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

Dave Martin dave.martin at linaro.org
Fri Jan 14 12:59:54 EST 2011

On Fri, Jan 14, 2011 at 11:43 AM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> 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.

OK, that sounds like a practical compromise.

I'll roll this in and repost the patch.


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