[cfe-dev] Why clang needs to fork into itself?
richard at metafoo.co.uk
Tue Jan 28 23:58:31 PST 2014
On Tue, Jan 28, 2014 at 11:40 PM, Manuel Klimek <klimek at google.com> wrote:
> On Wed, Jan 29, 2014 at 8:05 AM, Ted Kremenek <kremenek at apple.com> wrote:
>> If this can be done, then great.
>> Yury Gribov's point about stack smashing is a good one. We implemented
>> such crash recovery mechanisms in libclang and libclang still takes down
>> Xcode sometimes because of stack overflows due to unbounded recursion, etc.
>> We've also noticed that when libclang "crashes" (and recovers) the overall
>> process can be in an undefined state. Our experience is that such
>> histrionics can provide an 80% solution, but we've never been all that
>> satisfied with them. It may, however, be good enough for generating crash
>> reports, but it seems like a lot of work to replace something we already
>> have that works very well in practice.
> One thing I'd be curious about:
> What would be the downside of using the parent clang *just* as crash
> reporter - and push all driver logic into the spawned process. The parent
> clang would then forward all command line arguments literally, and then
> just sit there, waiting for the crash, and if there is a crash scrap up all
> it needs for the bug report.
> That way, we could easily turn off forking and have equivalent behavior
> minus crash reporting.
> Is there a downside to that that I'm missing (apart from the time needed
> to implement it)?
We still lose all the simplicity benefits of having the driver run one
subprocess per compilation, and we'd need to turn off -disable-free in the
multiple-files case, and we'd need to worry about having a large VMSize if
we fork to spawn another process (for instance, the linker). Having a
separate process for the compiler from the driver is a much cleaner model.
That said, most users of the compiler don't actually *want* a driver. They
want just a .c* -> .o compiler, not a multi-source-file cross-language
compiler + assembler + linker + kitchen sink, and it makes sense to me to
optimize for the single-source, -c case.
On Jan 28, 2014, at 10:28 PM, Yuri <yuri at rawbw.com> wrote:
>> > On 01/28/2014 22:04, Yury Gribov wrote:
>> >> Makes sense but what if some important bits (say argv) are trashed by
>> stack overflow
>> > All information needed for crash reporting should be copied into the
>> fixed memory area, and it should be made read-only for the duration of run.
>> > Yuri
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