r278882 - If possible, set the stack rlimit to at least 8MiB on cc1 startup, and work

[Richard Smith via cfe-commits]

On Fri, Aug 19, 2016 at 12:58 PM, Joerg Sonnenberger via cfe-commits <
cfe-commits at lists.llvm.org> wrote:

> On Thu, Aug 18, 2016 at 11:33:49AM -0700, Richard Smith wrote:
> > On Wed, Aug 17, 2016 at 6:35 AM, Joerg Sonnenberger via cfe-commits <
> > cfe-commits at lists.llvm.org> wrote:
> >
> > > On Wed, Aug 17, 2016 at 01:05:08AM -0000, Richard Smith via cfe-commits
> > > wrote:
> > > > Author: rsmith
> > > > Date: Tue Aug 16 20:05:07 2016
> > > > New Revision: 278882
> > > >
> > > > URL: http://llvm.org/viewvc/llvm-project?rev=278882&view=rev
> > > > Log:
> > > > If possible, set the stack rlimit to at least 8MiB on cc1 startup,
> and
> > > work
> > > > around a Linux kernel bug where the actual amount of available stack
> may
> > > be a
> > > > *lot* lower than the rlimit.
> > >
> > > Can you please restrict this to Linux? I'm quite opposed to overriding
> > > system default limits, they exist for a reason.
> >
> >
> > No, that wouldn't make any sense. It's not up to the operating system how
> > an application decides to allocate memory (on the heap versus on the
> > stack), and Clang's stack usage isn't going to be significantly lower on
> > other kernels. If some BSD kernel's VM is unable to cope with this, we
> > could spawn a thread with a suitable amount of stack space instead.
>
> This is not about kernel bugs. It is about POLA -- programs are not
> supposed to alter process limits. If GCC does it, it is a GCC bug.
> That's no reason to introduce the same bug here. Using excessive stack
> space due to deep recursion might be a QoI issue, but it is
> fundamentally no different from any other out of memory condition. Those
> kill clang just as easily.


Hitting this limit does not imply that memory was exhausted, it instead
means the OS killed a process that was functioning just fine, for no good
reason. As you say, this limit exists for a reason -- and the reason is to
catch programs that are recursing infinitely or using an unexpected amount
of stack space. In Clang's case, using a lot of stack is not unexpected and
does not suggest infinite recursion. So the appropriate thing to do is to
relax or otherwise avoid the arbitrary and harmful limit.
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