[llvm] [LiveDebugValues] Generalize approach to find stack units, NFC (PR #124862)

[Emma Pilkington via llvm-commits]

https://github.com/epilk created https://github.com/llvm/llvm-project/pull/124862

On x86_64 this produces an identical set of stack units, however on other targets (e.g. AMDGPU) this can significantly improve compile time performance. Fixes SWDEV-508735.



>From 31b027d507a629e2f315a2f2cf18ddd808924472 Mon Sep 17 00:00:00 2001
From: Emma Pilkington <emma.pilkington95 at gmail.com>
Date: Tue, 28 Jan 2025 18:11:49 -0500
Subject: [PATCH] [LiveDebugValues] Generalize approach to find stack units,
 NFC

On x86_64 this produces an identical set of stack units, however on
other targets (e.g. AMDGPU) this can significantly improve compile time
performance. Fixes SWDEV-508735.
---
 .../LiveDebugValues/InstrRefBasedImpl.cpp     | 44 ++++++++++---------
 1 file changed, 23 insertions(+), 21 deletions(-)

diff --git a/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp b/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
index 2510b77c6d5be4..bd8d64fbb6c50f 100644
--- a/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
+++ b/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
@@ -2514,27 +2514,29 @@ bool InstrRefBasedLDV::mlocJoin(
 
 void InstrRefBasedLDV::findStackIndexInterference(
     SmallVectorImpl<unsigned> &Slots) {
-  // We could spend a bit of time finding the exact, minimal, set of stack
-  // indexes that interfere with each other, much like reg units. Or, we can
-  // rely on the fact that:
-  //  * The smallest / lowest index will interfere with everything at zero
-  //    offset, which will be the largest set of registers,
-  //  * Most indexes with non-zero offset will end up being interference units
-  //    anyway.
-  // So just pick those out and return them.
-
-  // We can rely on a single-byte stack index existing already, because we
-  // initialize them in MLocTracker.
-  auto It = MTracker->StackSlotIdxes.find({8, 0});
-  assert(It != MTracker->StackSlotIdxes.end());
-  Slots.push_back(It->second);
-
-  // Find anything that has a non-zero offset and add that too.
-  for (auto &Pair : MTracker->StackSlotIdxes) {
-    // Is offset zero? If so, ignore.
-    if (!Pair.first.second)
-      continue;
-    Slots.push_back(Pair.second);
+  // Find the exact, minimal, set of stack indexes that interfere with each
+  // other, much like reg units.
+  SmallVector<std::pair<MLocTracker::StackSlotPos, unsigned>> AllSlotsBySize(
+      MTracker->StackSlotIdxes.begin(), MTracker->StackSlotIdxes.end());
+  std::sort(AllSlotsBySize.begin(), AllSlotsBySize.end(),
+            [](auto L, auto R) { return L.first.first < R.first.first; });
+
+  BitVector SlotCoverage, ThisSlot;
+  for (auto Pair : AllSlotsBySize) {
+    unsigned Start = Pair.first.second;
+    unsigned End = Start + Pair.first.first;
+    if (End > SlotCoverage.size()) {
+      SlotCoverage.resize(End, false);
+      ThisSlot.resize(End, false);
+    }
+
+    ThisSlot.reset();
+    ThisSlot.set(Start, End);
+
+    if (!SlotCoverage.anyCommon(ThisSlot)) {
+      SlotCoverage |= ThisSlot;
+      Slots.push_back(Pair.second);
+    }
   }
 }