[Bug tree-optimization/58806] New attribute for functions that access memory only through their arguments

["glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58806

--- Comment #3 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #2)
> You cannot find the PR because it's already implemented via the "fn spec"
> attribute (conveniently not user-accessible because bike-shedding about
> whether separate attributes are required).  The documentation resides
> in gimple.c:gimple_call_arg_flags,

Thanks, I knew of the EAF_* flags (none look that promising for my particular
use), but not of this "fn spec" attribute.

> and I _think_ it doesn't quite provide
> what you want as it was designed to help the context and flow-insensitive
> points-to analysis which doesn't benefit from "this call does not clobber
> or read from escaped memory".  But what it provides is "the argument to
> this function does not escape" which is important.

I locally changed the name to fn_spec so I could try it. Nothing helps for
comment #0. The combination EAF_NOCLOBBER | EAF_NOESCAPE (aka 'r') is enough to
optimize comment #1. However, EAF_NOCLOBBER is exactly the opposite of what I
want since the whole point of g is to write to its argument, and I don't see
why EAF_NOESCAPE should be necessary since there is no other call afterwards.

So I guess more flags (ECF_* since they wouldn't apply to a specific argument)
would be good. I tried the attributes nothrow,nonnull,leaf on comment #0
because that's what gcc puts on memcpy and gcc knows that memcpy(data,...)
doesn't change *p, but that didn't work, I should look at where exactly the
difference between memcpy and init plays a role.