From ac5c9632b0f8255ad3f1584a31bc7a0ce9b7457b Mon Sep 17 00:00:00 2001
From: Ali Mohammad Pur <ali.mpfard@gmail.com>
Date: Sun, 6 Apr 2025 19:50:28 +0200
Subject: [PATCH] Meta: Add a link job pool with a configurable size

Parallel linking (esp. with bfd) uses a huge chunk of memory, make it
possible for users to limit the number of link jobs to at least
limit the pain.
---
 Documentation/BuildInstructionsLadybird.md | 11 +++++++++++
 Meta/CMake/lagom_options.cmake             |  1 +
 Meta/CMake/use_linker.cmake                |  5 +++++
 3 files changed, 17 insertions(+)

diff --git a/Documentation/BuildInstructionsLadybird.md b/Documentation/BuildInstructionsLadybird.md
index 624280b8b97..97d8e679a95 100644
--- a/Documentation/BuildInstructionsLadybird.md
+++ b/Documentation/BuildInstructionsLadybird.md
@@ -279,6 +279,17 @@ cmake --build --preset default MyBuildDir
 ninja -C MyBuildDir run-ladybird
 ```
 
+### Building with limited system memory
+
+The default build mode will run as many build steps in parallel as possible, which includes link steps;
+this may be an issue for users with limited system memory (or users building with fat LTO in general).
+If you wish to reduce the number of parallel link jobs, you may use the LAGOM_LINK_POOL_SIZE cmake option
+to set a maximum limit for the number of parallel link jobs.
+
+```
+cmake --preset default -B MyBuildDir -DLAGOM_LINK_POOL_SIZE=2
+```
+
 ### Running manually
 
 The Meta/ladybird.sh script will execute the `run-ladybird` and `debug-ladybird` custom targets.
diff --git a/Meta/CMake/lagom_options.cmake b/Meta/CMake/lagom_options.cmake
index c7ad2d30337..aaeded5d60c 100644
--- a/Meta/CMake/lagom_options.cmake
+++ b/Meta/CMake/lagom_options.cmake
@@ -13,6 +13,7 @@ serenity_option(ENABLE_FUZZERS_OSSFUZZ OFF CACHE BOOL "Build OSS-Fuzz compatible
 serenity_option(LAGOM_TOOLS_ONLY OFF CACHE BOOL "Don't build libraries, utilities and tests, only host build tools")
 serenity_option(ENABLE_LAGOM_CCACHE ON CACHE BOOL "Enable ccache for Lagom builds")
 serenity_option(LAGOM_USE_LINKER "" CACHE STRING "The linker to use (e.g. lld, mold) instead of the system default")
+serenity_option(LAGOM_LINK_POOL_SIZE "" CACHE STRING "The maximum number of parallel jobs to use for linking")
 serenity_option(ENABLE_LAGOM_COVERAGE_COLLECTION OFF CACHE STRING "Enable code coverage instrumentation for lagom binaries in clang")
 
 if (ANDROID OR APPLE)
diff --git a/Meta/CMake/use_linker.cmake b/Meta/CMake/use_linker.cmake
index 8909c57c6a3..904fbe169c7 100644
--- a/Meta/CMake/use_linker.cmake
+++ b/Meta/CMake/use_linker.cmake
@@ -30,3 +30,8 @@ if (LAGOM_USE_LINKER)
         set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} ${LINKER_FLAG}")
     endif()
 endif()
+
+if (LAGOM_LINK_POOL_SIZE)
+    set_property(GLOBAL PROPERTY JOB_POOLS link_pool=${LAGOM_LINK_POOL_SIZE})
+    set(CMAKE_JOB_POOL_LINK link_pool CACHE STRING "Linking job pool")
+endif()