[llvm] [AMDGPU] Convert SVS form scratch frame index fallback into SV form (PR #219239)

[Janek van Oirschot via llvm-commits]

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@@ -3285,12 +3287,31 @@ bool SIRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator MI,
           auto [ImmOffset, RemainderOffset] =
               TII->splitFlatOffset(FullOffset, AMDGPUAS::PRIVATE_ADDRESS,
                                    AMDGPU::FlatAddrSpace::FlatScratch);
-          BuildMI(*MBB, MI, DL, TII->get(AMDGPU::V_MOV_B32_e32), TmpVGPR)
-              .addImm(RemainderOffset);
+
+          Register UsedVAddr;
+          if (MachineOperand *VAddr =
+                  TII->getNamedOperand(*MI, AMDGPU::OpName::vaddr)) {
+            // SVS form: add RemainderOffset to vaddr.
+            Register Src = VAddr->getReg();
+            Register Dst = VAddr->isKill() ? Src
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JanekvO wrote:

AFAIU, if there is vgpr aliasing, the `killed` attribute is on the first occurrence of the vgpr in the instruction but it definitely is possible to reproduce with handcrafted mir so I did add something for it (+ test). Thank for the catch!

https://github.com/llvm/llvm-project/pull/219239