Re: * 4 converted to << 2 for networking code

["H. Peter Anvin" <hpa@zytor.com>]

Followup to:  <20010110161146.A3252@unthought.net>
By author:    =?iso-8859-1?Q?Jakob_=D8stergaard?= <jakob@unthought.net>
In newsgroup: linux.dev.kernel
> 
> On most processors <<2 is slower than *4.
> 

That's a funny statement.  Which processors do you include in "most"?
That has not been my experience.

> It's outright stupid to write <<2 when we mean *4 in order to optimize for one out of a
> gazillion supported architectures - even more so when the compiler
> for the one CPU where <<2 is faster, will actually generate a shift
> instead of a multiply as a part of the standard optimization.
> 
> One question for the GCC people:  Will gcc change <<2 to *4 on other 
> architectures ?    If so, then my case is not quite as strong of course.
> 

gcc should consider the statements equivalent, and generate whichever
pattern is preferred.  On an i386 that may mean take a pattern such as

	foo = (bar << 2) + quux;

... and generate ...

	lea ecx,[esi*4+ebx]

	-hpa
-- 
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
http://www.zytor.com/~hpa/puzzle.txt
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
Please read the FAQ at http://www.tux.org/lkml/