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

[Michael Maitland via llvm-commits]

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@@ -761,6 +762,64 @@ static void updatePhysDepsDownwards(const MachineInstr *UseMI,
   }
 }
 
+/// Estimates the number of cycles elapsed between DefMI and UseMI 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;
+  const MachineBasicBlock *ParentBB = DefMI->getParent();
+  if (ParentBB != UseMI->getParent())
+    return std::nullopt;
+
+  const auto DefIt =
+      llvm::find_if(ParentBB->instrs(),
+                    [DefMI](const MachineInstr &MI) { return DefMI == &MI; });
+  const auto UseIt =
+      llvm::find_if(ParentBB->instrs(),
+                    [UseMI](const MachineInstr &MI) { return UseMI == &MI; });
+
+  unsigned NumMicroOps = 0;
+  for (auto It = DefIt; It != UseIt; ++It) {
+    // In cases where the UseMI is a PHI at the beginning of the MBB, compute
+    // MicroOps until the end of the MBB.
+    if (It.isEnd())
+      break;
+
+    NumMicroOps += Sched.getNumMicroOps(&*It);
+  }
+  return NumMicroOps / Sched.getIssueWidth();
----------------
michaelmaitland wrote:

I wonder if it would be useful to use `defaultDefLatency` for the instructions we are iterating over here in the case that some of the instructions we are iterating over depend on eachother. Imagine a scenario:

```
defmi = ...
a = ...
b = use a
usemi = ...
```
In this case, what if we checked defaultDefLatency of a and used it to understand the number of cycles elapsed between a and b? For example, if `a` has a default latency of 10, then b can't really start in the next `NumMicroOps / IssueWidth` cycles, since it has to wait 10 additional cycles.

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