> By majority vote and a hint from the API name which is
> arg_max_access_size_given_by_arg_p this interprets a memory access
> size specified as given as other argument such as for strncpy
> in the testcase which has "1cO313" as specifying the _maximum_
> size read/written rather than the exact size. There are two
> uses interpreting it that way already and one differing. The
> following adjusts the differing and clarifies the documentation.
This makes sense to me. Bit sad is memcpy/memset may be common enough so
the extra disambiguation based on maximal object size may be useful.
>
> Bootstrap & regtest running on x86_64-unknown-linux-gnu.
>
> It looks like arg_access_size_given_by_type_p is interpreted in
> a similar way but the API doesn't reflect this (no _max). On IRC
> we discussed it would be nice to have both, exact and non-exact
> size (for example use 'a'..'j' for the alternate variant).
Hmm 'a'...'j' may work to stick this information in....
>
> I still wonder if my interpretation is correct in that '1'..'9'
> specify a _bound_ for the size. Honza, do you remember why
> you wrote the IPA modref users this way? Was that intentional?
> Thus is my reverse engineering of the desired semantics correct?
My fnspec implementation was based on what the code was doing
beforehand. In GCC 10 we have:
case BUILT_IN_STRCPY:
case BUILT_IN_STRNCPY:
case BUILT_IN_MEMCPY:
case BUILT_IN_MEMMOVE:
case BUILT_IN_MEMPCPY:
case BUILT_IN_STPCPY:
case BUILT_IN_STPNCPY:
case BUILT_IN_TM_MEMCPY:
case BUILT_IN_TM_MEMMOVE:
{
ao_ref dref;
tree size = NULL_TREE;
if (gimple_call_num_args (call) == 3)
size = gimple_call_arg (call, 2);
ao_ref_init_from_ptr_and_size (&dref,
gimple_call_arg (call, 1),
size);
return refs_may_alias_p_1 (&dref, ref, false);
}
So it seem that we handled strncpy the way claiming full size access
there too and I simply copied it into the new implementation.
While adding fnspec I just used adapted this code, so the difference
between modref and tree-ssa-alias implementation was not quite
intentional.
>
> Thanks,
> Richard.
>
> PR tree-optimization/109609
> * attr-fnspec.h (arg_max_access_size_given_by_arg_p):
> Clarify semantics.
> * tree-ssa-alias.cc (check_fnspec): Correctly interpret
> the size given by arg_max_access_size_given_by_arg_p as
> maximum, not exact, size.
>
> * gcc.dg/torture/pr109609.c: New testcase.
Patch looks good to me (modulo the fact that it would be nice to be also
able to handle memcpy/memset precisely).
Honza
