[llvm-branch-commits] [llvm] [LoopUnroll] Use MapVector for deterministic iteration order. (#188821) (PR #219380)

[Bernhard M. Wiedemann via llvm-branch-commits]

https://github.com/bmwiedemann created https://github.com/llvm/llvm-project/pull/219380

_note: this backport helps reproducible builds of openSUSE's `cosmic-bg` package and likely other rust software as well._  @efriedma-quic @fhahn 

NonLoopBlocksIDom determines the order we adjust the DT, impacting DRT child ordering.

Similarly, ExitInfos order impacts the order of SetDest, which applies DT updates.

The order impacts collectChildrenInLoop, which in turn impacts the we process BBs in LICM. And this in turn impacts the order in which we perform alias queries.

If the order is not deterministic, we sometimes get different NumNoAlias statistic counts for the same input, and possibly also different IR, although I have not confirmed the latter.

Compile-time impact is neutral:
https://llvm-compile-time-tracker.com/compare.php?from=77710f14202a8684e12d68d08d2cd3738bbd0e1d&to=066ac6284041aac454c8ffa2894b866acbf96fd3&stat=instructions:u

On interesting change to highlight for stage1-O3, although this is probably just noise : ClamAV 55233M 55190M (-0.08%)

PR: https://github.com/llvm/llvm-project/pull/188821
(cherry picked from commit 965f9d87adb0a7376454374fbc140ab69bd796a9)

>From 0aa219896a34269dce6b3f638222b4bc23678a03 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Fri, 27 Mar 2026 13:02:51 +0000
Subject: [PATCH] [LoopUnroll] Use MapVector for deterministic iteration order.
 (#188821)

NonLoopBlocksIDom determines the order we adjust the DT, impacting DRT
child ordering.

Similarly, ExitInfos order impacts the order of SetDest, which applies
DT updates.

The order impacts collectChildrenInLoop, which in turn impacts the we
process BBs in LICM. And this in turn impacts the order in which we
perform alias queries.

If the order is not deterministic, we sometimes get different NumNoAlias
statistic counts for the same input, and possibly also different IR,
although I have not confirmed the latter.

Compile-time impact is neutral:
https://llvm-compile-time-tracker.com/compare.php?from=77710f14202a8684e12d68d08d2cd3738bbd0e1d&to=066ac6284041aac454c8ffa2894b866acbf96fd3&stat=instructions:u

On interesting change to highlight for stage1-O3, although this is
probably just noise : ClamAV 55233M 55190M (-0.08%)

PR: https://github.com/llvm/llvm-project/pull/188821
(cherry picked from commit 965f9d87adb0a7376454374fbc140ab69bd796a9)
---
 llvm/lib/Transforms/Utils/LoopPeel.cpp   | 3 ++-
 llvm/lib/Transforms/Utils/LoopUnroll.cpp | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/Utils/LoopPeel.cpp b/llvm/lib/Transforms/Utils/LoopPeel.cpp
index 960ec9d4c7d6e..cf99baa6f0ed5 100644
--- a/llvm/lib/Transforms/Utils/LoopPeel.cpp
+++ b/llvm/lib/Transforms/Utils/LoopPeel.cpp
@@ -11,6 +11,7 @@
 
 #include "llvm/Transforms/Utils/LoopPeel.h"
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/Loads.h"
@@ -1121,7 +1122,7 @@ bool llvm::peelLoop(Loop *L, unsigned PeelCount, bool PeelLast, LoopInfo *LI,
   // later. Immediate dominator of such block might change, because we add more
   // routes which can lead to the exit: we can reach it from the peeled
   // iterations too.
-  DenseMap<BasicBlock *, BasicBlock *> NonLoopBlocksIDom;
+  MapVector<BasicBlock *, BasicBlock *> NonLoopBlocksIDom;
   for (auto *BB : L->blocks()) {
     auto *BBDomNode = DT.getNode(BB);
     SmallVector<BasicBlock *, 16> ChildrenToUpdate;
diff --git a/llvm/lib/Transforms/Utils/LoopUnroll.cpp b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
index 0f256398e5b1e..8eb13a9fff526 100644
--- a/llvm/lib/Transforms/Utils/LoopUnroll.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
@@ -17,6 +17,7 @@
 
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/ScopedHashTable.h"
 #include "llvm/ADT/SetVector.h"
@@ -515,7 +516,7 @@ llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
     BasicBlock *FirstExitingBlock = nullptr;
     SmallVector<BasicBlock *> ExitingBlocks;
   };
-  DenseMap<BasicBlock *, ExitInfo> ExitInfos;
+  MapVector<BasicBlock *, ExitInfo> ExitInfos;
   SmallVector<BasicBlock *, 4> ExitingBlocks;
   L->getExitingBlocks(ExitingBlocks);
   for (auto *ExitingBlock : ExitingBlocks) {