linux-kernel - Re: Patches for REALLY TINY 386 kernels

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Date:	Mon, 30 Jul 2007 18:59:33 +0100
From:	"Denis Vlasenko" <vda.linux@...glemail.com>
To:	"Andi Kleen" <andi@...stfloor.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, "Matt Mackall" <mpm@...enic.com>,
	"Jonathan Campbell" <jon@...dgrounds.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Patches for REALLY TINY 386 kernels

On Wednesday 18 July 2007 22:04, Andi Kleen wrote:
> Better just write less bloated code. Perhaps mandatory bloatometer
> runs during -rc*s for kernels with minimal config with public code pig shame lists
> similar to the regression lists are useful. Anyone volunteering?
>
> I suspect there is also much more low hanging fruit of this around.

Thousands "static int flag" variables taking 4 bytes where 1 byte
(actually 1 bit) would suffice. And when you do "flag = 1" -
store insns for bytes are also shorter by 3 bytes, _each_.

Unused code/data linked in
(-ffunction-sections -fdata-sections -Wl,--gc-sections may help)

int global_n; char global_c; int global_m;
and you lose 3 bytes to alignment.
(How to instruct linker to sort sections by alignment or at least for size?
Tried -ffunction-sections -fdata-sections -Wl,--sort-section,alignment
but it seems to only (try to) sort .data, not .data.var_name sections)

Massive inlining. Example: more than 80k of bloat in aic7*xx driver
because of gigantic inlined I/O access functions.

Sadistic alignment by gcc for structs/strings >= 32 bytes.
(gcc 4.2.1 is better, just don't forget -mpreferred-stack-boundary=2)
--
vda
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