Android and compatibility with deprecated armv7 instructions

[Måns Rullgård]

Janne Grunau <j at jannau.net> writes:

> Arnd Bergmann <arnd <at> arndb.de> writes:
>> 
>> On Saturday 05 July 2014, Catalin Marinas wrote:
>> > On 5 Jul 2014, at 19:43, Arnd Bergmann <arnd <at> arndb.de> wrote:
>> >
>> > > I think if a there is no way to turn off a feature, we can't really
>> > > treat it as "deprecated", since we are lacking the most important tool
>> > > to help users migrate away from it. Do you know why the architecture
>> > > folks believe it makes sense to have distinct steps 1 and 2?
>> > 
>> > During this phase you usually only get compiler warnings which are
>> > clearly not enough for already built apps.
>> > 
>> > But it looks like this is changing with ARMv8. SETEND is deprecated and
>> > disable bit available. IT instruction also deprecated with a disable bit
>> > (basically only allowing for one subsequent conditional instruction,
>> > though I don’t think we can easily get rid of this one in user space).
>> 
>> Ok. I can also see ways to emulate SETEND from kernel side by modifying
>> the user pt_regs,
>
> The single use of SETEND in FFmpeg/libav is not important and could easily
> be disabled on ARMv8. Is there a better way than parsing AT_PLATFORM for
> runtime detection of the CPU arch? A HWCAP for the missing SETEND would be
> nicer. That of course doesn't fix existing binaries.

SETEND is serialising on some v7 cores as well, so it really shouldn't
be used at all.

>> but I don't see how we could emulate IT without anything
>> short of a full interpretation of user instructions from the kernel.
>
> Avoiding deprecated forms of IT would be harder. It's used more often in
> libav (~50 times) so it should be only disabled if it's emulated or not
> available using an explicit HWCAP. Not using Thumb for those functions would
> be another option for libav. Again this doesn't fix existing binaries and it
> doesn't fix compiled code.

IT covering more than one conditional instruction is trivial to fix by
either a) adding IT before each instruction, b) replacing it with a
branch, or in some cases c) making all but the last instruction
conditional because the rest don't actually need to be.  Long runs of
conditional instructions have been frowned upon for quite some time
anyway.

-- 
Måns Rullgård
mans at mansr.com