Re: [Xenomai] [RFC] Consolidated exception prologue/epiloge for x86 and ARM

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

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

-- 
					    Gilles.