[LLVMdev] AArch64AddressTypePromotion does nothing (was Re: Contributing the Apple ARM64 compiler backend)

[Duncan P. N. Exon Smith]

> On 2014-Jun-30, at 16:53, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
>> 
>> On 2014-Jun-30, at 16:19, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
>> 
>> 
>>> On 2014-Jun-27, at 16:30, Jim Grosbach <grosbach at apple.com> wrote:
>>> 
>>> AArch64AddressTypePromotion.cpp does a fair bit of work to help make these things work out well. It could probably be generalized for non-AArch64 targets as per the comment in the file header.
>> 
>> I spent some time today generalizing AArch64AddressTypePromotion (I'll
>> send the patches when they're ready), but in the process discovered
>> that this pass does nothing (!) right now.  I assume this bug was
>> introduced when we changed the semantics of `Use` (IIRC, ARM64 was
>> still private at the time).
>> 
>> After the tiny patch inline below, the code looks even better:
>> 
>> --- old.s	2014-06-30 16:12:52.000000000 -0700
>> +++ new.s	2014-06-30 16:13:24.000000000 -0700
>> @@ -5,12 +5,10 @@
>> _foo:                                   ; @foo
>> 	.cfi_startproc
>> ; BB#0:                                 ; %entry
>> -	add	w8, w1, #1              ; =1
>> -	add	w9, w1, #2              ; =2
>> -	ldr	w8, [x0, w8, sxtw #2]
>> -	ldr	w9, [x0, w9, sxtw #2]
>> -	add	 w8, w9, w8
>> -	str	w8, [x0, w1, sxtw #2]
>> +	add	x8, x0, w1, sxtw #2
>> +	ldp	w9, w10, [x8, #4]
>> +	add	 w9, w10, w9
>> +	str	 w9, [x8]
>> 	ret
>> 	.cfi_endproc
>> 
>> I'll commit the fix once I've written up a testcase.
> 
> FYI, I committed this as r212073.
> 
> I'll do a quick audit of other calls of `uses()` and `use_begin()` in
> AArch64.

I found another problem in the same file (fixed in r212075), but
otherwise it looks clean -- most calls are on `SDNode`, which still
has the *old* semantics.