[llvm-branch-commits] LiveRangeShrink: Early exit when encountering a code motion barrier. (PR #136806)

[via llvm-branch-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-regalloc

Author: Peter Collingbourne (pcc)

<details>
<summary>Changes</summary>

Without this, we end up with quadratic behavior affecting functions with
large numbers of code motion barriers, such as CFI jump tables.


---
Full diff: https://github.com/llvm/llvm-project/pull/136806.diff


1 Files Affected:

- (modified) llvm/lib/CodeGen/LiveRangeShrink.cpp (+12-3) 


``````````diff
diff --git a/llvm/lib/CodeGen/LiveRangeShrink.cpp b/llvm/lib/CodeGen/LiveRangeShrink.cpp
index dae7e14e54aae..46e837c086c5d 100644
--- a/llvm/lib/CodeGen/LiveRangeShrink.cpp
+++ b/llvm/lib/CodeGen/LiveRangeShrink.cpp
@@ -95,14 +95,24 @@ static MachineInstr *FindDominatedInstruction(MachineInstr &New,
   return Old;
 }
 
+static bool isCodeMotionBarrier(MachineInstr &MI) {
+  return MI.hasUnmodeledSideEffects() && !MI.isPseudoProbe();
+}
+
 /// Builds Instruction to its dominating order number map \p M by traversing
 /// from instruction \p Start.
 static void BuildInstOrderMap(MachineBasicBlock::iterator Start,
                               InstOrderMap &M) {
   M.clear();
   unsigned i = 0;
-  for (MachineInstr &I : make_range(Start, Start->getParent()->end()))
+  bool SawStore = false;
+  for (MachineInstr &I : make_range(Start, Start->getParent()->end())) {
+    if (I.mayStore())
+      SawStore = true;
+    if (!I.isSafeToMove(SawStore) && isCodeMotionBarrier(I))
+      break;
     M[&I] = i++;
+  }
 }
 
 bool LiveRangeShrink::runOnMachineFunction(MachineFunction &MF) {
@@ -166,8 +176,7 @@ bool LiveRangeShrink::runOnMachineFunction(MachineFunction &MF) {
         // If MI has side effects, it should become a barrier for code motion.
         // IOM is rebuild from the next instruction to prevent later
         // instructions from being moved before this MI.
-        if (MI.hasUnmodeledSideEffects() && !MI.isPseudoProbe() &&
-            Next != MBB.end()) {
+        if (isCodeMotionBarrier(MI) && Next != MBB.end()) {
           BuildInstOrderMap(Next, IOM);
           SawStore = false;
         }

``````````

</details>


https://github.com/llvm/llvm-project/pull/136806