Re: [PATCH] Add workaround for PR64715

[Richard Biener <rguenther at suse dot de>]

On Thu, 26 Mar 2015, Jakub Jelinek wrote:

> On Thu, Mar 26, 2015 at 10:32:18AM +0100, Richard Biener wrote:
> > I think I simply didn't want to change more testcases at that point,
> > but I can't see how followup passes at the very point wouldn't
> > clean things up very quickly (via fre or copyprop).  Eventually
> > it was -Og and pass_fold_builtins interaction (thinking of
> > __builtin_constant_p (__builtin_object_size (...)) or so).
> > If so I'm sure I added a testcase.
> 
> Can't find such a testcase.
> Anyway, the shorter version of the patch regresses:
> 
> struct S { char a[10]; char b[10]; };
> int
> foo (int x)
> {
>   struct S s;
>   char *p = &s.a[4];
>   if (x)
>     p += 1;
>   return __builtin_constant_p (__builtin_object_size (p, 1));
> }
> 
> with -Og, while it works with the longer version.
> 
> Though, say for
>   return __builtin_constant_p (__builtin_object_size (p, 1) + 1);
> 
> we already return 0 for -Og (and not -O2) already in 4.9.

Yeah, I think for -Og ideally both pass_object_sizes and
pass_fold_builtins should run from pass_ccp.  We do have
some pass ordering issue in -Og otherwise.  It's pipeline
would then also simplify to

Index: gcc/passes.def
===================================================================
--- gcc/passes.def      (revision 221689)
+++ gcc/passes.def      (working copy)
@@ -314,13 +314,7 @@ along with GCC; see the file COPYING3.
       NEXT_PASS (pass_lower_complex);
       NEXT_PASS (pass_lower_vector_ssa);
       /* Perform simple scalar cleanup which is constant/copy 
propagation.  */
-      NEXT_PASS (pass_ccp);
-      NEXT_PASS (pass_object_sizes);
-      /* Fold remaining builtins.  */
-      NEXT_PASS (pass_fold_builtins);
-      /* Copy propagation also copy-propagates constants, this is 
necessary
-         to forward object-size and builtin folding results properly.  */
-      NEXT_PASS (pass_copy_prop);
+      NEXT_PASS (pass_ccp_with_objsize_and_fab);
       NEXT_PASS (pass_dce);
       NEXT_PASS (pass_asan);
       NEXT_PASS (pass_tsan);

well, at least if ccp would also propagate copies...  (I'd really
like to unify CCP and copyprop and have some flags to control
what it performs).  So the above would probably need a copyprop
before that pass_ccp_with_objsize_and_fab, or teaching ccp
to also propagate copies (lattice value CONSTANT, value == SSA_NAME
and mask == ~0).

I wouldn't care about this for GCC 5.

Richard.