x86 weirdness

Erwin Rol mailinglists at erwinrol.com
Tue Jul 31 12:16:29 EDT 2012

On 31-7-2012 17:51, Jean-Christophe PLAGNIOL-VILLARD wrote:
> On 17:16 Tue 31 Jul     , Erwin Rol wrote:
>> Hey Sascha,
>> On 31-7-2012 15:13, Sascha Hauer wrote:
>>> Hi Erwin,
>>> On Tue, Jul 31, 2012 at 12:49:29PM +0200, Erwin Rol wrote:
>>>> Hey x86-asm-experts,
>>> Surely you do not mean me ;)
>>>> On 31-7-2012 0:36, Erwin Rol wrote:
>>>> So when I use the memset code the asm looks like this below. As you
>>>> can see the memset gets its 3 parameters from the stack (see
>>>> comments below in the asm src). But when you look at the location
>>>> when memset is called it looks like the caller tries to pass the
>>>> parameters in registers. After looking into the Makefile I noticed
>>>> the arch/x86/boot/* sources get compiled with -mregparm=3, but the
>>>> other sources use normal calling convention. This means code from
>>>> arch/x86/boot/* can not call any other barebox functions when they
>>>> are not of type void f(void). The file arch/x86/boot/regs.c also
>>>> calls memset but it seems the compiler notices that regs.c is 16bit
>>>> code and so it inlines a 16bit memset version (I at least don't see
>>>> a call function).
>>>> Is there any GCC or LD magic we can use to prevent error like this?
>>>> Wouldn't it be better to _not_ change the calling convention in the
>>>> Makefile, but mark the functions that need it with __attribute__ ?
>>> Maybe it's easier and more straight forward to just use a for-loop here
>>> instead of playing magic games with attributes and or flags? Are there
>>> more functions affected or is it just memset?
>> There seem to be two memset calls, that could be problematic. Both
>> should be easily replaceable with for-loops (like I already did by
>> the first).
>> We just have to be very careful to not call any functions from
>> outside the boot dir (expect for the main barebox entry point).
>> Don't you love the weird x86 world :-)
> so I guess we should update the x86 to use the pbl so we can have two explicit
> part of binary and we can avoid those issue

One thing to know about the x86 support is that it uses (16bit) bios 
calls to load data from disk and print info on the (vga) screen. These 
calls are wrapped with special code to change from 32bit to 16bit and 
back. So I have no idea of "pbl" works with this very special x86 code.

- Erwin

> Best Regards,
> J.

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