Re: [Mesa3d-dev] gl_sqrt in mmath.c

[Josh V. <ho...@na...>]

David Konerding <de...@ko...> writes:

> > But with '-fstrict-aliasing', which was used in my auto-generated
> > Makefile for Mesa, the results for sqrt are for some numbers up to
> > 10 times greater than the once generated by gl_sqrt, while the
> > speed for normal sqrt is still higher.
>
> Did you read the man page for gcc and learn what -fstrict-aliasing
> does?  I wouldn't use that feature for math-intensive routines.

I don't know of any reason why math-intensive routines would be
affected by -fstrict-aliasing.  However, it does matter for what gcc
calls 'type punning' (accessing one type as if it were another type),
which Mesa does a lot of.

GCC will handle this ok:

int get_float_bits(float f)
{
        union { int i; float f; } n;

        n.f = f;

        return n.i;
}

But it is allowed to screw this up:

int get_float_bits(float f)
{
        return *(int *)&f;
}

Most of the type puns in mmath use unions, but it looks like there are
several occurrences of *(type *)&var scattered around (one is at the
begining of gl_sqrt()).  Someone should get rid of them so we can
compile with strict aliasing.


Josh