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Subject: Re: [ub] launder and aliasing
From: Hubert Tong (hubert.reinterpretcast_at_[hidden])
Date: 2016-02-26 08:26:27


On Fri, Feb 26, 2016 at 7:12 AM, Robert Haberlach <rh633_at_[hidden]> wrote:

> I apologize in advance if this was discussed before; I didn't check the
> entire archive. Consider
>
> static_assert(alignof(float) >= alignof(int) && sizeof(float) >=
> sizeof(int));
> int foo(float f) {
> return *std::launder(reinterpret_cast<int*>(&f)); }
>
> As it stands, invocation of foo is undefined, as the argument to launder
> is not pointing to an object of type int (or similar) within its lifetime,
> violating launder's requirement.
>
> launder is designed to inhibit address propagation analysis, which is the
> only concerning optimization. Moreover, as long as all usual conditions are
> met (alignment, size & trap values), this should be fine on any
> implementation. If so, can we make the above formally well-defined?
>
This is not fine. Type-based aliasing analysis (TBAA) is allowed to
determine that the write to "f" and the read from the dereference is not
related.
Consider the effects of inlining:
int main(void) { return !foo(0.5f); }

becomes (loosely):

int main(void) {
  int __ret;
  {
    float __f = 0.5f;
    __ret = *std::launder(reinterpret_cast<int *>(&__f));
  }
  return !__ret;
}

The implementation is allowed to observe that there are no TBAA-compatible
reads of the value of __f within its lifetime. The likely result is that
the initialization of __f would be optimized away.

-- HT

Being able to use launder in such scenarios would render circumlocutions
> via memcpy superfluous.
>
> Robert
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> ub_at_[hidden]
> http://www.open-std.org/mailman/listinfo/ub
>



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