[Buildroot] [PATCH v2 04/30] gst-ffmpeg: use the new BR2_ARM_CPU_ARM* options

[Arnout Vandecappelle <arnout@mind.be>]

On 23/10/14 09:49, Thomas Petazzoni wrote:
> Dear Arnout Vandecappelle,
> 
> On Wed, 22 Oct 2014 18:14:21 +0200, Arnout Vandecappelle wrote:
> 
>>  I wouldn't want to actually use it unless tested on real hardware, however.
> 
> Well, if you grep through the gst-ffmpeg tree, you will that the
> ARMV6T2 condition is only used in two places in
> gst-libs/ext/libav/libavcodec/arm/asm.S :
> 
> .macro  mov32   rd, val
> #if HAVE_ARMV6T2
>         movw            \rd, #(\val) & 0xffff
>     .if (\val) >> 16
>         movt            \rd, #(\val) >> 16
>     .endif
> #else
>         ldr             \rd, =\val
> #endif
> .endm
> 
> .macro  movrel rd, val
> #if HAVE_ARMV6T2 && !CONFIG_PIC && !defined(__APPLE__)
>         movw            \rd, #:lower16:\val
>         movt            \rd, #:upper16:\val
> #else
>         ldr             \rd, =\val
> #endif
> .endm
> 
> So when you have ARMV6T2, it uses movw/movt instead of ldr. That's the
> only difference with the non-ARMV6T2 mode. Then you can read
> http://community.arm.com/groups/processors/blog/2010/07/27/how-to-load-constants-in-assembly-for-arm-architecture,
> which says:
> 
> """
> As mentioned earlier, there are other ways to load a constant. In the
> latest version of the ARM architecture, ARMv7, two new instructions
> were introduced to improve the situation:
> 
> - movw, or move wide, will move a 16-bit constant into a register,
>   implicitly zeroing the top 16 bits of the target register.
> 
> - movt, or move top, will move a 16-bit constant into the top half of a
>   given register without altering the bottom 16 bits. Now moving an
>   arbitrary 32-bit constant is as simple as this:
> """
> 
> So I think it's safe to say than --enable-armv6t2 can be used on ARMv7
> as well.

 Okay, that's good enough for me.


 Regards,
 Arnout




-- 
Arnout Vandecappelle                          arnout at mind be
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