Re: [PATCH] middle-end/106075 - non-call EH and DSE

[Richard Biener <rguenther@suse.de>]

On Tue, 17 Jan 2023, Jan Hubicka wrote:

> > On Tue, 17 Jan 2023, Jan Hubicka wrote:
> > 
> > > > The following fixes a long-standing bug with DSE removing stores as
> > > > dead even though they are live across non-call exceptional flow.
> > > > This affects both GIMPLE and RTL DSE and the fix is similar in
> > > > making externally throwing statements uses of non-local stores.
> > > > Note this doesn't fix the GIMPLE side when the throwing statement
> > > > does not involve a load or a store because then the statement does
> > > > not have virtual operands and thus is not visited by GIMPLE DSE.
> > > 
> > > Thanks for looking into this.
> > > My main motivation for poking on this is the patch to add fnspec to
> > > throw/catch machinery
> > > https://gcc.gnu.org/pipermail/gcc-patches/2022-June/597124.html
> > > 
> > > The eh6.C testcase is misoptimized by current trun with the patch.
> > > I think I can adjust it for the throwing function to have no vops and it
> > > will still get misoptimized by DSE.
> > 
> > I think conceptually the "throw" may not be 'const' - it has to be
> > 'pure' at most since the program point the control is transfered to
> > can inspect memory.
> 
> We don't use same argumentation about other control flow statements.
> The following:
> 
> fn()
> {
>   try {
>     i_read_no_global_memory ();
>   } catch (...)
>   {
>     reutrn 1;
>   }
>   return 0;
> }
> 
> should be detected as const.  Marking throw pure would make fn pure too.

I suppose i_read_no_global_memory is const here.  Not sure why that
should make it pure?  Only if anything throws externally (not catched
in fn) should force it to be pure, no?

Of course for IPA purposes whether 'fn' is to be considered const
or pure depends on whether its exceptions are catched in the context
where that's interesting - that is, whether the EH side-effect is
explicitely or implicitely modeled.

> With noncall exceptions a=b/c also can transfer to place that inspect
> memory.  We may want all statements with can_throw_extenral to have VUSE
> on them (like with return) since they may cause function to return, but
> I think fnspec is wrong place to model this.

Yes, I think all control transfer instructions need a VUSE.

Richard.

