[libcxx-commits] [libcxx] [libcxxabi] [libcxx][cmake] Allow using FatLTO in libc++ builds (PR #112277)

[Paul Kirth via libcxx-commits]

https://github.com/ilovepi created https://github.com/llvm/llvm-project/pull/112277

We'd like to build libc++ using FatLTO (see https://llvm.org/docs/FatLTO.html
for details). This gives us more control over how libc++ can be consumed
by users of our toolchain, like the Fuchsia SDK.


>From 1dafa521d5a1e10e3f79f63a661b2e14acff5a4a Mon Sep 17 00:00:00 2001
From: Paul Kirth <paulkirth at google.com>
Date: Mon, 14 Oct 2024 15:06:38 -0700
Subject: [PATCH] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20initia?=
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Created using spr 1.3.4
---
 libcxx/CMakeLists.txt        |  4 ++++
 libcxx/src/CMakeLists.txt    | 10 ++++++++++
 libcxxabi/src/CMakeLists.txt | 10 ++++++++++
 3 files changed, 24 insertions(+)

diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index f1942e963ccc31..5a68237f7336c5 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -102,6 +102,10 @@ option(LIBCXX_ENABLE_WIDE_CHARACTERS
    support the C functionality for wide characters. When wide characters are
    not supported, several parts of the library will be disabled, notably the
    wide character specializations of std::basic_string." ON)
+ option(LIBCXX_ENABLE_FATLTO
+   "Whether to compile libc++ with FatLTO enabled." ON)
+ option(LIBCXX_ENABLE_LTO
+   "Whether to compile libc++ with LTO enabled." ON)
 
 # To use time zone support in libc++ the platform needs to have the IANA
 # database installed. Libc++ will fail to build if this is enabled on a
diff --git a/libcxx/src/CMakeLists.txt b/libcxx/src/CMakeLists.txt
index b187677ff2db52..670db758f53173 100644
--- a/libcxx/src/CMakeLists.txt
+++ b/libcxx/src/CMakeLists.txt
@@ -173,6 +173,16 @@ if (APPLE AND LLVM_USE_SANITIZER)
   endif()
 endif()
 
+
+if(LIBCXX_ENABLE_LTO)
+  list(APPEND LIBCXX_COMPILE_FLAGS "-flto")
+  list(APPEND LIBCXX_LINK_FLAGS "-flto")
+endif()
+if(LIBCXX_ENABLE_FATLTO)
+  list(APPEND LIBCXX_COMPILE_FLAGS "-ffat-lto-objects")
+  list(APPEND LIBCXX_LINK_FLAGS "-ffat-lto-objects")
+endif()
+
 split_list(LIBCXX_COMPILE_FLAGS)
 split_list(LIBCXX_LINK_FLAGS)
 
diff --git a/libcxxabi/src/CMakeLists.txt b/libcxxabi/src/CMakeLists.txt
index 480e528b819bb9..822ede39c6a525 100644
--- a/libcxxabi/src/CMakeLists.txt
+++ b/libcxxabi/src/CMakeLists.txt
@@ -143,6 +143,15 @@ if ( APPLE )
   endif()
 endif()
 
+if(LIBCXX_ENABLE_LTO)
+  list(APPEND LIBCXXABI_COMPILE_FLAGS "-flto")
+  list(APPEND LIBCXXABI_LINK_FLAGS "-flto")
+endif()
+if(LIBCXX_ENABLE_FATLTO)
+  list(APPEND LIBCXXABI_COMPILE_FLAGS "-ffat-lto-objects")
+  list(APPEND LIBCXXABI_LINK_FLAGS "-ffat-lto-objects")
+endif()
+
 split_list(LIBCXXABI_COMPILE_FLAGS)
 split_list(LIBCXXABI_LINK_FLAGS)
 
@@ -154,6 +163,7 @@ endif()
 
 include(WarningFlags)
 
+
 # Build the shared library.
 add_library(cxxabi_shared_objects OBJECT EXCLUDE_FROM_ALL ${LIBCXXABI_SOURCES} ${LIBCXXABI_HEADERS})
 cxx_add_warning_flags(cxxabi_shared_objects ${LIBCXXABI_ENABLE_WERROR} ${LIBCXXABI_ENABLE_PEDANTIC})