Re: [PATCH] Make strbuf_cmp inline, constify its arguments and optimize it a bit

From: Jeff King <peff@peff.net>
Date: Oct 8, 2007, 02:19 UTC
Message-ID: <20071008021945.GC20050@coredump.intra.peff.net>
In-Reply-To: <20071007215749.GD2765@steel.home>

On Sun, Oct 07, 2007 at 11:57:49PM +0200, Alex Riesen wrote:

Show 6 quoted lines

> > > Can't the result of the expression be reused in compiled?
> > > Isn't it a common expression?
> > 
> > No, since the call to memcmp might change a->len or b->len.  A
> 
> Huh?! How's that? It is not even given them!

But they are non-local variables (they are part of structs passed in as pointers), so that translation unit has no idea how they are allocated. They could be globals that memcmp mucks with as a side effect.

That being said, standards-conforming compilers _can_ realize that memcmp is a special, standards-defined function with no side effects and act accordingly. gcc provides the 'pure' function attribute for this purpose, which is used by glibc.

-Peff