[llvm] r200695 - Don't use -ffunction-sections if -fno-function-sections is not supported in the compiler.

[Evgeniy Stepanov]

Well, it's not that easy. Nothing is easy with cmake. I'll try some
more today, and either fix or revert the change.


On Tue, Feb 4, 2014 at 1:49 AM, Justin Bogner <mail at justinbogner.com> wrote:
> Evgeniy Stepanov <eugeni.stepanov at gmail.com> writes:
>> Don't use -ffunction-sections if -fno-function-sections is not
>> supported in the compiler.
>>
>> This will disable -ffunction-sections in older versions of Clang where it
>> breaks build of sanitizer runtime library.
>>
>> Modified:
>>     llvm/trunk/cmake/modules/HandleLLVMOptions.cmake
>>
>> Modified: llvm/trunk/cmake/modules/HandleLLVMOptions.cmake
>> URL:
>> http://llvm.org/viewvc/llvm-project/llvm/trunk/cmake/modules/HandleLLVMOptions.cmake?rev=200695&r1=200694&r2=200695&view=diff==============================================================================
>> --- llvm/trunk/cmake/modules/HandleLLVMOptions.cmake (original)
>> +++ llvm/trunk/cmake/modules/HandleLLVMOptions.cmake Mon Feb  3 07:57:09 2014
>> @@ -350,13 +350,23 @@ if (UNIX AND
>>    append("-fcolor-diagnostics" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
>>  endif()
>>
>> +# Clang prior to 3.5 ignored -fno-function-sections.
>> +# It's pretty hard to test directly, so we rely on the version number.
>> +if( ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang") AND (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.5) )
>> +  set(LLVM_COMPILER_HAS_BROKEN_FNO_FUNCTION_SECTIONS ON)
>> +endif()
>> +
>
> Please don't do it this way. Relying on version numbers to check for a
> feature is the wrong thing to do. It doesn't account for the 5000
> revisions that are called 3.5 and don't support this flag and just
> generally isn't nice to packagers.
>
> It also wouldn't be very scalable if other compilers needed to be
> handled as well, though that doesn't seem to be the case in this
> particular example.
>
> Can't we check if the compiler supports the flag directly?
>
>>  # Add flags for add_dead_strip().
>>  # FIXME: With MSVS, consider compiling with /Gy and linking with /OPT:REF?
>>  # But MinSizeRel seems to add that automatically, so maybe disable these
>>  # flags instead if LLVM_NO_DEAD_STRIP is set.
>>  if(NOT CYGWIN AND NOT WIN32)
>>    if(NOT ${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
>> -    append("-ffunction-sections -fdata-sections" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
>> +    # Don't add -ffunction-section if it can be disabled with -fno-function-sections.
>> +    # Doing so will break sanitizers.
>> +    if (NOT LLVM_COMPILER_HAS_BROKEN_FNO_FUNCTION_SECTIONS)
>> +      append("-ffunction-sections -fdata-sections" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
>> +    endif()
>>    endif()
>>  endif()
>>
>>
>>
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