From e42a7f9f7ad5458262a66832a2e0f2444c8b6e75 Mon Sep 17 00:00:00 2001
From: Rob Vermaas <rob.vermaas@relational.ai>
Date: Thu, 30 Jul 2026 13:07:33 +0200
Subject: [PATCH] Restore AllocOptPass in the -O1 scalar optimizer pipeline
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Backport the `else if (O.getSpeedupLevel() >= 1)` branch of
buildScalarOptimizerPipeline that upstream added in JuliaLang/julia#52850
("Add passes to -O1 pipeline to reduce allocations in reinterpret").

That branch was carried to release-1.10 (#57731) and release-1.11 (#57732)
on 2025-03-11, but never landed on release-1.12 — it is present on master,
release-1.13 and v1.11.9, and absent on release-1.12, backports-release-1.12
and v1.12.6. We could not find a revert or a rationale for the omission.

Without AllocOptPass at -O1, a `Ref` holding an immutable value which never
escapes and is used only via `pointer_from_objref` is heap-allocated:

    julia 1.10.11  -O1  @allocated = 0
    julia 1.11.9   -O1  @allocated = 0
    julia 1.12.6   -O1  @allocated = 32   <-- regression
    julia 1.12.6   -O2  @allocated = 0

raicode runs its test suite at -O1 and asserts that several hot paths
(TypeNormalization's byte-string hashing and fast_reinterpret, among
others) allocate nothing, so -O1 no longer gives a usable "this code does
not allocate" signal. Production builds run at -O2/-O3 and are unaffected.

The added pass list is byte-identical to v1.11.9's. All six passes are
already used elsewhere in this file and their headers are already included,
so this adds no new dependencies.
---
 src/pipeline.cpp | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/src/pipeline.cpp b/src/pipeline.cpp
index 39f896ba656d2..eb7fa684c5c74 100644
--- a/src/pipeline.cpp
+++ b/src/pipeline.cpp
@@ -465,6 +465,19 @@ static void buildScalarOptimizerPipeline(FunctionPassManager &FPM, PassBuilder *
             FPM.addPass(IRCEPass());
             FPM.addPass(InstCombinePass());
             FPM.addPass(JumpThreadingPass());
+        } else if (O.getSpeedupLevel() >= 1) {
+            // RAI: backport of the -O1 branch added upstream by JuliaLang/julia#52850,
+            // which was carried to release-1.10 (#57731) and release-1.11 (#57732) but
+            // never to release-1.12. Without AllocOptPass here, a non-escaping Ref that
+            // is only used via pointer_from_objref is heap-allocated at -O1, while it is
+            // elided on 1.10/1.11 and at -O2+ on every version. Our test suite runs at
+            // -O1 and asserts such code is allocation-free.
+            JULIA_PASS(FPM.addPass(AllocOptPass()));
+            FPM.addPass(SROAPass(SROAOptions::ModifyCFG));
+            FPM.addPass(MemCpyOptPass());
+            FPM.addPass(SCCPPass());
+            FPM.addPass(InstCombinePass());
+            FPM.addPass(ADCEPass());
         }
         if (O.getSpeedupLevel() >= 3) {
             FPM.addPass(GVNPass());