Re: [PATCH] avoid calling alloca(0)

[Jeff Law <law at redhat dot com>]

On 11/18/2016 12:58 AM, Jakub Jelinek wrote:
> On Thu, Nov 17, 2016 at 05:32:31PM -0700, Martin Sebor wrote:
> > > The point is warning on an alloca (0) may not be as clear cut as it
> > > might seem.  It's probably a reasonable thing to do on the host, but on
> > > a target, which might be embedded and explicitly overriding the builtin
> > > alloca, warning on alloca (0) is less of a slam dunk.
> >
> > I confess I haven't heard of such an implementation before but after
> > some searching I have managed to find a few examples of it online,
> > such as in QNX or in the BlackBerry SDK, or in the GCC shipped by
> > Texas Instruments.  For instance:
>
> In the libiberty/alloca.c, I don't see anything special about alloca (0).
> Yes, alloca (0) reclaims garbage, but so does alloca (1) and alloca (4035).
> alloca (0) just returns NULL after it.  The diffutils link is the same
> alloca.c as in libiberty.  Returning NULL or returning a non-unique pointer
> for alloca (0) is just fine, it is the same thing as returning NULL or
> returning a non-unique pointer from malloc (0).  We definitely don't want
> to warn for malloc (0), and IMNSHO don't want to warn for alloca (0),
> because it behaves the same.
Maybe that's the key.  We don't want to warn for

alloca (0);

But we should warn for

whatever = alloca (0);


The former is a clear request for GC of the alloca space.  The latter it 
much more likely a request for space where something went wrong 
computing the amount of space.

jeff