On Wed, Aug 26, 2020 at 11:45 AM Erick Ochoa
<erick.oc...@theobroma-systems.com> wrote:
>
>
>
> On 26/08/2020 10:36, Erick Ochoa wrote:
> >
> >
> > On 25/08/2020 22:03, Richard Biener wrote:
> >> On August 25, 2020 6:36:19 PM GMT+02:00, Erick Ochoa
> >> <erick.oc...@theobroma-systems.com> wrote:
> >>>
> >>>
> >>> On 25/08/2020 17:19, Erick Ochoa wrote:
> >>>>
> >>>>
> >>>> On 25/08/2020 17:10, Richard Biener wrote:
> >>>>> On August 25, 2020 3:09:13 PM GMT+02:00, Erick Ochoa
> >>>>> <erick.oc...@theobroma-systems.com> wrote:
> >>>>>> Hi,
> >>>>>>
> >>>>>> I'm trying to understand how the escape analysis in IPA-PTA works.
> >>> I
> >>>>>> was
> >>>>>> testing a hypothesis where if a structure contains an array of
> >>>>>> characters and this array of characters is passed to fopen, the
> >>>>>> structure and all subfields will escape.
> >>>>>>
> >>>>>> To do this, I made a program that has a global structure variable
> >>> foo2
> >>>>>> that is has a field passed as an argument to fopen. I also made
> >>> another
> >>>>>>
> >>>>>> variable foo whose array is initialized by the result of rand.
> >>>>>>
> >>>>>> However, after compiling this program with -flto
> >>> -flto-partition=none
> >>>>>> -fipa -fdump-ipa-pta -fdump-tree-all-all -Ofast (gcc --version
> >>> 10.2.0)
> >>>>>>
> >>>>>> E.g.
> >>>>>>
> >>>>>> #include <stdio.h>
> >>>>>> #include <math.h>
> >>>>>> #include <string.h>
> >>>>>>
> >>>>>> struct foo_t {
> >>>>>> char buffer1[100];
> >>>>>> char buffer2[100];
> >>>>>> };
> >>>>>>
> >>>>>> struct foo_t foo;
> >>>>>> struct foo_t foo2;
> >>>>>>
> >>>>>> int
> >>>>>> main(int argc, char** argv)
> >>>>>> {
> >>>>>>
> >>>>>> fopen(foo2.buffer1, "r");
> >>>>>> for (int i = 0; i < 100; i++)
> >>>>>> {
> >>>>>> foo.buffer1[i] = rand();
> >>>>>> }
> >>>>>> int i = rand();
> >>>>>> int retval = foo.buffer1[i % 100];
> >>>>>> return retval;
> >>>>>> }
> >>>>>>
> >>>>>> I see the PTA dump state the following:
> >>>>>>
> >>>>>> ESCAPED = { STRING ESCAPED NONLOCAL foo2 }
> >>>>>> foo = { ESCAPED NONLOCAL }
> >>>>>> foo2 = { ESCAPED NONLOCAL }
> >>>>>>
> >>>>>> which I understand as
> >>>>>> * something externally visible might point to foo2
> >>>>>> * foo2 might point to something externally visible
> >>>>>> * foo might point to something externally visible
> >>>>>
> >>>>> Yes. So it's exactly as your hypothesis.
> >>>>>
> >>>>>> I have seen that global variables are stored in the .gnu.lto_.decls
> >>> LTO
> >>>>>>
> >>>>>> file section. In the passes I have worked on I have ignored global
> >>>>>> variables. But can foo and foo2 be marked as escaping because the
> >>>>>> declarations are not streamed in yet? Or is there another reason I
> >>> am
> >>>>>> not seeing? I am aware of aware of the several TODOs at the
> >>> beginning
> >>>>>> of
> >>>>>> gcc/tree-ssa-structalias.c but I am unsure if they contribute to
> >>> these
> >>>>>> variables being marked as escaping. (Maybe TODO 1 and TODO 2?)
> >>>>>
> >>>>> Not sure what the problem is. Foo2 escapes because it's address is
> >>>>> passed to a function.
> >>>>>
> >>>>
> >>>> foo2 is not the problem, it is foo. foo is not passed to a function
> >>> and
> >>>> it is also escaping.
> >>>
> >>>
> >>> Sorry, I meant: foo might point to something which is externally
> >>> visible. Which I don't think is the case in the program. I understand
> >>> this might be due to the imprecision in the escape-analysis and what
> >>> I'm
> >>> trying to find out is the source of imprecision.
> >>
> >> Foo is exported and thus all function calls can store to it making it
> >> point to escaped and nonlocal variables.
> >
> > Hi Richard,
> >
> > I'm still not sure why foo can point to escaped and nonlocal variables.
> >
> > My understanding is that ipa-visibility can mark variables and functions
> > as not externally visible. Which I think is equivalent to excluding
> > these symbols from the export table. I have printed the results of
> > vnode->externally_visible and vnode->externally_visible_p() of foo and
> > foo2 and both predicates return 0. (This is in a pass just before
> > IPA-PTA). I think this means that both variables are not exported.
> > IPA-PTA still says that foo can point to escaped memory.
> >
> > ESCAPED = { NULL STRING ESCAPED NONLOCAL foo2 } // correct
> > foo = { ESCAPED NONLOCAL } same as _4 // my question
> > foo2 = { ESCAPED NONLOCAL } // correct
> >
> > I then later declared foo as static. Which I think should mark the
> > variable for internal linkage only. IPA-PTA still says that foo can
> > point to escaped memory.
> >
> > I also used the -fwhole-program flag and IPA-PTA still says that foo can
> > point to escaped memory.
> >
> > Am I failing to specify that foo must *not* be exported? Or IPA-PTA does
> > not considering the visibility of symbols? Or there is something else
> > I'm missing?
>
> Hi Richard,
>
> I think the reason why the global variables escape is because probably
> is_ipa_escape_point is not being used in all the places. According to
> the comments in tree-ssa-structalias.c
>
> The is_global_var bit which marks escape points is overly conservative
> in IPA mode. Split it to is_escape_point and is_global_var - only
> externally visible globals are escape points in IPA mode.
> There is now is_ipa_escape_point but this is only used in a few
> selected places.
That comment is accurate. IPA PTA is "tuned" for correctness, no extensive
optimality testing has been done for it (it's also not enabled by default).
> I had read this before, but the test I had made originally tested to see
> if a subfield escapes the whole global structure also escapes. I think
> there's another comment in tree-ssa-structalias.c that comments on this:
>
> /* ??? Force us to not use subfields for globals in IPA mode.
> Else we'd have to parse arbitrary initializers. */
>
> Thanks!
>
>
> >
> > Thanks!
> >
> >>
> >> Richard.
> >>
> >>>>
> >>>>> ?
> >>>>>
> >>>>> Richard.
> >>>>>
> >>>>>> Just FYI, I've been reading:
> >>>>>> * Structure Aliasing in GCC
> >>>>>> * Gimple Alias Improvements for GCC 4.5
> >>>>>> * Memory SSA - A Unified Approach for Sparsely Representing Memory
> >>>>>> Operations
> >>>>>>
> >>>>>> Thanks, I appreciate all help!
> >>>>>
> >>
