[oe] mono build error with swp instruction

[Philip Balister]

Cliff Brake wrote:
> I ran into an interesting build error this morning that might be worth
> documenting ...
> 
> When building mono with the latest toolchains built with OE, I get:
> 
> /tmp/ccYsXwlg.s:3067: Error: selected processor does not support `swp
> r2,r0,[r4]'
> /tmp/ccYsXwlg.s:3161: Error: selected processor does not support `swp
> r2,r0,[r4]'
> /tmp/ccYsXwlg.s:3272: Error: selected processor does not support `swp
> r3,r4,[r5]'
> 
> In the libgc code, there is the following:
> 
> #    ifdef ARM32
>         inline static int GC_test_and_set(volatile unsigned int *addr) {
>           int oldval;
>           /* SWP on ARM is very similar to XCHG on x86.  Doesn't lock the
>            * bus because there are no SMP ARM machines.  If/when there are,
>            * this code will likely need to be updated. */
>           /* See linuxthreads/sysdeps/arm/pt-machine.h in glibc-2.1 */
>           __asm__ __volatile__("swp %0, %1, [%2]"
>       		  	     : "=&r"(oldval)
>       			     : "r"(1), "r"(addr)
> 			     : "memory");
>           return oldval;
>         }
> 
> The solution was to set ARM_INSTRUCTION_SET = "arm" in the mono recipe.

On armv6 and higher, you want to use LDREX and STREX instead of SWP. 
This makes your code SMP ready (and a few other benefits I suspect) :)

Philip


> 
> Thanks,
> Cliff
> 
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