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From: Szabolcs Nagy <nsz@port70.net>
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Subject: Re: thoughts on reallocarray, explicit_bzero?
Date: Mon, 19 May 2014 20:12:58 +0200
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* Rich Felker <dalias@libc.org> [2014-05-19 12:55:23 -0400]:
> On Mon, May 19, 2014 at 06:25:57PM +0200, Szabolcs Nagy wrote:
> > static size_t sizemul(size_t a, size_t b)
> > {
> > 	return b>1 && a>1 && a>-1/b ? -1 : a*b;
> > }
> 
> On 32-bit this can easily be optimized to just one conditional instead
> of three:
> 
> uint64_t tmp = (uint64_t)a * b;
> return tmp>SIZE_MAX ? SIZE_MAX : tmp;
> 
> Of course that requires an ifdef, which is perhaps ugly.

or without ifdef

static size_t sizemul(size_t a, size_t b)
{
	if (2*sizeof a <= sizeof 1ULL) {
		unsigned long long m = 1ULL*a*b;
		return m>>8*sizeof a ? -1 : m;
	}
	return (a|b)>>4*sizeof a && b && a>-1/b ? -1 : a*b;
}