[llvm] [AMDGPU] Fix negative immediate offset for unbuffered smem loads (PR #79553)

[Matt Arsenault via llvm-commits]

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@@ -2096,21 +2099,47 @@ bool AMDGPUDAGToDAGISel::SelectSMRDBaseOffset(SDValue Addr, SDValue &SBase,
   }
   if (!N0 || !N1)
     return false;
+
+  bool Selected = false;
   if (SelectSMRDOffset(N1, SOffset, Offset, Imm32Only, IsBuffer)) {
     SBase = N0;
-    return true;
-  }
-  if (SelectSMRDOffset(N0, SOffset, Offset, Imm32Only, IsBuffer)) {
+    Selected = true;
+  } else if (SelectSMRDOffset(N0, SOffset, Offset, Imm32Only, IsBuffer)) {
     SBase = N1;
+    Selected = true;
+  }
+
+  if (Selected) {
+    // For unbuffered smem loads, it is illegal and undefined for the Immediate
+    // Offset to be negative if the resulting (Offset + (M0 or SOffset or zero)
+    // is negative. Handle the case where the Immediate Offset is negative and
+    // there is no SOffset.
+    //
+    // FIXME: Also handle M0 or SOffset case?
----------------
arsenm wrote:

Not sure what the fixme is about; I also doubt the rules are different if it's m0 or a numbered SGPR 

https://github.com/llvm/llvm-project/pull/79553