On Thu, Nov 16, 2017 at 06:14:35PM -0700, Jeff Law wrote:
> > However, this routine currently miss a very obvious case as the following:
> >
> > char s[100] = {'a','b','c','d’};
> >
> > __builtin_strcmp(s, "abc") != 0
> >
> > So, I have to change this routine to include such common case.
> >
> > do you think using this routine is good? or do you have other
> > suggestions (since I am still not very familiar with the internals of
> > GCC, might not find the best available one now…)
> The range information attached to an SSA_NAME is global data. ie, it
> must hold at all locations where the object in question might be
> referenced. This implies that it will sometimes (often?) be less
> precise than you might like.
>
> I am currently working towards an embeddable context sensitive range
> analyzer that in theory could be used within tree-ssa-strlen pass to
> give more precise range information. I'm hoping to wrap that work up in
> the next day or so so that folks can use it in gcc-8.
Well, one thing is a range of integral SSA_NAME at a certain point, the
other is the length of the C string pointed by a certain pointer (that is
something the strlen pass tracks), and another case is the content of that
string, which you'd presumably need to optimize the strcmp at compile time.
I think current strlen pass ought to find out that strlen (s) is 4 at that
point, if it doesn't, please file a PR with a testcase.
Jakub
