/llvm/lib/Target/AMDGPU/SILowerControlFlow.cpp b/llvm/lib/Target/AMDGPU/SILowerControlFlow.cpp
index f178324dbbe246..2e6f6463d61dae 100644
--- a/llvm/lib/Target/AMDGPU/SILowerControlFlow.cpp
+++ b/llvm/lib/Target/AMDGPU/SILowerControlFlow.cpp
@@ -292,7 +292,6 @@ void SILowerControlFlow::emitIf(MachineInstr &MI) {
   LIS->InsertMachineInstrInMaps(*SetExec);
   LIS->InsertMachineInstrInMaps(*NewBr);
 
-  LIS->removeAllRegUnitsForPhysReg(AMDGPU::EXEC);
   MI.eraseFromParent();
 
   // FIXME: Is there a better way of adjusting the liveness? It shouldn't be
@@ -362,9 +361,6 @@ void SILowerControlFlow::emitElse(MachineInstr &MI) {
   RecomputeRegs.insert(SrcReg);
   RecomputeRegs.insert(DstReg);
   LIS->createAndComputeVirtRegInterval(SaveReg);
-
-  // Let this be recomputed.
-  LIS->removeAllRegUnitsForPhysReg(AMDGPU::EXEC);
 }
 
 void SILowerControlFlow::emitIfBreak(MachineInstr &MI) {
@@ -932,8 +928,6 @@ bool SILowerControlFlow::runOnMachineFunction(MachineFunction &MF) {
       case AMDGPU::SI_INIT_EXEC:
       case AMDGPU::SI_INIT_EXEC_FROM_INPUT:
         lowerInitExec(MBB, MI);
-        if (LIS)
-          LIS->removeAllRegUnitsForPhysReg(AMDGPU::EXEC);
         Changed = true;
         break;
 
@@ -951,6 +945,11 @@ bool SILowerControlFlow::runOnMachineFunction(MachineFunction &MF) {
   optimizeEndCf();
 
   if (LIS) {
+    if (Changed) {
+      // These will need to be recomputed for insertions and removals.
+      LIS->removeAllRegUnitsForPhysReg(AMDGPU::EXEC);
+      LIS->removeAllRegUnitsForPhysReg(AMDGPU::SCC);
+    }
     for (Register Reg : RecomputeRegs) {
       LIS->removeInterval(Reg);
       LIS->createAndComputeVirtRegInterval(Reg);

[jayfoad]

Do you have a test case that fails verification without this?

[arsenm]

Needs test

[arsenm]

exec is reserved and has no liveness tracking, not sure why it was handled before