[llvm] [llvm][GVNSink] Avoid non-determistic iteration order over NeededPHIs (PR #205952)

[Paul Kirth via llvm-commits]

https://github.com/ilovepi created https://github.com/llvm/llvm-project/pull/205952

The iteration order of DenseSet is not guaranteed, which affects the
output of code generated with GVNSink enabled. This can cause code to be
emitted in differing order, affect section ordering and in some cases
was reported to sometimes result in larger binaries due to increased
padding between sections.

This patch addresses this by using SetVector, which has a deterministic
iteration order.

>From 54935858a4ddce8d2fe1ccaad6c2f1529e4df5c5 Mon Sep 17 00:00:00 2001
From: Paul Kirth <paulkirth at google.com>
Date: Thu, 25 Jun 2026 23:41:41 +0000
Subject: [PATCH] [llvm][GVNSink] Avoid non-determistic iteration order over
 NeededPHIs

The iteration order of DenseSet is not guaranteed, which affects the
output of code generated with GVNSink enabled. This can cause code to be
emitted in differing order, affect section ordering and in some cases
was reported to sometimes result in larger binaries due to increased
padding between sections.

This patch addresses this by using SetVector, which has a deterministic
iteration order.
---
 llvm/lib/Transforms/Scalar/GVNSink.cpp | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Transforms/Scalar/GVNSink.cpp b/llvm/lib/Transforms/Scalar/GVNSink.cpp
index 95e0f7d9a6b60..67196ef9715f1 100644
--- a/llvm/lib/Transforms/Scalar/GVNSink.cpp
+++ b/llvm/lib/Transforms/Scalar/GVNSink.cpp
@@ -35,10 +35,10 @@
 
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseMap.h"
-#include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/Hashing.h"
 #include "llvm/ADT/PostOrderIterator.h"
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
@@ -262,7 +262,7 @@ template <> struct llvm::DenseMapInfo<ModelledPHI> {
   }
 };
 
-using ModelledPHISet = DenseSet<ModelledPHI>;
+using ModelledPHISet = SetVector<ModelledPHI>;
 
 namespace {
 
@@ -645,7 +645,7 @@ GVNSink::analyzeInstructionForSinking(LockstepReverseIterator<false> &LRI,
   ModelledPHI NewPHI(NewInsts, ActivePreds, RPOTOrder);
 
   // Does sinking this instruction render previous PHIs redundant?
-  if (NeededPHIs.erase(NewPHI))
+  if (NeededPHIs.remove(NewPHI))
     RecomputePHIContents = true;
 
   if (RecomputePHIContents) {
@@ -693,7 +693,6 @@ GVNSink::analyzeInstructionForSinking(LockstepReverseIterator<false> &LRI,
         PHI.areAnyIncomingValuesConstant())
       return std::nullopt;
 
-    NeededPHIs.reserve(NeededPHIs.size());
     NeededPHIs.insert(PHI);
     PHIContents.insert_range(PHI.getValues());
   }