[compiler-rt] build compiler-rt runtimes without LTO

[inglorion]

Summary

Currently, we build the compiler-rt runtimes with link-time optimization if LTO is configured for the LLVM project. This will break external programs that don't invoke the linker in such a way that it supports LLVM's LTO. To avoid this, this change causes the compiler-rt runtimes to be compiled with -fno-lto. This also makes the check-profile tests work on systems when doing a lld LTO build on a system where the system linker does not support LLVM LTO.

[rnk]

I agree with this in principle. The high-level LLVM_ENABLE_LTO flag shouldn't affect compiler-rt. We should have a different way of getting compiler-rt bitcode libraries if we want them.

Are you sure it's best to do -flto=thin/full -fno-lto instead of filtering out -flto[^ ]*?

[inglorion]

@rnk, thanks for taking a look. I have an alternative implementation that records which flags were added to enable LTO and provides functions to filter those out. That might lead to cleaner command lines. On the other hand, this is simpler and follows what we already do for SANITIZER_COMMON_FLAGS (in compiler-rt/CMakeLists.txt), so I decided to see what people think about this implementation, first.

[davidxl]

Even the system linker supports lto, there might be issues with bit code version mismatches etc. In general it is good idea to not build compiler rt with lto. It is a different story if we want to support cross module inlining of some compiler-rt functions in the future. For now it is fine, but with a brief comment there.

For the same reason, profile rt should not be built with -fprofile-generate or -fprofile-instr-generate, but we did not filter those out -- perhaps a different patch.

[inglorion]

added comment, as suggested by @davidxl

[davidxl]

lgtm

compiler-rt/trunk/cmake/Modules/AddCompilerRT.cmake

if(NOT type MATCHES "^(STATIC|SHARED)$")
     return()
   endif()
   cmake_parse_arguments(LIB
     ""
     "PARENT_TARGET"
     "OS;ARCHS;SOURCES;CFLAGS;LINK_FLAGS;DEFS;LINK_LIBS;OBJECT_LIBS"
     ${ARGN})
   set(libnames)
+  # Until we support this some other way, build compiler-rt runtime without LTO
+  # to allow non-LTO projects to link with it.
+  if(COMPILER_RT_HAS_FNO_LTO_FLAG)
+    set(NO_LTO_FLAGS "-fno-lto")
+  else()
+    set(NO_LTO_FLAGS "")
+  endif()
   if(APPLE)
     foreach(os ${LIB_OS})
       if(type STREQUAL "STATIC")
         set(libname "${name}_${os}")
       else()
         set(libname "${name}_${os}_dynamic")
         set(extra_link_flags_${libname} ${DARWIN_${os}_LINK_FLAGS} ${LIB_LINK_FLAGS})
       endif()
       list_intersect(LIB_ARCHS_${libname} DARWIN_${os}_ARCHS LIB_ARCHS)
       if(LIB_ARCHS_${libname})
         list(APPEND libnames ${libname})
-        set(extra_cflags_${libname} ${DARWIN_${os}_CFLAGS} ${LIB_CFLAGS})
+        set(extra_cflags_${libname} ${DARWIN_${os}_CFLAGS} ${NO_LTO_FLAGS} ${LIB_CFLAGS})
         set(output_name_${libname} ${libname}${COMPILER_RT_OS_SUFFIX})
         set(sources_${libname} ${LIB_SOURCES})
         format_object_libs(sources_${libname} ${os} ${LIB_OBJECT_LIBS})
       endif()
     endforeach()
   else()
     foreach(arch ${LIB_ARCHS})
       if(NOT CAN_TARGET_${arch})
foreach(arch ${LIB_ARCHS})
           set(output_name_${libname} ${name}_dynamic-${arch}${COMPILER_RT_OS_SUFFIX})
         else()
           set(output_name_${libname} ${name}-${arch}${COMPILER_RT_OS_SUFFIX})
         endif()
       endif()
       set(sources_${libname} ${LIB_SOURCES})
       format_object_libs(sources_${libname} ${arch} ${LIB_OBJECT_LIBS})
       set(libnames ${libnames} ${libname})
-      set(extra_cflags_${libname} ${TARGET_${arch}_CFLAGS} ${LIB_CFLAGS})
+      set(extra_cflags_${libname} ${TARGET_${arch}_CFLAGS} ${NO_LTO_FLAGS} ${LIB_CFLAGS})
     endforeach()
   endif()
 
   if(NOT libnames)
     return()
   endif()
 
   if(LIB_PARENT_TARGET)