[PATCH 1/7]: SVE: Add CLOBBER_HIGH expression

[Jeff Law]

On 11/22/2017 04:31 AM, Alan Hayward wrote:
> 
>> On 21 Nov 2017, at 03:13, Jeff Law <law@redhat.com> wrote:
>>>
>>>>
>>>> You might also look at TARGET_HARD_REGNO_CALL_PART_CLOBBERED.  I'd
>>>> totally forgotten about it.  And in fact it seems to come pretty close
>>>> to what you need…
>>>
>>> Yes, some of the code is similar to the way
>>> TARGET_HARD_REGNO_CALL_PART_CLOBBERED works. Both that code and the
>>> CLOBBER expr code served as a starting point for writing the patch. The main difference
>>> here, is that _PART_CLOBBERED is around all calls and is not tied to a specific Instruction,
>>> it’s part of the calling abi. Whereas clobber_high is explicitly tied to an expression (tls_desc).
>>> It meant there wasn’t really any opportunity to resume any existing code.
>> Understood.  Though your first patch mentions that you're trying to
>> describe partial preservation "around TLS calls".  Presumably those are
>> represented as normal insns, not call_insn.
>>
>> That brings me back to Richi's idea of exposing a set of the low subreg
>> to itself using whatever mode is wide enough to cover the neon part of
>> the register.
>>
>> That should tell the generic parts of the compiler that you're just
>> clobbering the upper part and at least in theory you can implement in
>> the aarch64 backend and the rest of the compiler should "just work"
>> because that's the existing semantics of a subreg store.
>>
>> The only worry would be if a pass tried to get overly smart and
>> considered that kind of set a nop -- but I think I'd argue that's simply
>> wrong given the semantics of a partial store.
>>
> 
> So, the instead of using clobber_high(reg X), to use set(reg X, reg X).
> It’s something we considered, and then dismissed.
> 
> The problem then is you are now using SET semantics on those registers, and it
> would make the register live around the function, which might not be the case.
> Whereas clobber semantics will just make the register dead - which is exactly
> what we want (but only conditionally).
?!?  A set of the subreg is the *exact* semantics you want.  It says the
low part is preserved while the upper part is clobbered across the TLS
insns.

jeff