[PATCH 1/1 v2] ARM: Thumb-2: Symbol manipulation macros for function body copying

Russell King - ARM Linux linux at arm.linux.org.uk
Fri Jan 14 12:43:39 EST 2011

On Fri, Jan 14, 2011 at 11:15:19AM -0600, Dave Martin wrote:
> There's another problem which I hadn't spotted before:
> In the Thumb case, we risk violating the alignment constraints of the
> code which gets copied (actually, this is also true of the ARM case,
> but less likely to bite).  This matters because the code may contain
> literals and other data words -- quite likely given the requirement
> for self-containedness.

There's a solution to this - require that the thumb function is
preceded by a .align 3 (which according to the GAS documentation I
have means for ARM, it aligns the PC to 1 << 3 not 3 bytes.)

Also require 8-byte alignment from the allocation function, and make
fncpy() bug if the destination isn't 8-byte aligned.  Same for the
source function argument (but ignoring bit 0 of course.)

We don't then have to mess with rounding allocation sizes up, nor worry
about aligning the destination according to the source or any other
games like that.

The down-side is wastage of maybe 7 bytes a function, but that's
probably going to happen anyway.

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