[Bug c++/70178] Loop-invariant memory loads from std::string innards are not hoisted

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70178

--- Comment #3 from rguenther at suse dot de <rguenther at suse dot de> ---
On Fri, 11 Mar 2016, zackw at panix dot com wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70178
> 
> --- Comment #2 from Zack Weinberg <zackw at panix dot com> --- That was 
> my working hypothesis as well.  Isn't there some way we can annotate 
> s->data to reassure the compiler that *this* char* doesn't alias?  I 
> don't know enough about the guts of std::string to know whether 
> 'restrict' is accurate, but it certainly shouldn't be pointing to 
> anything but string contents.

We can use 'restrict', yes, but it will only have an effect if used
on an object a function parameter points to.

We do use it on valarray for example.  Not sure if it is valid for
std::string.

Note that using restrict on data members is poorly specified (if at all)
and I can't find it specified for C++ at all (just for C).  I only
find in 17.2 (C++14) that for C standard library functions
"any use of the restrict qualifier shall be omitted" ...

We do use the restrict feature for fortran arrray descriptors and
it works there (as function parameter).