AMDGPU: Try to reuse dest reg for s_add_i32 frame indexes (#111201)

Hack around the register scavenger doing the wrong thing.
It does not find the result register as available in the
case the frame index add isn't also reading the dest register.
This is the quick fix for a regression where the scavenge would
create a broken spill of SGPR to memory. I believe this is still
broken for cases we cannot use the result register.

I'm confused about what position the scavenger iterator
is supposed to be in, and what RestoreAfter is for. The scavenger
is missing a full set of forward/backward APIs and there seems
to be an off by one somewhere.

Author: arsenm

llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp

@@ -2678,12 +2678,18 @@ bool SIRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator MI,
 
       Register TmpReg;
 
+      // FIXME: Scavenger should figure out that the result register is
+      // available. Also should do this for the v_add case.
+      if (OtherOp.isReg() && OtherOp.getReg() != DstOp.getReg())
+        TmpReg = DstOp.getReg();
+
       if (FrameReg && !ST.enableFlatScratch()) {
         // FIXME: In the common case where the add does not also read its result
         // (i.e. this isn't a reg += fi), it's not finding the dest reg as
         // available.
-        TmpReg = RS->scavengeRegisterBackwards(AMDGPU::SReg_32_XM0RegClass, MI,
-                                               false, 0);
+        if (!TmpReg)
+          TmpReg = RS->scavengeRegisterBackwards(AMDGPU::SReg_32_XM0RegClass,
+                                                 MI, false, 0);
         BuildMI(*MBB, *MI, DL, TII->get(AMDGPU::S_LSHR_B32))
             .addDef(TmpReg, RegState::Renamable)
             .addReg(FrameReg)
@@ -2711,7 +2717,8 @@ bool SIRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator MI,
 
         if (!TmpReg && MaterializedReg == FrameReg) {
           TmpReg = RS->scavengeRegisterBackwards(AMDGPU::SReg_32_XM0RegClass,
-                                                 MI, false, 0);
+                                                 MI, /*RestoreAfter=*/false, 0,
+                                                 /*AllowSpill=*/false);
           DstReg = TmpReg;
         }