This is the mail archive of the gcc-patches@gcc.gnu.org mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: Bit twiddling builtins

> Date: Mon, 3 Feb 2003 14:49:21 -0800
> From: Richard Henderson <rth@redhat.com>
> Cc: Falk Hueffner <falk.hueffner@student.uni-tuebingen.de>,
>    gcc-patches@gcc.gnu.org

> On Mon, Feb 03, 2003 at 01:54:49PM -0800, Geoff Keating wrote:
> > On powerpc, one of the most important uses of clz is that you can write
> > 
> > __builtin_clz(x) >> 5
> > 
> > instead of
> > 
> > x == 0
> > 
> > which allows removal of the branch; but this doesn't work if clz
> > doesn't return 32 at 0.
> 
> So what?  How do you plan on communicating this to the user program?

I don't really care about the user program; I'm much more interested
in communicating with the middle-end of the compiler so it can apply
the above transformation itself and the user program doesn't have to know.

-- 
- Geoffrey Keating <geoffk@geoffk.org>
Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]