[cfe-dev] Slowness of clang build

[David Blaikie]

On Sat, Dec 29, 2012 at 10:50 AM, Edward Diener
<eldlistmailingz at tropicsoft.com> wrote:
> On 12/29/2012 12:02 PM, David Blaikie wrote:
>>
>> On Sat, Dec 29, 2012 at 8:47 AM, Edward Diener
>> <eldlistmailingz at tropicsoft.com> wrote:
>>>
>>> I am periodically retrieving the latest sources from the llvm/clang
>>> repository, as specified in the Getting Started page.
>>>
>>> When I do a build of clang, through the 'make' command from the build
>>> directory, the build runs very slow. Evidently the default is a
>>> Debug+Assert
>>> build. Compiling a single source file averages 5-10 seconds and overall
>>> the
>>> build takes hours considering how many modules must be built. It is all
>>> successful in the end and when I am finished I have the latest clang,
>>> which
>>> is great.
>>>
>>> I have plenty of memory, plenty of disk space, a fairly fast CPU, and am
>>> doing the build on Fedora 17 with hardly any other applications running.
>>
>>
>> Could you be more specific about the CPU, cores, and available memory?
>
>
> I have 8 GB of memory and 4 CPUs.

I'd expect a full/clean build to take maybe 10 minutes across 4 cores
(but if it's 4 cpus each with multiple cores, then less time/more
parallelism).

>
>
>> (have you watched the utilization while building Clang? Is the build
>> topping out on RAM/using swap? using all your CPU cores?)
>>
>> The simplest thing to do is probably to pass "-j N" where N is the
>> number of cores you have available (by default the make build isn't
>> parallelized).
>
>
> Use "make -j 4" ?

Correct.

>
>
>>
>> Other ideas:
>> * what compiler are you using to perform the build?
>
>
> I just execute 'make' from my buid directory. I have no idea what compiler
> clang uses. Is there a way to find out ? I would expect it uses 'clang'
> itself.

That'd only be possible if you have an existing clang installation?
(otherwise there's the bootstrap problem: how could it build clang
with clang if it hasn't build clang yet?) Assuming you didn't do
anything special, it should be using your system compiler (which means
that's probably not the performance problem - since you'd only get
really heinous performance from a debug/asserts build of Clang being
used to build Clang, and you wouldn't have a debug+asserts built
compiler as your system compiler)

>
>
>> If it's a previous
>> Debug+Asserts build of Clang, then that's really going to slow things
>> down - don't use that, use Release+Asserts or Release-Asserts.
>
>
> How do I specify Release+Asserts in the 'make' command line ?
>
>
>> * you could try the cmake+ninja build (ninja will run maximally
>> parallel by default & exploits more parallelism than the make build)
>
>
> Again I am just following clang's own instructions in their getting started
> page. I have not tried cmake.

The getting started docs do briefly mention cmake.

>
>
>> * to do a release build of clang (which might speed things up a little
>> since you won't be dealing with debug info, etc) I think it's
>> "--disable-optimized" when configuring the make based build
>
>
> Is this a configure option ? Do I go back and rerun ./configure passing it
> '--disable-optimized' ?

That's the rough idea, yes. I'm not sure I've remembered the exact
spelling of the option off-hand, though.

>
>
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