[Mesa-dev] [PATCH] mesa: Enable LTO compilation

[Emil Velikov]

On 30 May 2016 at 21:10, Jason Ekstrand <jason at jlekstrand.net> wrote:

> On Mon, May 30, 2016 at 12:27 PM, Emil Velikov <emil.l.velikov at gmail.com>
> wrote:
>
>> Hi there,
>> On 30 May 2016 at 18:28, ⚛ <0xe2.0x9a.0x9b at gmail.com> wrote:
>> > This patch enables compilation with -flto.
>> >
>> > The performance benefits of LTO (GCC 4.9 & 6.1) are about 1% for
>> glxgears.
>> > Performance changes in OpenGL games haven't been measured yet, my
>> feeling is
>> > that they are negligible.
>> >
>> Please try to send patches via git send-email for the future.
>>
>> While not a hard requirement, it would be nice if you setup your email
>> client/git to show your name/nickname.
>> It's always better to address someone directly, as opposed to "hey you".
>>
>> About the patch itself:
>>  - Please add a summary message for the change. Peter's has a nice
>> article on the topic [1].
>>  - We normally try to include performance metrics in the commit
>> message. It makes it easier to justify said change.
>>  - Have you tried debugging the resulting binary ? As Matt mentioned
>> there used to be issues with LTO, so if those are still unresolved we
>> want to _avoid_ forcing that for everyone.
>>
>
> Regarding the above two comments:  The patch he attached does not enable
> LTO, it just disables it for mapi so that *if* you put -flto in your
> CFLAGS, it doesn't blow up.  If this patch were enabling LTO by default,
> questions about performance and debugability would be reasonable.  However,
> given that the patch is purely protective, the only real question required
> to justify the patch is "is LTO actually a problem for mapi?"
>
>
Seems like I'm the next person to fall trap of the commit message... that's
embarrassing.

Wrt the approach used... perhaps one should be checking for the same flag
like they use ? It should still work though - just good practise.


>  - Are you use the mapi hunk is needed ? last time I've tried (some
>> months ago, copying the tweaks from the Clearlinux spec file [2])
>> things built without issues.
>>
>
> You are claiming it isn't.
>
> That said, I tried to build with -flto recently and it blew up.  I don't
> remember exactly why.
>
Do you (others) recall any details about how you enabled it (tweak *FLAGS,
CC and/or other), if there was anything special in the configure options
and/or which the GCC version ?

I'll give it another try in a minute, just in case I butchered something.

-Emil
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