[Bug middle-end/102930] equal values appear to be different due to missing correct rounding in libc

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102930

--- Comment #4 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Mon, 25 Oct 2021, vincent-gcc at vinc17 dot net via Gcc-bugs wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102930
> 
> --- Comment #3 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
> (In reply to Jakub Jelinek from comment #2)
> > I think there is nothing that can be done about this, and something like
> > this has been there since forever.  While perhaps some double routines in
> > glibc used to be at some point correctly roundded, many others (e.g. float
> > or long double) never were, there is always a chance you get different
> > results from compile time folding vs. runtime evaluation.
> 
> In this case, for its optimizations, GCC shouldn't assume that these results
> are equal.

Indeed.  It's fine that different executions of a call to such a function 
*in the abstract machine* return different values.  But each execution of 
such a call in the abstract machine must return some particular value 
(possibly different for different executions); it's not valid to optimize 
the code in a way that's inconsistent with a single return value for each 
call to the function in the abstract machine.  (Likewise for other cases 
discussed in other bugs such as out-of-range floating-point-to-integer 
conversions.)