[llvm] [AMDGPU] Tweak sched model latencies for VALU fast-forward cases (PR #198475)

[Jay Foad via llvm-commits]

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@@ -830,6 +830,22 @@ void GCNSubtarget::adjustSchedDependency(
     Dep.setLatency(InstrInfo.getSchedModel().computeOperandLatency(
         DefI, DefOpIdx, UseI, UseOpIdx));
   }
+
+  // Handle "fast-forward" cases for VALU dependencies via a lanemask-like SGPR:
+  //   V_CMP* -> V_CNDMASK_B32
+  //   V_ADD/SUB with carryout -> V_ADD/SUB with carryin (on GFX10+)
+  switch (UseI->getOpcode()) {
+  case AMDGPU::V_ADDC_U32_e64:
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jayfoad wrote:

There are also _dpp and _sdwa forms, but I'm not sure it's really worth handling those too? It is a pain to list them all and the returns would be diminishing. Is there some simple way to query the "base" form of an opcode independent of encoding?

https://github.com/llvm/llvm-project/pull/198475