[AMDGPU] Fix Livereg computation during epilogue insertion

[cdevadas]

Summary

The LivePhysRegs calculated in order to find a scratch register in the epilogue code wrongly uses 'LiveIns'. Instead, it should use the 'Liveout' sets.
For the liveness, also considering the operands of the terminator (return) instruction which is the insertion point for the scratch-exec-copy instruction.

[arsenm]

LGTM

test/CodeGen/AMDGPU/callee-special-input-sgprs.ll

488

Why did this change?

[arsenm]

r364470

test/CodeGen/AMDGPU/callee-special-input-sgprs.ll

488

I fixed this to add the new scratch use and keep the -NOT

lib/Target/AMDGPU/SIFrameLowering.cpp

for (const SIMachineFunctionInfo::SGPRSpillVGPRCSR &Reg
          : FuncInfo->getSGPRSpillVGPRs()) {
     if (!Reg.FI.hasValue())
       continue;
 
     const SIRegisterInfo &TRI = TII->getRegisterInfo();
     if (ScratchExecCopy == AMDGPU::NoRegister) {
       // See emitPrologue
       LivePhysRegs LiveRegs(*ST.getRegisterInfo());
-      LiveRegs.addLiveIns(MBB);
+      LiveRegs.addLiveOuts(MBB);
+      LiveRegs.stepBackward(*MBBI);
 
       ScratchExecCopy
         = findScratchNonCalleeSaveRegister(MF, LiveRegs,
                                            *TRI.getWaveMaskRegClass());
 
       const unsigned OrSaveExec = ST.isWave32() ?
       AMDGPU::S_OR_SAVEEXEC_B32 : AMDGPU::S_OR_SAVEEXEC_B64;
 

test/CodeGen/AMDGPU/callee-special-input-sgprs.ll

 ; GCN-NOT: s6
 ; GCN-NOT: s7
 ; GCN-NOT: s8
 ; GCN-NOT: s9
 ; GCN-NOT: s10
 ; GCN-NOT: s11
 ; GCN-NOT: s12
 ; GCN-NOT: s13
-; GCN-NOT: s[6:7]
 ; GCN-NOT: s[8:9]
 ; GCN-NOT: s[10:11]
 ; GCN-NOT: s[12:13]
 define hidden void @func_indirect_use_every_sgpr_input() #1 {
   call void @use_every_sgpr_input()
   ret void
 }