[llvm] [AMDGPU] Unused sdst writing to null (PR #133229)

[Jay Foad via llvm-commits]

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@@ -39,7 +39,8 @@ define amdgpu_ps void @v_add_u64(ptr addrspace(1) %out, i64 %a, i64 %b) {
 ; GCN-LABEL: v_add_u64:
 ; GCN:       ; %bb.0: ; %entry
 ; GCN-NEXT:    v_add_co_u32 v2, vcc_lo, v2, v4
-; GCN-NEXT:    v_add_co_ci_u32_e32 v3, vcc_lo, v3, v5, vcc_lo
+; GCN-NEXT:    s_delay_alu instid0(VALU_DEP_1)
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jayfoad wrote:

There are two things going on here:
1. In general `AMDGPUInsertDelayAlu` _should_ look at implicit ops, like you said.
2. But in this specific case of carry out -> carry in, the hardware has a fast forward that means there is no delay, so no `s_delay_alu` is needed.

https://github.com/llvm/llvm-project/pull/133229