Re: [PATCH 02/10] ARM: ftrace: document mcount formats

Uwe Kleine-König
2010-02-23T20:28:05Z

Hi Rabin,

> 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.)
I rechecked the document describing the calling convention. It writes
about ip:

Both the ARM- and Thumb-state BL instructions are unable to address the
full 32-bit address space, so it may be necessary for the linker to
insert a veneer between the calling routine and the called subroutine.
Veneers may also be needed to support ARM-Thumb inter-working or dynamic
linking. Any veneer inserted must preserve the contents of all
registers except IP (r12) and the condition code flags; a conforming
program must assume that a veneer that alters IP may be inserted at any
branch instruction that is exposed to a relocation that supports inter-
working or long branches.

So the last sentence isn't completly accurate. But describing the
actual situation is a bit overkill, so I'm OK with your suggestion.

Best regards and thanks
Uwe

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