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Date: Mon, 02 Mar 2015 18:39:22 +0100
From: Jan Kiszka <jan.kiszka@siemens.com>
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Subject: Re: [Xenomai] [RFC] Consolidated exception prologue/epiloge for x86
 and ARM
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To: Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>
Cc: Xenomai <xenomai@xenomai.org>

On 2015-02-27 22:24, Gilles Chanteperdrix wrote:
> On Fri, Feb 27, 2015 at 10:21:30PM +0100, Gilles Chanteperdrix wrote:
>> On Fri, Feb 27, 2015 at 09:37:45PM +0100, Jan Kiszka wrote:
>>> On 2015-02-27 21:27, Gilles Chanteperdrix wrote:
>>>> On Fri, Feb 27, 2015 at 09:12:14PM +0100, Jan Kiszka wrote:
>>>>> Hi,
>>>>>
>>>>> just pushed a first implementation of the general model that I proposed
>>>>> for exception handling. You can find it at
>>>>>
>>>>> http://git.xenomai.org/ipipe-jki.git/log/?h=queues/trap-rework
>>>>
>>>> Also, the clean way to pass virtual + physical flags is to use
>>>> arch_mangle_bits. Using two longs (potentially 128 bits then) is
>>>> completely useless since one of the longs simply has one significant
>>>> bit.
>>>
>>> The costs of mangling is higher than using two regs for passing that
>>> data as-is, both binary and LOC-wise (tried it). Plus the code is more
>>> readable.
>>
>> That is false on ARM. On ARM gcc does not pass structs by values in
>> registers. The values get passed on stack.
> 
> Sorry, misread the assembler. They are passed by registers, however
> the registers get uselessly saved on stack, then restored to other
> registers. 
> 
> struct foo {
> 	int x;
> 	int y;
> };
> 
> int f(struct foo f)
> {
> 	return f.x + f.y;
> }
> 
> Gives, with -Os:
> 00000000 <f>:
>    0:   b082            sub     sp, #8
>    2:   ab02            add     r3, sp, #8
>    4:   e903 0003       stmdb   r3, {r0, r1}
>    8:   e89d 0009       ldmia.w sp, {r0, r3}
>    c:   4418            add     r0, r3
>    e:   b002            add     sp, #8
>   10:   4770            bx      lr
> 

Ouch. I missed that this sneaked in.

The complications with the existing mangle functions are that they do
not play well with what I need for the existing
ipipe_restore_root_nosync. I can open-code the latter (size increases),
extend the former to alternatively return architectural flags (instead
of boolean), or provide another wrapper to convert the virt bit into flags.

Hmm, or - and that's probably cleanest - I simply align
ipipe_restore_root_nosync to ipipe_restore_root argument-wise. The
latter takes "x" (stall) as boolean, the former as architectural flags.
That's highly confusing anyway. And it seems there are no users to break
in Xenomai, despite that it is exported to modules.

Jan

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