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

[Ramkumar Ramachandra via llvm-commits]

================
@@ -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();
----------------
artagnon wrote:

Sorry, I'm having trouble understanding your example. Here's what I think I understand:

```
c = ...
a = ...
b = use a
d = use c
```

Here, if c has a default def latency of N cycles, a has that of M cycles, b can start after M - 1 cycles, and d can start after N - 3 cycles, assuming issue-width = num-micro-ops = 1. What do you mean by "instructions that depends on each other"? How can one instruction depend on another, which in turn depends on the first? Wouldn't this break basic dominance criteria? Also, if I understand correctly, I think we're in SSA form at this point, so we don't have to worry about re-definitions.

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