[Mesa-dev] [PATCH] mesa: Enable LTO by default on i965/libdricore release builds.

[Eric Anholt]

Chad Versace <chad.versace at linux.intel.com> writes:

> On 08/08/2013 01:43 PM, Eric Anholt wrote:
>> We can't just smash it on globally due to (probably resolvable) issues
>> with the asm in glapi.  And we don't want to penalize developers with
>> longer build times for their normal debug environment.
>>
>> Due to libdricore making almost all of our symbols public, the effect is
>> very small -- cairo-gl with INTEL_NO_HW=1 shows -0.798709% +/- 0.333703%
>> change in runtime (n=30).
>> ---
>>
>> If we were to avoid dricore, there's an additional 5% improvement available
>> (see the "megadriver" branch of my tree).
>>
>>   configure.ac                          | 25 +++++++++++++++++++++++++
>>   src/mesa/Makefile.am                  |  4 ++--
>>   src/mesa/drivers/dri/i965/Makefile.am |  1 +
>>   src/mesa/libdricore/Makefile.am       |  8 +++++++-
>>   src/mesa/program/Makefile.am          |  4 ++--
>>   5 files changed, 37 insertions(+), 5 deletions(-)
>>
>> diff --git a/configure.ac b/configure.ac
>> index 62d06e0..26c230d 100644
>> --- a/configure.ac
>> +++ b/configure.ac
>> @@ -314,6 +314,7 @@ AC_ARG_ENABLE([debug],
>>       [enable_debug="$enableval"],
>>       [enable_debug=no]
>>   )
>> +enable_lto=yes
>>   if test "x$enable_debug" = xyes; then
>>       DEFINES_FOR_BUILD="$DEFINES_FOR_BUILD -DDEBUG"
>>       if test "x$GCC_FOR_BUILD" = xyes; then
>> @@ -330,7 +331,31 @@ if test "x$enable_debug" = xyes; then
>>       if test "x$GXX" = xyes; then
>>           CXXFLAGS="$CXXFLAGS -g -O0"
>>       fi
>> +
>> +    # Disable LTO by default on debug builds, since it's so expensive at
>> +    # compile time.
>> +    enable_lto=no
>> +fi
>
> I'd like to emit a configuration error if someone tries to enable LTO
> in a debug build. According the gcc-4.8 manpage, that's a really bad
> idea.
>
>             Link-time optimization does not work well with generation of
>             debugging information.  Combining -flto with -g is currently
>             experimental and expected to produce wrong results.
>
> Other than that, the patch looks good to me.

Yeah, debug symbols appear to be quite scrambled.  As is, I don't think
we can ship with it -- breaking backtraces (and thus profiling) is not
acceptable.
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