linux-kernel - Re: [PATCH] proc: calculate end pointer for /proc/*/* lookup at compile time

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Message-ID: <20190130055501.GA2115@avx2>
Date:   Wed, 30 Jan 2019 08:55:01 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] proc: calculate end pointer for /proc/*/* lookup at
 compile time

On Tue, Jan 29, 2019 at 02:18:48PM -0800, Andrew Morton wrote:
> On Mon, 14 Jan 2019 23:04:23 +0300 Alexey Dobriyan <adobriyan@...il.com> wrote:
> 
> > Compilers like to transform loops like
> > 
> > 	for (i = 0; i < n; i++) {
> > 		[use p[i]]
> > 	}
> > 
> > into
> > 	for (p = p0; p < end; p++) {
> > 		...
> > 	}
> > 
> > Do it by hand, so that it results in overall simpler loop
> > and smaller code.
> > 
> > Space savings:
> > 
> > 	$ ./scripts/bloat-o-meter ../vmlinux-001 ../obj/vmlinux
> > 	add/remove: 0/0 grow/shrink: 2/1 up/down: 4/-9 (-5)
> > 	Function                                     old     new   delta
> > 	proc_tid_base_lookup                          17      19      +2
> > 	proc_tgid_base_lookup                         17      19      +2
> > 	proc_pident_lookup                           179     170      -9
> > 
> > Note: this trick bloats readdir, so don't do it :-\
> 
> I don't understand the Note:.  Can you please expand?

The same could be done to proc_pident_readdir(), but the code becomes
bigger for some reason.