[cfe-dev] Why clang needs to fork into itself?

Sean Silva silvas at purdue.edu
Wed Jan 29 15:10:23 PST 2014


On Wed, Jan 29, 2014 at 3:26 PM, Alp Toker <alp at nuanti.com> wrote:

>
> On 29/01/2014 20:06, Sean Silva wrote:
>
>
>>
>>
>> On Tue, Jan 28, 2014 at 12:45 PM, Nico Weber <thakis at chromium.org<mailto:
>> thakis at chromium.org>> wrote:
>>
>>     I believe there was a thread about not forking the driver a while
>>     ago, but I'm unable to find it. As far as I remember, Chris
>>     Lattner wanted to get rid of it for aesthetic reasons and to save
>>     the milliseconds of overhead it adds (I think doing this has
>>     originally been the plan, see the "fork/exec" section on
>>     http://lists.cs.uiuc.edu/pipermail/cfe-dev/2009-December/007211.html
>> ).
>>     At the end, the decision was made to keep the subprocess for cc1,
>>     but I don't remember all the reasons. I think crash reporting was
>>     part of the discussion, but there was half a plan to keep that
>>     feature with in-process crash reporting somehow.
>>
>>     Maybe someone still has a copy of that thread in their inbox?
>>     Keywords "lattner dgregor cc1 crash fork exec" or similar might
>>     find it.
>>
>>
>> The thread you linked to predates my LLVM involvement, but I remember
>> another thread (long time ago) that I was involved in where basically it
>> was asked "is this a measurable slowdown?" (my me or someone else, I
>> forget) and nobody could provide real numbers supporting that it was a
>> measurable slowdown (even on Windows). Can't see to find the thread in my
>> email though.
>>
>> Do we have real performance numbers demonstrating a slowdown at this
>> point?
>>
>
> Hi Sean,
>
> I have a patch now that pushes all self-invocations in-process without
> doing anything else clever such as reusing FileManager stat caches or
> in-memory object files (6 line diff, posted in another part of this thread).
>
> Haven't had a chance to time it yet, but it doesn't look like an
> earth-shattering win on OS X (5 reruns of make check show around 2 seconds
> saved from a 30 second run as a very early indication).
>
> On the other hand it's been fun to debug the driver with this setup and I
> suspect there'll be a bigger win on Windows.
>
> Another patch I'm cooking up just to see "what happens", is to use the
> IPCChannel facility recently added to LLVM to avoid the driver invocation
> itself.
>

What is this IPCChannel? I can't seem to find it.

-- Sean Silva


>
> Alp.
>
>
>
>> -- Sean Silva
>>
>>
>>
>>     On Tue, Jan 28, 2014 at 7:19 AM, Rafael Espíndola
>>     <rafael.espindola at gmail.com <mailto:rafael.espindola at gmail.com>>
>>
>>     wrote:
>>
>>         On 27 January 2014 17:27, Reid Kleckner <rnk at google.com
>>         <mailto:rnk at google.com>> wrote:
>>         > As an alternative, on Windows we could rig up some kind of
>>         SEH filter to do
>>         > crash recovery.  Then we could save the subprocess
>>         invocation and speed
>>         > things up.
>>
>>         Breakpad is used by both chrome and firefox for this. If going
>>         this
>>         path, please make sure the same technique is used for all systems.
>>
>>         Cheers,
>>         Rafale
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>>
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>>
>>
>>
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>>
>>
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>
> --
> http://www.nuanti.com
> the browser experts
>
>
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