Re: [PATCH v6 1/9] ppc64 (le): prepare for -mprofile-kernel

On Wed, Jan 27, 2016 at 11:28:09PM +1030, Alan Modra wrote:

On Wed, Jan 27, 2016 at 09:19:27PM +1100, Michael Ellerman wrote:

quoted

Can we use r11 instead? eg:

_GLOBAL(_mcount)
	mflr	r11
	mtctr	r11
	mtlr	r0
	bctr

Depends on what you need to support.  As Torsten says, the code to
call _mcount when -mprofile-kernel is emitted before the prologue of a
function (similar to -m32), but after the ELFv2 global entry point
code.  If you trash r11 here you're killing the static chain pointer,
used by C nested functions or other languages that use a static chain,
eg. Pascal.  r11 has *not* been saved for ELFv2.

Even if nested functions aren't supported in the Linux kernel(?), I think
it was an earlier version of mcount when r11 usage ruined my day.

r12 might be a better choice for a temp reg.

Good idea. r12 holds the function entry point and is used to calculate the
new TOC value just _before_ the call. It should be available.
I'll try, thanks for the hint.

	Torsten