[libc-commits] [lldb] [llvm] [lld] [clang] [libcxx] [flang] [libc] [CMAKE] Enable FatLTO as a build option for LLVM (PR #80480)

[via libc-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Paul Kirth (ilovepi)

<details>
<summary>Changes</summary>

Since LLVM supports `-ffat-lto-objects` we should enable this as an option in the LLVM build. FatLTO should improve the time it takes to build tests for LTO enabled builds of the compiler by not linking w/ the bitcode portion of the object files, which should speed up build times for LTO builds without disabling optimizations.

---
Full diff: https://github.com/llvm/llvm-project/pull/80480.diff


3 Files Affected:

- (modified) clang/cmake/caches/Fuchsia-stage2.cmake (+1) 
- (modified) llvm/cmake/modules/AddLLVM.cmake (+9-2) 
- (modified) llvm/cmake/modules/HandleLLVMOptions.cmake (+6) 


``````````diff
diff --git a/clang/cmake/caches/Fuchsia-stage2.cmake b/clang/cmake/caches/Fuchsia-stage2.cmake
index eee37c5e7901f..d5a1662cbf4aa 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 5e98961855282..26ba092a82948 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 ${test_name} APPEND_STRING PROPERTY
                     LINK_FLAGS " /opt:lldlto=0")
diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
index 0699a8586fcc7..05bbe98ef96f8 100644
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -32,6 +32,8 @@ endif()
 set(LLVM_ENABLE_LTO OFF CACHE STRING "Build LLVM with LTO. May be specified as Thin or Full to use a particular kind of LTO")
 string(TOUPPER "${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
@@ -1251,6 +1253,10 @@ 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)

``````````

</details>


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