[llvm] MTM: improve operand latency when missing sched info (PR #101389)

[Ramkumar Ramachandra via llvm-commits]

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@@ -761,6 +762,60 @@ static void updatePhysDepsDownwards(const MachineInstr *UseMI,
   }
 }
 
+/// Estimates the number of cycles elapsed between DefMI and UseMI, DefMI
+/// inclusive and UseMI exclusive, if they're non-null and in the same
+/// BasicBlock. Returns std::nullopt when UseMI is in a different MBB than
+/// DefMI.
+static std::optional<unsigned>
+estimateDefUseCycles(const TargetSchedModel &Sched, const MachineInstr *DefMI,
+                     const MachineInstr *UseMI) {
+  if (!DefMI || !UseMI || DefMI == UseMI)
+    return 0;
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artagnon wrote:

The UseMI is missing in many call sites, and I've only included `DefMI == UseMI` for completeness, to independently reason about the function.

https://github.com/llvm/llvm-project/pull/101389