Re: avoiding recursive calls of calloc due to optimization

[Jakub Jelinek <jakub at redhat dot com>]

On Mon, Sep 21, 2015 at 10:26:23AM -0700, Andrew Pinski wrote:
> On Mon, Sep 21, 2015 at 10:20 AM, Daniel Gutson
> <daniel.gutson@tallertechnologies.com> wrote:
> > This is derived from https://gcc.gnu.org/ml/gcc-help/2015-03/msg00091.html
> >
> > Currently, gcc provides an optimization that transforms a call to
> > malloc and a call to memset into a call to calloc.
> > This is fine except when it takes place within the calloc() function
> > implementation itself, causing a recursive call.
> > Two alternatives have been proposed: -fno-malloc-builtin and disable
> > optimizations in calloc().
> > I think the former is suboptimal since it affects all the code just
> > because of the implementation of one function (calloc()),
> > whereas the latter is suboptimal too since it disables the
> > optimizations in the whole function (calloc too).
> > I think of two alternatives: either make -fno-calloc-builtin to
> > disable the optimization, or make the optimization aware of the
> > function context where it is operating and prevent it to do the
> > transformation if the function is calloc().
> >
> > Please help me to find the best alternative so we can implent it.
> 
> Did you try the optimize attribute?
> 
> Also you can try the following:
> 
> size_t ns = size*elements;
> If (ns / elements != size)
>   return NULL;
> void *ptr = malloc (ns);
> asm ("":"+r"(ptr));
> memset (ptr, 0, ns);
> 
> Notice I put in a check for overflow in there.

You can use
  size_t ns;
  if (__builtin_mul_overflow (size, elements, &ns))
    return ns;
for that part ;)

	Jakub