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    SubjectRe: [PATCH 02/10] ARM: ftrace: document mcount formats
    On Mon, Feb 22, 2010 at 08:20:09PM +0100, Uwe Kleine-König wrote:
    > On Mon, Feb 22, 2010 at 11:36:24PM +0530, Rabin Vincent wrote:
    > > On Sat, Feb 13, 2010 at 09:37:48PM +0100, Uwe Kleine-König wrote:
    > > > On Sun, Feb 14, 2010 at 01:18:30AM +0530, Rabin Vincent wrote:
    > > > > + * With both the mcount types, we need to restore the original lr before
    > > > > + * returning. In the __gnu_mcount_nc, version we're allowed to clobber ip.
    > > > > + * No other registers should be clobbered.
    > > > > + */
    > > > Very nice.
    > > >
    > > > Maybe make the last two sentences:
    > > >
    > > > In the __gnu_mcount_nc case the ip register is clobbered which is OK as
    > > > the calling convention for ARM allow clobbering this value for
    > > > subroutines and it doesn't contain parameters.
    > >
    > > Won't quoting calling conventions here be misleading? The mcounts don't
    > > following normal convention for the other bits (we can't clobber even
    > > the registers that are normally caller-saved, we need to restore lr,
    > > etc.)
    > actually mcount is a function in the middle of a function call. So it
    > must be transparent for both the caller and the callee. As the called
    > function needs to know the return address, mcount must not clobber it.
    > And as ip is don't care and can be clobbered mcount can use it as it
    > likes.

    OK. How about this for the last paragraph:

    mcount can be thought of as a function called in the middle of a
    subroutine call. As such, it needs to be transparent for both the
    caller and the callee: the original lr needs to be restored when
    leaving mcount, and no registers should be clobbered. (In the
    __gnu_mcount_nc implementation, we clobber the ip register. This
    is OK because the ARM calling convention allows it to be clobbered
    in subroutines and doesn't use it to hold parameters.)

    Rabin
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