>
> <http://sourceware.org/ml/libc-alpha/2013-05/msg00389.html>
>
> The function is in glibc's math/atest-exp2.c file.
I see, I was curious what made LLVM developers to implement the feature about
making
memory writes unreachable. While I see wild interpretation of the documentation
allows it, I would not anticipate much benefits.
It is easy to add a warning (we analyze the function for autodetecting the
attribute
and it is easy to warn when the properties are not matched).
With lowlevel stuff, like GLIBC, it is IMO perfectly sane to make memoizing
function
const/pure when knowing it is never going to be called from same unit.
I.e. to arrange headers to make the function not const/pure when compiling the
unit containing it and const/pure for the external interface. The memoization
is completelly hidden from the compiler when linking application with GLIBC.
>
> >BTW we deduce all loops to be finite within const/pure functions that is also
> >bit crazy effect of the attribute.
>
> Uhm, okay.
>
> >The memoization you mention is IMO not really safe even with current GCC.
> >With
> >bit of trickery one can convince GCC to early inline the memoizing const
> >function in some cases and not in others. Optimizers will then expect your
> >memoizing cache to not change across the non-inlined calls that may lead to
> >wrong code.
>
> Oh.
>
> >At the moment I can not think of anything that would break if you had
> >pure/const
> >function modifying global memory and restoring it before returning.
>
> Well, with a bit of cheating, that's actually easy:
>
> int f1(void) __attribute__((const));
> void f2(int);
> void lock(void);
> void unlock(void);
>
> void
> g()
> {
> for (int i = 0; i < 10; ++i) {
> lock();
> f2(f1());
> unlock();
> }
> }
Yeah, that is cheating though ;) Either your program should be non-threaded
or you pure/const function should be thread safe ;)
Honza
