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

[Ramkumar Ramachandra via llvm-commits]

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@@ -761,6 +762,46 @@ static void updatePhysDepsDownwards(const MachineInstr *UseMI,
   }
 }
 
+/// Returns the distance between DefMI and UseMI if they're non-null and in the
+/// same BasicBlock, 0 otherwise.
+static unsigned computeDefUseDist(const MachineInstr *DefMI,
+                                  const MachineInstr *UseMI) {
+  if (!DefMI || !UseMI || DefMI == UseMI)
+    return 0;
+  const MachineBasicBlock *ParentBB = DefMI->getParent();
+  if (ParentBB != UseMI->getParent())
+    return 0;
+  auto DefIt = llvm::find_if(
+      *ParentBB, [DefMI](const MachineInstr &MI) { return DefMI == &MI; });
+  auto UseIt = llvm::find_if(
+      *ParentBB, [UseMI](const MachineInstr &MI) { return UseMI == &MI; });
+  return std::distance(DefIt, UseIt);
+}
+
+/// Wraps Sched.computeOperandLatency, accounting for the case when
+/// InstrSchedModel and InstrItineraries are not available: in this case,
+/// Sched.computeOperandLatency returns DefaultDefLatency, which is a very rough
+/// approximate; to improve this approximate, offset it by the approximate
+/// cycles elapsed from DefMI to UseMI (since the MIs could be re-ordered by the
+/// scheduler, and we don't have this information, this distance cannot be known
+/// exactly). When scheduling information is available,
+/// Sched.computeOperandLatency returns a much better estimate (especially if
+/// UseMI is non-null), so we just return that.
+static unsigned computeOperandLatency(const TargetSchedModel &Sched,
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artagnon wrote:

Doesn't the IssueWidth tell us how many micro-ops will be issued per cycle? How do we know the micro-ops from the MI?

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