[Bug c++/97151] GCC fails to optimize away uselessly allocated arrays (C++)

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97151

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Marat Radchenko from comment #4)
> (In reply to Martin Liška from comment #3)
> > It's caused by what was mentioned as 2) which transforms to:
> > 
> >   _6 = operator new [] (40);
> >   __builtin_memset (_6, 0, 40);
> >   operator delete [] (_6);
> > 
> > So there's a use of _6 and we probably can't remove it as memset can
> > potentially access NULL pointer if operator new [] fails.
> 
> 1. If operator new[] fails, there would be an exception instead of NULL, no?
> 
> 2. It doesn't matter, you may guard accessing array with NULL check and
> still observe described behavior.
> 
> What puzzles me the most is why removal of delete[] allows it to optimize.

Very likely delete[] is keeping the memset itself live.  In particular
early CDDCE can elide the loop without the delete[] but with the delete[]
we see

Marking useful stmt: operator delete [] (_7);

so that's a no-go there.  Using malloc/free we manage to delete the loop early
because its argument is not considered necessary.  It looks like points-to
analysis fails to fully handle new/delete and causes the allocated storage
to point to escaped memory:

_7 = &HEAP(10)

new[] handled OK

ESCAPED = _7

delete[] not handled.  One "easy" way would be for the C++ FE to annotate
delete[] with fnspec ".w" to mark the argument as not escaping.  A hackish
patch in PTA also fixes it:

diff --git a/gcc/tree-ssa-structalias.c b/gcc/tree-ssa-structalias.c
index 44fe52e0f65..f676bf91e95 100644
--- a/gcc/tree-ssa-structalias.c
+++ b/gcc/tree-ssa-structalias.c
@@ -4857,6 +4857,8 @@ find_func_aliases_for_call (struct function *fn, gcall
*t)
         point for reachable memory of their arguments.  */
       else if (flags & (ECF_PURE|ECF_LOOPING_CONST_OR_PURE))
        handle_pure_call (t, &rhsc);
+      else if (fndecl && DECL_IS_REPLACEABLE_OPERATOR_DELETE_P (fndecl))
+       ;
       else
        handle_rhs_call (t, &rhsc);
       if (gimple_call_lhs (t))