Re: New -Wall issues

["Chris Fairles" <chris dot fairles at gmail dot com>]

> I'm analyzing some new -Wall failures in the v3 testsuite, and I'm also
> seeing one in your area, 20_util/duration/cons/1.cc, can you have a
> look?

I've looked at this one before, just to provide a reduced testcase:

#include <chrono>

int main() {
  using std::chrono::duration;
  duration<int> d;
  int i = d.count();
  return i;
}

compiled with "-std=c++0x -Wall -O1" gives:
'd.std::chrono::duration<int, std::ratio<1l, 1l> >::__r' is used
uninitialized in this function

Note that with -O0, no warning is issued. It appears with -O1,-O2 and -O3.

The duration default constructor is defined as:
duration() = default;

>From my readings (8.4p9, 12.6.2p4, 8.5p6) __r is default-initialized
which means, for a non-class, non-array type no initialization is
performed leaving __r with an indeterminate value.

My question is, why doesn't -O0 also emit the warning? But first, is
__r really not being initialized? Checking the return value of the
testcase with -O0, I get random values indicating it is indeed not
being initialized ... but -O0 doesn't emit the warning. Now with -01,
the output is always 0, meaning its being zero-initialized or
equivalent but -O1 does emit the warning which is counter-intuitive.

In conclusion, the check 'd.count() == 0' is invalid for a
non-initialized duration<int> because d.count() should return an
indeterminent value and thus it should be removed (i'll provide a
patch in a separate thread) but I'm wondering if another testcase
should check for the warning (i.e. dg-warning) if its suppose to be
emitted with -Wall at all optimization levels.

Out of curiosity, I punched out the tree dump for both -O0 and -O1.
The final -O0 code looks like (modified for brevity):

duration<int, ratio<1,1>>::duration() { return; }
duration<int, ratio<1,1>>::count() { int i; i = this->__r; return i; }

int main() () {
  int i;
  struct duration d;
  int D.5078;
  int i.0;

<bb 2>:
  __comp_ctor  (&d);
  i.0 = count (&d);
  i = i.0;
  D.5078 = i;
  return D.5078;
}

whereas the final -O1 code looks like:

int main() () {
  const int d$__r;

<bb 2>:
  return d$__r;
}

To me, it looks like __r is uninitialized in both cases. the latter it
seems obvious (although, I don't really know what "d$" means). In the
former case, I don't know what "struct duration d;" does with __r and
from what I understand __comp_ctor(&d) just calls duration<int,
ratio<1,1>>::duration() { return; } which doesn't do anything with
__r.

Cheers,
Chris