[SROA] Use stable sort for slices to avoid non-determinism

[henke9600]

Found this while trying to build a LLVM toolchain reproducibly from both Debian 12 and FreeBSD 14. With these changes they come out bit-by-bit identical.

Previously there was a mix of stable and unstable sorts for slices, now only stable sorts are used.

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@llvm/pr-subscribers-llvm-transforms

Author: None (henke9600)

Changes

Found this while trying to build a LLVM toolchain reproducibly from both Debian 12 and FreeBSD 14. With these changes they come out bit-by-bit identical.

Previously there was a mix of stable and unstable sorts for slices, now only stable sorts are used.

---

Full diff: https://github.com/llvm/llvm-project/pull/91609.diff

1 Files Affected:

• (modified) llvm/lib/Transforms/Scalar/SROA.cpp (+2-5)

diff --git a/llvm/lib/Transforms/Scalar/SROA.cpp b/llvm/lib/Transforms/Scalar/SROA.cpp
index 096c6d1b1fad2..857cf6b77ea20 100644
--- a/llvm/lib/Transforms/Scalar/SROA.cpp
+++ b/llvm/lib/Transforms/Scalar/SROA.cpp
@@ -627,11 +627,8 @@ class AllocaSlices {
   /// everything so that the usual ordering properties of the alloca's slices
   /// hold.
   void insert(ArrayRef<Slice> NewSlices) {
-    int OldSize = Slices.size();
     Slices.append(NewSlices.begin(), NewSlices.end());
-    auto SliceI = Slices.begin() + OldSize;
-    llvm::sort(SliceI, Slices.end());
-    std::inplace_merge(Slices.begin(), SliceI, Slices.end());
+    llvm::stable_sort(Slices);
   }
 
   // Forward declare the iterator and range accessor for walking the
@@ -5122,7 +5119,7 @@ bool SROA::splitAlloca(AllocaInst &AI, AllocaSlices &AS) {
   }
 
   if (!IsSorted)
-    llvm::sort(AS);
+    llvm::stable_sort(AS);
 
   /// Describes the allocas introduced by rewritePartition in order to migrate
   /// the debug info.

[nikic]

LGTM

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