[Bug c++/86173] Default construction of a union (in std::optional)

2019-09-27 Thread glisse at gcc dot gnu.org 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86173

--- Comment #5 from Marc Glisse  ---
A similar example was reported on https://stackoverflow.com/q/57964217/1918193

[Bug c++/86173] Default construction of a union (in std::optional)

2018-06-20 Thread glisse at gcc dot gnu.org 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86173

--- Comment #4 from Marc Glisse  ---
Recent related commits: r261758 r261735 (they don't fix the issue).

[Bug c++/86173] Default construction of a union (in std::optional)

2018-06-18 Thread ebotcazou at gcc dot gnu.org 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86173

--- Comment #3 from Eric Botcazou  ---
> There's also the following weak heuristic that might kick in if
> CONSTRUCTOR_NO_CLEARING would be set:
> 
> else if (num_ctor_elements - num_nonzero_elements
>  > CLEAR_RATIO (optimize_function_for_speed_p (cfun))
>  && num_nonzero_elements < num_ctor_elements / 4)
>   /* If there are "lots" of zeros, it's more efficient to clear
>  the memory and then set the nonzero elements.  */
>   cleared = true;
> 
> with CONSTRUCTOR_NO_CLEARING this heuristic is off by not honoring
> the constructor elements being not present (but for the testcase it
> doesn't matter).  CCing Eric for this specific issue (not the C++ one).

Ugh.  I didn't write this but, yes, there is an oversight, the check on the
flag should probably be on entry instead:

  if (CONSTRUCTOR_NO_CLEARING (ctor))
cleared = false;
  else if (int_size_in_bytes (TREE_TYPE (ctor)) < 0)
   ...
  else if

[Bug c++/86173] Default construction of a union (in std::optional)

2018-06-18 Thread rguenth at gcc dot gnu.org 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86173

Richard Biener  changed:

   What|Removed |Added

 Status|UNCONFIRMED |NEW
   Last reconfirmed||2018-06-18
 CC||ebotcazou at gcc dot gnu.org
 Ever confirmed|0   |1

--- Comment #2 from Richard Biener  ---
This is because of gimplification interpreting a CONSTURCTOR with missing
elements as to clear them unless CONSTRUCTOR_NO_CLEARING is set (which isn't).

So the GENERIC _doesn't_ look good since it says (implicitely) that 'a' is
zeroed.

Confirmed as C++ issue.

There's also the following weak heuristic that might kick in if
CONSTRUCTOR_NO_CLEARING would be set:

else if (num_ctor_elements - num_nonzero_elements
 > CLEAR_RATIO (optimize_function_for_speed_p (cfun))
 && num_nonzero_elements < num_ctor_elements / 4)
  /* If there are "lots" of zeros, it's more efficient to clear
 the memory and then set the nonzero elements.  */
  cleared = true;

with CONSTRUCTOR_NO_CLEARING this heuristic is off by not honoring
the constructor elements being not present (but for the testcase it
doesn't matter).  CCing Eric for this specific issue (not the C++ one).

[Bug c++/86173] Default construction of a union (in std::optional)

2018-06-16 Thread glisse at gcc dot gnu.org 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86173

--- Comment #1 from Marc Glisse  ---
Note that constructing optional from std::nullopt does avoid the memset.