[llvm] [AMDGPU] Preserve WWM ownership of allocated AGPRs (PR #223278)

[Yaxun Liu via llvm-commits]

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@@ -92,17 +92,20 @@ bool SILowerWWMCopies::isSCCLiveAtMI(const MachineInstr &MI) {
   return LR.liveAt(Idx);
 }
 
-// If \p Reg is assigned with a physical VGPR, add the latter into wwm-spills
-// for preserving its entire lanes at function prolog/epilog.
-void SILowerWWMCopies::addToWWMSpills(MachineFunction &MF, Register Reg) {
-  if (Reg.isPhysical())
-    return;
-
-  // FIXME: VRM may be null here.
-  MCRegister PhysReg = VRM->getPhys(Reg);
-  assert(PhysReg && "should have allocated a physical register");
+// Record the physical register assigned to a WWM copy destination. It remains
+// owned by the WWM allocation after the temporary allocation mask is cleared.
+void SILowerWWMCopies::reserveWWMRegister(MachineFunction &MF, Register Reg) {
+  Register PhysReg = Reg;
+  if (Reg.isVirtual()) {
+    assert(VRM && "expected VirtRegMap after WWM register allocation");
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yxsamliu wrote:

At `-O0`, fast allocation changes the operands to physical registers before this pass. That path does not use `VirtRegMap`; only the virtual-register path needs it.

https://github.com/llvm/llvm-project/pull/223278