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

[Richard Biener <rguenther@suse.de>]

On Wed, 18 Jan 2023, Jan Hubicka wrote:

> > On Tue, 17 Jan 2023, Jan Hubicka wrote:
> > 
> > > > > 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
> > > Suppose we have:
> > > 
> > > void
> > > i_read_no_global_memory ()
> > > {
> > >   throw(0);
> > > }
> > > 
> > > If cxa_throw itself was annotated as 'p' rahter than 'c' ipa-modref will
> > > believe that cxa_throw will read any global memory and will propagate it
> > > to all callers. So fn() will be also marked as reading all global
> > > memory.
> > 
> > Sure - but for the purpose of local optimizations in 
> > i_read_no_global_memory cxa_throw has to appear to read memory.
> 
> Yes, I think every stmt that can throw externally need VUSE (just like
> return_stmt needs it).  Even if throw(0) was replaced by a=b/c with
> -fnon-call-exceptions.  It is still not clear to me why this should
> imply that we need 'p' instead of 'c' in fnspecs.
> 
> So I think we should try to make the following to work:
> 
> diff --git a/gcc/tree-ssa-operands.cc b/gcc/tree-ssa-operands.cc
> index 57e393ae164..d24f1721eb2 100644
> --- a/gcc/tree-ssa-operands.cc
> +++ b/gcc/tree-ssa-operands.cc
> @@ -951,6 +951,9 @@ operands_scanner::parse_ssa_operands ()
>    enum gimple_code code = gimple_code (stmt);
>    size_t i, n, start = 0;
>  
> +  if (stmt_can_throw_external (fn, stmt))
> +    append_vuse (gimple_vop (fn));
> +
>    switch (code)
>      {
>      case GIMPLE_ASM:

It's going to be a bit tricky since in many places we use
gimple_vuse () != NULL to check whether an assignment is a
load/store.  But yes, the above is sort-of what we'd need to do.

> > Having a VUSE there dependent on whether the function performs any
> > load or store would be quite ugly.  Instead modref could special-case
> > cxa_throw and not treat it as reading memory (like it already does
> > for the return stmt I suppose - that also has a VUSE).
> 
> modref looks into statements with VUSEs on them and checks what
> reads/stores are done.  So return statement with VUSE is walked and no
> load is recorded because no actual load is found.
> Similarly that would happen with __cxa_throw if it was 'c'.
> With 'p' it has nothing to analyze so it would trust the fact that
> cxa_throw itself reads some global state.

I see.  But does __cxa_throw stmt_can_throw_external ()?  Otherwise
the operand scanner elides VUSE on const function calls.

> > 
> > The problem is IIRC GIMPLE_RESX which doesn't derive from
> > gimple_statement_with_memory_ops_base.  There's a bugzilla I can't find
> > right now refering to this issue.
> 
> I never tried to play with gimple hiearchy. It is hard to fix resx?  I
> wonder if we have other cases.  I guess for a=b/c we are luck just
> because gimple_assign can also be load or store so it has memory_ops...

Fixing resx would come at the cost of deriving from _with_ops, but not
sure if that waste of space is too important.

Richard.

> Thanks,
> Honza
> 

-- 
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)