Subject: Re: [PATCH 2/2] ftrace: support for PowerPC

From: Scott Wood <[email-masked]>
Date: Thu, 15 May 2008 11:48:02 -0500

On Wed, May 14, 2008 at 10:28:57PM -0700, David Miller wrote:
> From: Steven Rostedt <[email-masked]>
> Date: Wed, 14 May 2008 23:49:44 -0400
> 
> > +#ifdef CONFIG_FTRACE
> > +#ifdef CONFIG_DYNAMIC_FTRACE
> > +_GLOBAL(mcount)
> > +_GLOBAL(_mcount)
> > +	stwu	r1,-48(r1)
> > +	stw	r3, 12(r1)
> > +	stw	r4, 16(r1)
> > +	stw	r5, 20(r1)
> > +	stw	r6, 24(r1)
> > +	mflr	r3
> > +	stw	r7, 28(r1)
> > +	mfcr	r5
> > +	stw	r8, 32(r1)
> > +	stw	r9, 36(r1)
> > +	stw	r10,40(r1)
> > +	stw	r3, 44(r1)
> > +	stw	r5, 8(r1)
> 
> Yikes, that's really expensive.
> 
> Can't you do a tail call and let the function you end
> up calling do all the callee-saved register pops onto
> the stack?

The PPC32 ABI seems to (unfortunately) suggest that, with mcount, all
registers are callee-saved (except for the modifiable-during-function-linkage
registers like r0, r11, and r12) -- so mcount has to save the registers that
the callee won't (because they're normally volatile).

-Scott