[llvm] [AMDGPU] CodeGen for GFX12 8/16-bit SMEM loads (PR #77633)

[Jay Foad via llvm-commits]

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@@ -5894,6 +5894,56 @@ void SITargetLowering::ReplaceNodeResults(SDNode *N,
       }
       return;
     }
+    case Intrinsic::amdgcn_s_buffer_load: {
+      // Lower llvm.amdgcn.s.buffer.load.(i8, u8) intrinsics. First, we generate
+      // s_buffer_load_u8 for signed and unsigned load instructions. Next, DAG
+      // combiner tries to merge the s_buffer_load_u8 with a sext instruction
+      // (performSignExtendInRegCombine()) and it replaces s_buffer_load_u8 with
+      // s_buffer_load_i8.
+      assert(Subtarget->hasScalarSubwordLoads() &&
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jayfoad wrote:

It's a failure to legalize, not a failure to select, since here we're custom promoting the illegal result type i8. But OK, it's still better than an assertion failure. Done.

https://github.com/llvm/llvm-project/pull/77633