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

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Message-Id: <551EA412-3D35-4BCD-ACAE-B6A7BE3CBF9B@kernel.crashing.org>
Date:	Tue, 10 Jul 2007 22:08:21 +0200
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andi Kleen <ak@...e.de>, 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
>>
>> Still it seems quite fragile to me agreed.
>
> The alternative, of course, is to compile to an .s file and insert
> .code16gcc into the .s file.  This makes the Makefile uglier, but  
> would
> be more resilient against oddball gcc changes.

This would be even more fragile.  The exact format of GCC's
assembler code output isn't defined at all, so in principle
this is a hopeless task.  In practice just putting the
.code16gcc directive on the first line would likely work
though, GCC never generates a .code32 AFAIK, but it isn't
guaranteed that this will work (or will keep working).

> I'm a bit surprised about the claim w.r.t. -fno-unit-at-a-time  
> (although
> I guess that is the default and one would thus typically not see  
> this.)

-fno-unit-at-a-time is the default on three year old compilers,
yes.  Newer compilers have unit-at-a-time enabled by default.

>  Got any pointers why that would cause a global asm() to be scrambled
> around?

An asm() outside of a function is a top-level thing, just
like functions and file-scope variable definitions, and with
unit-at-a-time compilation all such blocks can be reordered
(or even omitted, or inlined, or whatever).  Note that even
before unit-at-a-time was introduced there was no guarantee
of emitting everything in the order it appears in the source
file.


Segher

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