> > > According to compiler explorer testcase:
> > > struct a{int a,b,c,d,e;};
> > > void
> > > test(struct a * __restrict a, struct a *b)
> > > {
> > >   *a = (struct a){0,1,2,3,4};
> > >   *a = *b;
> > > }
> > > Is compiled correctly by GCC 5.4 and first miscopmiled by 6.1, so I
> > > think it is a regression. (For C++ not very important one as
> > > -fnon-call-exceptions is not very common for C++)
> > 
> > Ah, yes - RTL DSE probably is too weak for this and GIMPLE DSE
> > didn't handle aggregates well at some point.
> 
> Yep, we never handled it really correctly but were weaker on optimizing
> and thus also producing wrong code :)
> 
> Honza
> > 
> > Richard.
> > 
> > > 
> > > Honza
> > > > 
> > > > 	PR middle-end/106075
> > > > 	* dse.cc (scan_insn): Consider externally throwing insns
> > > > 	to read from not frame based memory.
> > > > 	* tree-ssa-dse.cc (dse_classify_store): Consider externally
> > > > 	throwing uses to read from global memory.
> > > > 
> > > > 	* gcc.dg/torture/pr106075-1.c: New testcase.
> > > > ---
> > > >  gcc/dse.cc                                |  5 ++++
> > > >  gcc/testsuite/gcc.dg/torture/pr106075-1.c | 36 +++++++++++++++++++++++
> > > >  gcc/tree-ssa-dse.cc                       |  8 ++++-
> > > >  3 files changed, 48 insertions(+), 1 deletion(-)
> > > >  create mode 100644 gcc/testsuite/gcc.dg/torture/pr106075-1.c
> > > > 
> > > > diff --git a/gcc/dse.cc b/gcc/dse.cc
> > > > index a2db8d1cc32..7e258b81f66 100644
> > > > --- a/gcc/dse.cc
> > > > +++ b/gcc/dse.cc
> > > > @@ -2633,6 +2633,11 @@ scan_insn (bb_info_t bb_info, rtx_insn *insn, int max_active_local_stores)
> > > >        return;
> > > >      }
> > > >  
> > > > +  /* An externally throwing statement may read any memory that is not
> > > > +     relative to the frame.  */
> > > > +  if (can_throw_external (insn))
> > > > +    add_non_frame_wild_read (bb_info);
> > > > +
> > > >    /* Assuming that there are sets in these insns, we cannot delete
> > > >       them.  */
> > > >    if ((GET_CODE (PATTERN (insn)) == CLOBBER)
> > > > diff --git a/gcc/testsuite/gcc.dg/torture/pr106075-1.c b/gcc/testsuite/gcc.dg/torture/pr106075-1.c
> > > > new file mode 100644
> > > > index 00000000000..b9affbf1082
> > > > --- /dev/null
> > > > +++ b/gcc/testsuite/gcc.dg/torture/pr106075-1.c
> > > > @@ -0,0 +1,36 @@
> > > > +/* { dg-do run { target *-*-linux* } } */
> > > > +/* { dg-additional-options "-fnon-call-exceptions" } */
> > > > +
> > > > +#include <unistd.h>
> > > > +#include <signal.h>
> > > > +#include <stdlib.h>
> > > > +
> > > > +int a = 1;
> > > > +short *b;
> > > > +void __attribute__((noipa))
> > > > +test()
> > > > +{
> > > > +  a=12345;
> > > > +  *b=0;
> > > > +  a=1;
> > > > +}
> > > > +
> > > > +void check (int i)
> > > > +{
> > > > +  if (a != 12345)
> > > > +    abort ();
> > > > +  exit (0);
> > > > +}
> > > > +
> > > > +int
> > > > +main ()
> > > > +{
> > > > +  struct sigaction s;
> > > > +  sigemptyset (&s.sa_mask);
> > > > +  s.sa_handler = check;
> > > > +  s.sa_flags = 0;
> > > > +  sigaction (SIGSEGV, &s, NULL);
> > > > +  test();
> > > > +  abort ();
> > > > +  return 0;
> > > > +}
> > > > diff --git a/gcc/tree-ssa-dse.cc b/gcc/tree-ssa-dse.cc
> > > > index 46ab57d5754..b2e2359c3da 100644
> > > > --- a/gcc/tree-ssa-dse.cc
> > > > +++ b/gcc/tree-ssa-dse.cc
> > > > @@ -960,6 +960,7 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
> > > >    auto_bitmap visited;
> > > >    std::unique_ptr<data_reference, void(*)(data_reference_p)>
> > > >      dra (nullptr, free_data_ref);
> > > > +  bool maybe_global = ref_may_alias_global_p (ref, false);
> > > >  
> > > >    if (by_clobber_p)
> > > >      *by_clobber_p = true;
> > > > @@ -1038,6 +1039,11 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
> > > >  		      last_phi_def = as_a <gphi *> (use_stmt);
> > > >  		    }
> > > >  		}
> > > > +	      /* If the stmt can throw externally and the store is
> > > > +		 visible in the context unwound to the store is live.  */
> > > > +	      else if (maybe_global
> > > > +		       && stmt_can_throw_external (cfun, use_stmt))
> > > > +		return DSE_STORE_LIVE;
> > > >  	      /* If the statement is a use the store is not dead.  */
> > > >  	      else if (ref_maybe_used_by_stmt_p (use_stmt, ref))
> > > >  		{
> > > > @@ -1116,7 +1122,7 @@ dse_classify_store (ao_ref *ref, gimple *stmt,
> > > >  	 just pretend the stmt makes itself dead.  Otherwise fail.  */
> > > >        if (defs.is_empty ())
> > > >  	{
> > > > -	  if (ref_may_alias_global_p (ref, false))
> > > > +	  if (maybe_global)
> > > >  	    return DSE_STORE_LIVE;
> > > >  
> > > >  	  if (by_clobber_p)
> > > > -- 
> > > > 2.35.3
> > > 
> > 
> > -- 
> > Richard Biener <rguenther@suse.de>
> > SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
> > Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
> > HRB 36809 (AG Nuernberg)
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
HRB 36809 (AG Nuernberg)