On Feb 15, 2012, at 1:19 AM, Dave Zarzycki wrote:
> On Feb 14, 2012, at 7:41 PM, Howard Hinnant <[email protected]> wrote:
>
>> void
>> __split_buffer<_Tp, _Allocator>::__destruct_at_begin(pointer __new_begin,
>> false_type)
>> {
>> - while (__begin_ < __new_begin)
>> + while (__begin_ != __new_begin)
>> __alloc_traits::destroy(__alloc(), __begin_++);
>> }
>
> Howard,
>
> Are there other places in libcxx where we should replace relational pointer
> comparisons with equality pointer comparisons?
Almost certainly, but I have not found those places yet.
> Also, what does the latest C++ specification say about what compilers may or
> may not do with relational pointer comparisons? What does clang do? I've been
> searching the web and I don't feel like I've found a solid answer yet.
I don't believe the C++ standard will say anything.
I believe what is happening here is the loop:
while (__begin_ != __new_begin)
__alloc_traits::destroy(__alloc(), __begin_++);
When __alloc_traits::destroy is a no-op, is being transformed into:
__begin_ = __new_begin;
on the rationale that if the loop is an infinite loop, then we've got undefined
behavior, and the only way for the loop to exit is when __begin_ == __new_begin.
With
while (__begin_ < __new_begin)
The loop can exit with __begin_ >= __new_begin, and the compiler can't tell
that it will always be the case that __begin_ == __new_begin on loop exit.
Disclaimer: This is all speculation on my part. I know next to nothing about
compiler optimizations.
Howard
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