[llvm] [DWARFLinker] Move __debug_str to the last section (PR #163603)

[Peter Rong via llvm-commits]

https://github.com/DataCorrupted created https://github.com/llvm/llvm-project/pull/163603

Context: MachO defined section offset to be a 32-bit integer, meaning that each section must start within 4GB of the segment.

https://github.com/llvm/llvm-project/blob/c4d7c42b1de2117f104e202ac0f4b808bce4d28b/llvm/include/llvm/BinaryFormat/MachO.h#L580-L593

Problem: With larger and larger dSYM we are running out of space. In our internal builds, __debug_str and __debug_line are the two biggest section each having more than 1GB, combined they pushed the last few sections out of the 4GB range.

This diff tries to mitigate that by moving one of the biggest sections to the last, buying our time to find a more permeant solutions

>From 82884a35eab4b80b85358034fb1c2f2fc5b961f2 Mon Sep 17 00:00:00 2001
From: Peter Rong <PeterRong at meta.com>
Date: Wed, 15 Oct 2025 10:39:04 -0700
Subject: [PATCH] [DWARFLinker] Move __debug_str to the last section

Signed-off-by: Peter Rong <PeterRong at meta.com>
---
 llvm/lib/DWARFLinker/Classic/DWARFLinker.cpp  |  2 +-
 .../dsymutil/X86/swift-ast-section-order.test | 19 +++++++++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/tools/dsymutil/X86/swift-ast-section-order.test

diff --git a/llvm/lib/DWARFLinker/Classic/DWARFLinker.cpp b/llvm/lib/DWARFLinker/Classic/DWARFLinker.cpp
index 8052773812a2c..ab7cd4d8d6273 100644
--- a/llvm/lib/DWARFLinker/Classic/DWARFLinker.cpp
+++ b/llvm/lib/DWARFLinker/Classic/DWARFLinker.cpp
@@ -3129,7 +3129,6 @@ Error DWARFLinker::link() {
     // Emit everything that's global.
     if (TheDwarfEmitter != nullptr) {
       TheDwarfEmitter->emitAbbrevs(Abbreviations, Options.TargetDWARFVersion);
-      TheDwarfEmitter->emitStrings(DebugStrPool);
       TheDwarfEmitter->emitStringOffsets(StringOffsetPool.getValues(),
                                          Options.TargetDWARFVersion);
       TheDwarfEmitter->emitLineStrings(DebugLineStrPool);
@@ -3150,6 +3149,7 @@ Error DWARFLinker::link() {
           break;
         }
       }
+      TheDwarfEmitter->emitStrings(DebugStrPool);
     }
   };
 
diff --git a/llvm/test/tools/dsymutil/X86/swift-ast-section-order.test b/llvm/test/tools/dsymutil/X86/swift-ast-section-order.test
new file mode 100644
index 0000000000000..9d6818220c9d4
--- /dev/null
+++ b/llvm/test/tools/dsymutil/X86/swift-ast-section-order.test
@@ -0,0 +1,19 @@
+# Check that the __debug_str section is last in the output
+
+# RUN: rm -rf %t && mkdir %t
+# RUN: dsymutil -oso-prepend-path %p/.. %p/../Inputs/swift-ast.macho.x86_64 -o %t/swift-ast.dSYM -verbose -no-swiftmodule-timestamp
+# RUN: llvm-dwarfdump --show-section-sizes %t/swift-ast.dSYM/Contents/Resources/DWARF/swift-ast.macho.x86_64 | FileCheck %s
+
+# This test verifies that the __debug_str section appears last in the section list.
+# This is important for compatibility with tools that expect this ordering.
+# The ordering should be consistent regardless of whether the classic or parallel linker is used.
+
+# CHECK: SECTION
+# CHECK: __swift_ast
+# CHECK: __debug_abbrev
+# CHECK: __apple_namespac
+# CHECK: __apple_names
+# CHECK: __apple_types
+# CHECK: __apple_objc
+# CHECK-NOT: __apple
+# CHECK: __debug_str