This is the mail archive of the
mailing list for the GCC project.
Re: Aliasing: look through pointer's def stmt
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Marc Glisse <marc dot glisse at inria dot fr>
- Cc: Jeff Law <law at redhat dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Fri, 25 Oct 2013 11:13:08 +0200
- Subject: Re: Aliasing: look through pointer's def stmt
- Authentication-results: sourceware.org; auth=none
- References: <alpine dot DEB dot 2 dot 02 dot 1310250707430 dot 14734 at stedding dot saclay dot inria dot fr> <526A0269 dot 8030407 at redhat dot com> <alpine dot DEB dot 2 dot 10 dot 1310250809590 dot 4165 at laptop-mg dot saclay dot inria dot fr> <CAFiYyc27=sN1itn+N09rpmm8tWGt-Khu7oASLd0ODsdZh=ofzw at mail dot gmail dot com>
On Fri, Oct 25, 2013 at 10:59 AM, Richard Biener
> On Fri, Oct 25, 2013 at 8:11 AM, Marc Glisse <email@example.com> wrote:
>> On Thu, 24 Oct 2013, Jeff Law wrote:
>>> On 10/24/13 23:23, Marc Glisse wrote:
>>>> I noticed that in some cases we were failing to find aliasing
>>>> information because we were only looking at an SSA_NAME variable,
>>>> missing the fact that it was really an ADDR_EXPR. The attached patch
>>>> passes bootstrap+testsuite, does it make sense? (I am a bit afraid of
>>>> losing some type information for instance)
>>>> I didn't investigate the 2 tests where I had to remove dg-bogus, because
>>>> removing dg-bogus sounds like a bonus...
>>>> 2013-10-25 Marc Glisse <firstname.lastname@example.org>
>>>> * tree-ssa-alias.c (ao_ref_init_from_ptr_and_size): Look for an
>>>> ADDR_EXPR in the defining statement.
>>> Shouldn't the ADDR_EXPR have been propagated into the use?
>> Maybe when the address is a constant, but here it comes from malloc.
> points-to should have "propagated" the alias info, so no, looking at
> def-stmts in random places like this isn't ok. Where does alias info
> get lost?
It doesn't get lost, but this is the missed optimization that malloc
results still alias with global pointers (here a function argument).
Taking into account the offset shouldn't change anything.
I suppose you are looking for the memcpy to be folded into an
assignment? Which ao_ref_from_ptr_and_size is critical for
the transform - it doesn't seem to be the one in memory op folding.
>> Marc Glisse