linux-kernel - Re: [x86 setup 13/33] Header file to produce 16-bit code with gcc

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Message-Id: <D6BECE27-25FA-4914-B293-4070392258D2@kernel.crashing.org>
Date:	Tue, 10 Jul 2007 21:57:11 +0200
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Andi Kleen <ak@...e.de>
Cc:	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [x86 setup 13/33] Header file to produce 16-bit code with gcc

>>> gcc for i386 can be used with the assembly prefix ".code16gcc" to
>>> generate
>>> 16-bit (real-mode) code.  This header file provides the assembly
>>> prefix.
>>
>> This only works correctly with newer GCCs if you pass the
>> -fno-toplevel-reorder option (and it only works on older
>> GCC versions by accident).
>
> And on older ones (gcc-3.3-hammer / 3.4 / 4.0 ) -fno-unit-at-a-time

...which inhibits even more optimisations.  Still not too
important, but the code size difference probably is
noticeable.

> Still it seems quite fragile to me agreed.

Well at least with -fno-toplevel-reorder it is guaranteed
to work (not the same thing as "is working", heh, but fairly
close).

It seems to me GCC should grow an option to insert .code16gcc
by itself (-m16 perhaps?)  Or GAS could get a flag to include
a file as a prelude before the main thing.  Neither will help
Linux right now of course.


Segher

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