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

[Matt Arsenault via llvm-commits]

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@@ -865,6 +865,42 @@ void GCNSubtarget::adjustSchedDependency(
     Dep.setLatency(InstrInfo.getSchedModel().computeOperandLatency(
         DefI, DefOpIdx, UseI, UseOpIdx));
   }
+
+  // Handle "fast-forward" cases for VALU dependencies via a lanemask-like SGPR:
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arsenm wrote:

Don't we have nearly identical code in the scheduler DAG mutation?

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