[clang] [llvm] [CMAKE] Enable FatLTO as a build option for LLVM (PR #80480)

[Paul Kirth via llvm-commits]

https://github.com/ilovepi updated https://github.com/llvm/llvm-project/pull/80480

>From 0c010db4bdf8808d95895db1ba4112fcb04f2d8b Mon Sep 17 00:00:00 2001
From: Paul Kirth <paulkirth at google.com>
Date: Tue, 22 Aug 2023 15:24:03 +0000
Subject: [PATCH 1/2] [CMAKE] Enable FatLTO as a build option for LLVM

---
 clang/cmake/caches/Fuchsia-stage2.cmake    |  1 +
 llvm/cmake/modules/AddLLVM.cmake           | 11 +++++++++--
 llvm/cmake/modules/HandleLLVMOptions.cmake |  6 ++++++
 3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/clang/cmake/caches/Fuchsia-stage2.cmake b/clang/cmake/caches/Fuchsia-stage2.cmake
index eee37c5e7901fa..d5a1662cbf4aa6 100644
--- a/clang/cmake/caches/Fuchsia-stage2.cmake
+++ b/clang/cmake/caches/Fuchsia-stage2.cmake
@@ -11,6 +11,7 @@ set(LLVM_ENABLE_RUNTIMES "compiler-rt;libcxx;libcxxabi;libunwind" CACHE STRING "
 
 set(LLVM_ENABLE_BACKTRACES OFF CACHE BOOL "")
 set(LLVM_ENABLE_DIA_SDK OFF CACHE BOOL "")
+set(LLVM_ENABLE_FATLTO ON CACHE BOOL "")
 set(LLVM_ENABLE_HTTPLIB ON CACHE BOOL "")
 set(LLVM_ENABLE_LIBCXX ON CACHE BOOL "")
 set(LLVM_ENABLE_LIBEDIT OFF CACHE BOOL "")
diff --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake
index 0f1734a64ee6e6..21455c304381bf 100644
--- a/llvm/cmake/modules/AddLLVM.cmake
+++ b/llvm/cmake/modules/AddLLVM.cmake
@@ -1621,8 +1621,15 @@ function(add_unittest test_suite test_name)
   # The runtime benefits of LTO don't outweight the compile time costs for tests.
   if(LLVM_ENABLE_LTO)
     if((UNIX OR MINGW) AND LINKER_IS_LLD)
-      set_property(TARGET ${test_name} APPEND_STRING PROPERTY
-                    LINK_FLAGS " -Wl,--lto-O0")
+      if(LLVM_ENABLE_FATLTO)
+        # When using FatLTO, just use relocatable linking.
+        set_property(TARGET ${test_name} APPEND_STRING PROPERTY
+                      LINK_FLAGS " -Wl,--no-fat-lto-objects")
+        set_property(TARGET ${test_name} APPEND_STRING PROPERTY COMPILE_FLAGS " -fno-lto")
+      else()
+        set_property(TARGET ${test_name} APPEND_STRING PROPERTY
+                      LINK_FLAGS " -Wl,--lto-O0")
+      endif()
     elseif(LINKER_IS_LLD_LINK)
       set_property(TARGET ${test_name} APPEND_STRING PROPERTY
                     LINK_FLAGS " /opt:lldlto=0")
diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
index 08ff49ded57a14..27b3ebf9a3466f 100644
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -32,6 +32,8 @@ endif()
 set(LLVM_ENABLE_LTO OFF CACHE STRING "Build LLVM with LTO. May be specified as Thin or Full to use a particular kind of LTO")
 string(TOUPPER "${LLVM_ENABLE_LTO}" uppercase_LLVM_ENABLE_LTO)
 
+option(LLVM_ENABLE_FATLTO "Build LLVM with -ffat-lto-objects." OFF)
+
 # Ninja Job Pool support
 # The following only works with the Ninja generator in CMake >= 3.0.
 set(LLVM_PARALLEL_COMPILE_JOBS "" CACHE STRING
@@ -1259,6 +1261,10 @@ elseif(LLVM_ENABLE_LTO)
   endif()
 endif()
 
+if(LLVM_ENABLE_FATLTO AND (FUCHSIA OR UNIX))
+    append("-ffat-lto-objects" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+endif()
+
 # Set an AIX default for LLVM_EXPORT_SYMBOLS_FOR_PLUGINS based on whether we are
 # doing dynamic linking (see below).
 set(LLVM_EXPORT_SYMBOLS_FOR_PLUGINS_AIX_default OFF)

>From 9c10222866c1c1f696f6ce48732adf120d7f1f39 Mon Sep 17 00:00:00 2001
From: Paul Kirth <paulkirth at google.com>
Date: Fri, 2 Feb 2024 13:09:18 -0800
Subject: [PATCH 2/2] Add -ffat-lto-objects to CMAKE_C_FLAGS and
 CMAKE_CXX_FLAGS

---
 llvm/cmake/modules/HandleLLVMOptions.cmake | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
index 27b3ebf9a3466f..e23d8d46272c4b 100644
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -1262,7 +1262,10 @@ elseif(LLVM_ENABLE_LTO)
 endif()
 
 if(LLVM_ENABLE_FATLTO AND (FUCHSIA OR UNIX))
+  append("-ffat-lto-objects" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+  if(NOT LINKER_IS_LLD_LINK)
     append("-ffat-lto-objects" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+  endif()
 endif()
 
 # Set an AIX default for LLVM_EXPORT_SYMBOLS_FOR_PLUGINS based on whether we are