Fix handling of gimple_clobber in ipa_modref

[Christophe Lyon]

Hi Honza,


On Sat, 26 Sep 2020 at 22:03, Jan Hubicka <hubicka@ucw.cz> wrote:
>
> > On September 26, 2020 12:04:24 AM GMT+02:00, Jan Hubicka <hubicka@ucw.cz> wrote:
> > >Hi,
> > >while adding check for gimple_clobber I reversed the return value
> > >so instead of ignoring the statement ipa-modref gives up.  Fixed thus.
> > >This explains the drop between originally reported disambinguations
> > >stats and ones I got later.
> >
> > I don't think you can ignore clobbers. They are barriers for code motion.
>
> Hi,
> this is fix I have installed after lto-bootstrapping/regtesting.
> The statistics for cc1plus are almost unchanged that is sort of expected
> given that I only measure late optimization by getting dump from LTO.
>
> Thank for pointing this out, it may have triggered a nasty wrong code
> bug :)
>
> Honza
>
> Alias oracle query stats:
>   refs_may_alias_p: 63013346 disambiguations, 73204989 queries
>   ref_maybe_used_by_call_p: 141350 disambiguations, 63909728 queries
>   call_may_clobber_ref_p: 23597 disambiguations, 29430 queries
>   nonoverlapping_component_refs_p: 0 disambiguations, 37763 queries
>   nonoverlapping_refs_since_match_p: 19444 disambiguations, 55671 must overlaps, 75884 queries
>   aliasing_component_refs_p: 54749 disambiguations, 753947 queries
>   TBAA oracle: 24159888 disambiguations 56277876 queries
>                16064485 are in alias set 0
>                10340953 queries asked about the same object
>                125 queries asked about the same alias set
>                0 access volatile
>                3920604 are dependent in the DAG
>                1791821 are aritificially in conflict with void *
>
> Modref stats:
>   modref use: 10444 disambiguations, 46994 queries
>   modref clobber: 1421468 disambiguations, 1954304 queries
>   4907798 tbaa queries (2.511277 per modref query)
>   396785 base compares (0.203031 per modref query)
>
> PTA query stats:
>   pt_solution_includes: 976073 disambiguations, 13607833 queries
>   pt_solutions_intersect: 1026016 disambiguations, 13185678 queries
>
>         * ipa-modref.c (analyze_stmt): Do not skip clobbers in early pass.
>         * ipa-pure-const.c (analyze_stmt): Update comment.
>

After commit g:67a5c215940f4b21bac1aa489ce1f2fb3d52a53a (r11-3468), I
see a failure in fortran on armeb-linux-gnueabihf,
it's still present after r11-3491 (g4383c595ce5cc6ef6bcb45d2c9caf43002afbc4f):
FAIL: gfortran.dg/PR94022.f90   -Os  execution test
with GCC configured with:
--target armeb-none-linux-gnueabihf
--with-mode arm
--with-cpu cortex-a9
--with-fpu neon-fp16

Can you have a look?

> diff --git a/gcc/ipa-modref.c b/gcc/ipa-modref.c
> index 73a7900883a..728c6c1523d 100644
> --- a/gcc/ipa-modref.c
> +++ b/gcc/ipa-modref.c
> @@ -676,13 +676,16 @@ static bool
>  analyze_stmt (modref_summary *summary, gimple *stmt, bool ipa,
>               vec <gimple *> *recursive_calls)
>  {
> -  /* There is no need to record clobbers.  */
> -  if (gimple_clobber_p (stmt))
> +  /* In general we can not ignore clobbers because they are barries for code
> +     motion, however after inlining it is safe to do becuase local optimization
> +     passes do not consider clobbers from other functions.
> +     Similar logic is in ipa-pure-consts.  */
> +  if ((ipa || cfun->after_inlining) && gimple_clobber_p (stmt))
>      return true;
> +
>    /* Analyze all loads and stores in STMT.  */
>    walk_stmt_load_store_ops (stmt, summary,
>                             analyze_load, analyze_store);
> -  /* or call analyze_load_ipa, analyze_store_ipa */
>
>    switch (gimple_code (stmt))
>     {
> @@ -705,7 +708,7 @@ analyze_stmt (modref_summary *summary, gimple *stmt, bool ipa,
>     }
>  }
>
> -/* Analyze function F.  IPA indicates whether we're running in tree mode (false)
> +/* Analyze function F.  IPA indicates whether we're running in local mode (false)
>     or the IPA mode (true).  */
>
>  static void
> diff --git a/gcc/ipa-pure-const.c b/gcc/ipa-pure-const.c
> index bdbccd010dc..1af3206056e 100644
> --- a/gcc/ipa-pure-const.c
> +++ b/gcc/ipa-pure-const.c
> @@ -742,6 +742,8 @@ check_stmt (gimple_stmt_iterator *gsip, funct_state local, bool ipa)
>    /* Do consider clobber as side effects before IPA, so we rather inline
>       C++ destructors and keep clobber semantics than eliminate them.
>
> +     Similar logic is in ipa-modref.
> +
>       TODO: We may get smarter during early optimizations on these and let
>       functions containing only clobbers to be optimized more.  This is a common
>       case of C++ destructors.  */