Thanks for the tips, Corey!
corey taylor wrote:
On Nov 27, 2007 9:09 PM, Raymond Sheh <rsheh@xxxxxxxxxxxxxxx> wrote:
whole thing goes random with -O3. Adding -fno-strict-aliasing makes it
work again but compiling with only -fstrict-aliasing doesn't break it so
it looks like a combination of things.
You don't see warnings with -fstrict-aliasing?
Now that you mention it, turning on "-W -Wall -fstrict-aliasing" does
give a warning about "dereferencing type-punned pointer will break
strict-aliasing rules" (but not if I don't turn on -W -Wall) ... for
some reason I always thought "-W -Wall" warned about everything
regardless of optimiser settings ... chalk one up to experience I guess!
float floatblah(float in)
{
unsigned long retval =
byte_manipulation_function_that_happens_to_take_ulongs(*((unsigned
long*)(&in)));
return *((float*)(&retval));
}
You realize of course, this is invalid for 64-bit since float is
32-bit and unsigned long is 64-bit on 64-bit linux.
Yup ... I'm not sure what the alternative is though given at the end of
the day this has to go through htonl/ntohl in preparation for network
transmission and I'm not aware of any word-length-transparent way of
doing THAT ... of course, it's possibly more due to my ignorance than
there actually not being a way, if there is I'd be most happy to find
out! :-)
Why can't you use the union for type-punning here?
union { float a; unsigned long b; };
a = in;
b = im_not_typing_that(b);
return a;
corey
I could and will probably end up doing that ultimately ... I just was of
the impression that the sort of casting I was using was reasonably
widely accepted and was a little confused when it worked fine up until
when I tried compiling it on a GCC 4.x system (at which point it gave me
random stack-corrupting segfaults - always a pain to debug!) ... ah well
... I notice this sort of casting in a few other libraries I use - and
I've just found that their autoconf'd makefiles all seem to have
-fno-strict-aliasing in them so I might just do that for now so I can
actually get some results!
Cheers!
- Raymond
