[Bug ipa/98834] [10/11 Regression] Code path incorrectly determined to be unreachable

["jamborm at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98834

Martin Jambor <jamborm at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hubicka at gcc dot gnu.org

--- Comment #6 from Martin Jambor <jamborm at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #4)
> Started with r10-3106-g46dfa8ad6c18feb45d35734eae38798edb7c38cd
> Anyway, I wonder if this isn't similar to the cases where the inliner
> optimistically assumed that __builtin_constant_p will fold to true but
> didn't actually fold it that way, and then later on that didn't happen?

It is exactly that.  We have one __builtin_constant_p that is called
if another, previous __builtin_constant_p returns false.  Inlining
assumes the previous one will be folded to true because it knows it is
called on a constant, albeit one read from an aggregate, and therefore
assumes the latter will never happen.  But after inlining, the
situation looks like this (some statements omitted):

  <bb 2> [local count: 1073741824]:
  MEM <intD.9> [(struct simdD.2582 *)&__xD.2752] = 0;
  MEM <intD.9> [(struct simdD.2582 *)&__xD.2752 + 4B] = 0;
  ...
  __xD.2820 = __xD.2752._M_dataD.2591;
  ...
  __xx_10 = MEM <intD.9> [(struct _TupleD.2456 *)&__xD.2820];
  _11 = __builtin_constant_p (__xx_10);

And we need a SRA + CCP to make the constant travel from the first
assignment to the __builtin_constant_p (__xx_10).  And SRA does take
place only after VRP which already decides to fold
__builtin_constant_p to false.

A quick and dirty (and potentially regression causing) way to fix that
is to declare __builtin_constant_p not working on aggregate values:

diff --git a/gcc/ipa-fnsummary.c b/gcc/ipa-fnsummary.c
index 18bbae145b9..c319323b31e 100644
--- a/gcc/ipa-fnsummary.c
+++ b/gcc/ipa-fnsummary.c
@@ -1638,7 +1638,8 @@ set_cond_stmt_execution_predicate (struct
ipa_func_body_info *fbi,
       || gimple_call_num_args (set_stmt) != 1)
     return;
   op2 = gimple_call_arg (set_stmt, 0);
-  if (!decompose_param_expr (fbi, set_stmt, op2, &index, &param_type,
&aggpos))
+  if (!decompose_param_expr (fbi, set_stmt, op2, &index, &param_type, &aggpos)
+      || aggpos.agg_contents)
     return;
   if (!aggpos.by_ref)
     add_builtin_constant_p_parm (summary, index);

(the last condition would then also need to be turned into a comment
...and possibly an assert).  But it feels like too big a hammer.

Alternatively, we could just resolve the builtin at IPA time.  We'd
need to store the predicate derived from its argument to
ipa_call_summary and then at IPA time redirect it to some
__builtin_true_p when IPA figures out it has to be true, which we
would then redirect in cgraph_edge::redirect_call_stmt_to_callee.
Honza, would that be a good idea?