[DebugInfo][InstrRef] Avoid some quadratic behaviour in LiveDebugVariables

This is a performance patch -- LiveDebugVariables can behave quadratically
if a lot of debug instructions are inserted back into the same place, and
we have to repeatedly step-over hte ones we've already inserted.

To get around it, whenever we insert a debug instruction at a slot index,
check whether there are more debug instructions to insert at this point,
and insert them too. That avoids the repeated lookup and stepping through.
It relies on the container for unlinked debug instructions being recorded
in-order, which is how LiveDebugVariables currently does it.

Differential Revision: https://reviews.llvm.org/D114587

1 files changed, 37 insertions, 14 deletions

diff --git a/llvm/lib/CodeGen/LiveDebugVariables.cpp b/llvm/lib/CodeGen/LiveDebugVariables.cpp
index dcd546f..5f976bf 100644
--- a/llvm/lib/CodeGen/LiveDebugVariables.cpp
+++ b/llvm/lib/CodeGen/LiveDebugVariables.cpp
@@ -1875,34 +1875,57 @@ void LDVImpl::emitDebugValues(VirtRegMap *VRM) {
 
   LLVM_DEBUG(dbgs() << "********** EMITTING INSTR REFERENCES **********\n");
 
-  // Re-insert any debug instrs back in the position they were. Ordering
-  // is preserved by vector. We must re-insert in the same order to ensure that
-  // debug instructions don't swap, which could re-order assignments.
-  for (auto &P : StashedDebugInstrs) {
-    SlotIndex Idx = P.Idx;
+  // Re-insert any debug instrs back in the position they were. We must
+  // re-insert in the same order to ensure that debug instructions don't swap,
+  // which could re-order assignments. Do so in a batch -- once we find the
+  // insert position, insert all instructions at the same SlotIdx. They are
+  // guaranteed to appear in-sequence in StashedDebugInstrs because we insert
+  // them in order.
+  for (auto StashIt = StashedDebugInstrs.begin();
+       StashIt != StashedDebugInstrs.end(); ++StashIt) {
+    SlotIndex Idx = StashIt->Idx;
+    MachineBasicBlock *MBB = StashIt->MBB;
+    MachineInstr *MI = StashIt->MI;
+
+    auto EmitInstsHere = [this, &StashIt, MBB, Idx,
+                          MI](MachineBasicBlock::iterator InsertPos) {
+      // Insert this debug instruction.
+      MBB->insert(InsertPos, MI);
+
+      // Look at subsequent stashed debug instructions: if they're at the same
+      // index, insert those too.
+      auto NextItem = std::next(StashIt);
+      while (NextItem != StashedDebugInstrs.end() && NextItem->Idx == Idx) {
+        assert(NextItem->MBB == MBB && "Instrs with same slot index should be"
+               "in the same block");
+        MBB->insert(InsertPos, NextItem->MI);
+        StashIt = NextItem;
+        NextItem = std::next(StashIt);
+      };
+    };
 
     // Start block index: find the first non-debug instr in the block, and
     // insert before it.
-    if (Idx == Slots->getMBBStartIdx(P.MBB)) {
+    if (Idx == Slots->getMBBStartIdx(MBB)) {
       MachineBasicBlock::iterator InsertPos =
-          findInsertLocation(P.MBB, Idx, *LIS, BBSkipInstsMap);
-      P.MBB->insert(InsertPos, P.MI);
+          findInsertLocation(MBB, Idx, *LIS, BBSkipInstsMap);
+      EmitInstsHere(InsertPos);
       continue;
     }
 
     if (MachineInstr *Pos = Slots->getInstructionFromIndex(Idx)) {
       // Insert at the end of any debug instructions.
       auto PostDebug = std::next(Pos->getIterator());
-      PostDebug = skipDebugInstructionsForward(PostDebug, P.MBB->instr_end());
-      P.MBB->insert(PostDebug, P.MI);
+      PostDebug = skipDebugInstructionsForward(PostDebug, MBB->instr_end());
+      EmitInstsHere(PostDebug);
     } else {
       // Insert position disappeared; walk forwards through slots until we
       // find a new one.
-      SlotIndex End = Slots->getMBBEndIdx(P.MBB);
+      SlotIndex End = Slots->getMBBEndIdx(MBB);
       for (; Idx < End; Idx = Slots->getNextNonNullIndex(Idx)) {
         Pos = Slots->getInstructionFromIndex(Idx);
         if (Pos) {
-          P.MBB->insert(Pos->getIterator(), P.MI);
+          EmitInstsHere(Pos->getIterator());
           break;
         }
       }
@@ -1911,8 +1934,8 @@ void LDVImpl::emitDebugValues(VirtRegMap *VRM) {
       // insert! It's not safe to discard any debug instructions; place them
       // in front of the first terminator, or in front of end().
       if (Idx >= End) {
-        auto TermIt = P.MBB->getFirstTerminator();
-        P.MBB->insert(TermIt, P.MI);
+        auto TermIt = MBB->getFirstTerminator();
+        EmitInstsHere(TermIt);
       }
     }
   }