Fix PR41573, remove pass_remove_useless_stmts

[Richard Guenther]

On Wed, Oct 7, 2009 at 2:29 PM, Michael Matz <matz@suse.de> wrote:
> Hi,
>
> remove_useless_stmts folds statements at a time where not the whole
> optimization engine is initialized yet, which causes the segfault of the
> above bugreport.  Now, instead of just fixing the symptom I thought maybe
> we can get rid of the whole pass instead.  Digging around it seems to have
> been rotting away for some years already, while occasionally making it
> remove less stmts or fixing other bugs, somewhen being split into
> cfg_remove_useless_stmts, which shortly after was removed again.  It
> survived tuplification living its meagre life on and on.
>
> To determine its meagreness I added some statistics code to see what it
> actually still removes and measured a whole bootstrap cycle with testsuite
> (all languages+Ada), and this is the result:
>
> number of compiler invocations: 70332
> overall number of repeats:     214136
>  (When something is changed by r_u_s it repeats the scan of all insns
>  again)
> overall insns visited:   1025945788
>  of those changed at all:   1111266 (a whopping promille)
> nr of conditional insns:    4136111
>  of those trivial true:       84084
>         trivial false:      152987
>        same destination:         8 (wow)
> nr unconditional gotos:   244390760
>  of those fallthru:          478812
> nr of GIMPLE_BIND:          2268987
>  of those removed:           304083
> nr of TRY_FINALLY:           201032
>  of those empty eval:         17130
>          empty cleanup:       2090
>         nonbranching body:    7095
> nr of TRY_CATCH:             178155
>  of those empty cleanup:          0
>    or non-throwing body:     61094
> nr of stale GIMPLE_NOP:        3883
> nr of other insns:        774766860
>
> So, it removes a few gotos, but that will also be done shortly after this
> pass in optimization (and could be done in the gimplifier already).  It
> removes some GIMPLE_BIND, where all of them will go away two passes later
> anyway.  It does some cleanup on the exception constructs, but with rths
> rewrite to expand them on trees something equivalent will be done three
> passes later.
>
> So, I removed the whole thing.  tramp3d compiles even a bit faster on -O0
> now.  Bootstrap times are unaffected.
>
> Doing this without regressions requires some minor surgery, mostly due to
> the pass formerly calling fold on all statements:
> * the builtin folders should use, well fold, not buildX directly, where
>  applicable
> * exception lowering should accept empty cleanups and do the obvious thing
> * exception region copying (used by omp lowering) should deal gracefully
>  with empty labels (left over when expanding trivial regions sometimes,
>  until cleanup_eh)
> * free_lang_data shouldn't null out DECL_INITIAL for static constants
>  (and the LTO streamer then needs to accept this)
> * LTO streamer should deal with GIMPLE_NOP (they're left until expand
>  with -O0)
>
> Regstrapped on x86_64-linux, no regressions, all languages+Ada.  Okay for
> trunk?

Ok with ...

> --- testsuite/gcc.dg/tree-ssa/foldstring-1.c    (revision 152447)
> +++ testsuite/gcc.dg/tree-ssa/foldstring-1.c    (working copy)
> @@ -1,5 +1,5 @@
>  /* { dg-do compile } */
> -/* { dg-options "-O1 -fdump-tree-useless" } */
> +/* { dg-options "-O1 -fdump-tree-fre" } */
>
>  void
>  arf ()
> @@ -7,5 +7,5 @@ arf ()
>   if (""[0] == 0)
>     blah ();
>  }
> -/* { dg-final { scan-tree-dump-times "= 0;" 1 "useless"} } */
> -/* { dg-final { cleanup-tree-dump "useless" } } */
> +/* { dg-final { scan-tree-dump-times "= 0;" 1 "fre"} } */
> +/* { dg-final { cleanup-tree-dump "fre" } } */

Changing that to scan the gimple dump, XFAILing it and file
a missed-optimization PR for fold / gimplification.

Oh, and please wait some 24h in case somebody has strong
reasons to keep that pass.

Thanks,
Richard.