On 9/21/06, Brian Paul <[EMAIL PROTECTED]> wrote:
> Jerome Glisse wrote:
> > On 9/21/06, Brian Paul <[EMAIL PROTECTED]> wrote:
> >
> >> Jerome Glisse wrote:
> >> > Hi,
> >> >
> >> > Roland pointed me to this yesterday (src/mesa/main/glheader.h):
> >> > /*
> >> > * Either define MESA_BIG_ENDIAN or MESA_LITTLE_ENDIAN.
> >> > * Do not use them unless absolutely necessary!
> >> > * Try to use a runtime test instead.
> >> > * For now, only used by some DRI hardware drivers for color/texel
> >> packing.
> >> > */
> >> >
> >> > So i am wondering what is best from a driver point of view,
> >> > use this macro or use a runtime test ? Maybe this comment
> >> > isn't revealent anymore. Any insight on this are welcome.
> >>
> >> I'd prefer to use a run-time test since it's a little more robust (one
> >> less preprocessor symbol to worry about). Though in some places we
> >> really do need a compile-time symbol.
> >>
> >> Here's the code used in core Mesa:
> >>
> >> const GLuint ui = 1;
> >> const GLubyte littleEndian = *((const GLubyte *) &ui);
> >>
> >> I should probably put that into a reusable inline function in
> >> imports.h ...
> >>
> >> -Brian
> >>
> >
> > Would be nice to have a reusable function (i am a bit lazy :))
>
> Done.
>
> Interestingly, these two variations produce different code (gcc -O2):
>
> static INLINE GLboolean
> _mesa_little_endian(void)
> {
> static const GLuint ui = 1; // NOTE: static
> return *((const GLubyte *) &ui);
> }
>
> static INLINE GLboolean
> _mesa_little_endian(void)
> {
> const GLuint ui = 1;
> return *((const GLubyte *) &ui);
> }
>
>
> If you have code such as:
>
> if (_mesa_little_endian())
> foo();
> else
> bar();
>
>
> The version without 'static' is evaluated at compile time and the
> branch code is optimized away. With 'static' the compiler still
> generates the conditional code.
>
> -Brian
>
Compiler parser voodoo :) Thanx, i will use that for now, and will
correct code.
best,
Jerome Glisse
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