Usage of ANSI-C |const| in X11 code...
lars at trolltech.com
Sat Mar 26 01:22:55 PST 2005
On Saturday 26 March 2005 03:40, Ian Romanick wrote:
> Roland Mainz wrote:
> > Adam Jackson wrote:
> >>On Friday 25 March 2005 16:12, Roland Mainz wrote:
> >>>AFAIK on some platforms (SPARC comes in mind) it makes a huge difference
> >>>as values are passed in registers (unless all registers are used up) and
> >>>not via the stack (and no stack version is provided).
> >>Yes, this is true on x86 too. However the register keyword has nothing
> >> to do with function calling conventions.
> > AFAIK it depends... some compilers may just modify the calling
> > conventions to fit their needs if they can control all the callers, too
> > (for example when the compiler has a global view about the whole
> > application in the Sun Workshop XIPO case or for |static| functions).
> I have some experience with this on GCC and libGL. Basically, GCC will
> inline, use register parameters, and do other tricks (e.g., use jumps
> instead of call / ret pairs) on functions that are static to a file and
> that are never referenced via a pointer.
> In addition, there are ways to force register based parameter passing on
> x86. Using the "register" keyword is *NOT* one of them. The way that I
> have used most often is the fastcall attribute. The regparm can also be
> used, but I'm less familiar with it. Basically, if a function is
> declared as:
> __attribute__((fastcall)) extern void foo( int arg1, int arg2,
> float arg3, int arg4 );
> the first 3 parameters will be passed in registers, and the remaining
> parameters will be passed on the stack. This is used extensively in the
> autogenerated code for GLX protocol.
> The way that I have done this is by creating a macro called FASTCALL
> based on some GCC pre-defines:
> #if defined(__i386__) && defined(__GCC__)
> # define FASTCALL __attribute__((fastcall))
> # define FASTCALL
> x86 is the only common arch that I know of that doesn't use register
> parameter passing by default, so I didn't bother to try and extend it
> for anything else. The other down side is that not all GCC builds seem
> to support the fastcall attribute even on x86. The result is a warning
> that the attribute is ignored. Since this attribute is only used on
> functions that are internal, I didn't worry about ABI issues (i.e., the
> function built with fastcall supported, but the caller built without).
IIRC __attribute__((regparm(3))) instead of __attribute__((fastcall)) is
supported on most gcc compilers (at least on 2.95 and upwards).
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