[Bug tree-optimization/102676] New: Failure to optimize out malloc/nothrow allocation that's only used for bool checking

[Bug tree-optimization/102676] New: Failure to optimize out malloc/nothrow allocation that's only used for bool checking

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102676

            Bug ID: 102676
           Summary: Failure to optimize out malloc/nothrow allocation
                    that's only used for bool checking
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

#include <new>
#include <stdlib.h>

bool f()
{
  return new(std::nothrow) int;
}

bool g()
{
    return malloc(1);
}

Both these functions can be optimized to `return true;`. This optimization is
done by LLVM, but not by GCC.

[Bug tree-optimization/102676] Failure to optimize out malloc/nothrow allocation that's only used for bool checking

[schwab@linux-m68k.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102676

--- Comment #1 from Andreas Schwab <schwab@linux-m68k.org> ---
Why is malloc(1) always true?

[Bug tree-optimization/102676] Failure to optimize out malloc/nothrow allocation that's only used for bool checking

[gabravier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102676

--- Comment #2 from Gabriel Ravier <gabravier at gmail dot com> ---
Well, I think the assumption LLVM is making is that the allocation, being
unused, can just be eliminated and considered to have always succeeded. I don't
see how that would contradict the standard, although I suppose some would
consider it a bad thing to do for the compiler (although in that case you might
as well rule out all optimizations that elide allocations).

[Bug tree-optimization/102676] Failure to optimize out malloc/nothrow allocation that's only used for bool checking

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102676

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-10-11
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
We don't eliminate a malloc that's just used in a conditional, I think there's
a related bugreport with

  p = malloc (n)
  if (!p)
    abort ();
  free (p);

or sth like that where we fail to elide the allocation.  Note in this case
failing allocation _would_ have a side-effect.