[LLVMdev] Unaligned vector memory access for ARM/NEON.

David Peixotto dpeixott at codeaurora.org
Thu Sep 6 16:40:32 PDT 2012

-----Original Message-----
From: Bob Wilson [mailto:bob.wilson at apple.com] 
Sent: Thursday, September 06, 2012 3:39 PM
To: David Peixotto
Cc: 'Peter Couperus'; 'Jim Grosbach'; 'Jakob Olesen'; llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Unaligned vector memory access for ARM/NEON.

On Sep 6, 2012, at 2:48 PM, David Peixotto <dpeixott at codeaurora.org> wrote:

> Hi Pete,
> We ran into the same issue with generating vector loads/stores for 
> vectors with less than word alignment. It seems we took a similar 
> approach to solving the problem by modifying the logic in
> As you and Jim mentioned, it looks like the vld1/vst1 instructions 
> should support element aligned access for any armv7 implementation 
> (I'm looking at Table A3-1 ARM Architecture Reference Manual - ARM DDI
> Right now I do not think we have the correct code setup in 
> ARMSubtarget to accurately represent this table. I would propose that 
> we keep the existing field for unaligned access and add a new field for
element-aligned access.
> The AllowsUnAlignedMem field remains as is and it could be used to 
> represent the SCTLR.A column in Table A3-1. The 
> AllowsElementAlignedNEON field would be used allow targets to generate 
> vld1/vst1 instructions for element-aligned accesses. By default it would
be set to true for armv7 targets with NEON.

That doesn't make sense to me.  Element-aligned Neon load/stores are always
valid.  The AllowsUnalignedMem setting is supposed to model the SCTLR.A
setting, which also applies to Neon load/stores with less-than-element
alignment.  Why would we need a new field?

I was proposing it as a way to keep the current behavior of the
AllowsUnAlignedMem field. Originally I just guarded the use of vld1/vst1 by
testing whether the target has NEON, but then I was getting a failure in
test/CodeGen/ARM/unaligned_load_store.ll because it did not want to see a
vld1 when using the -arm-strict-align even if the target has NEON. 

The difference in the flags is between allowing unaligned access for
half-word and word load/stores (e.g. ldr/str) and getting around an
unaligned access for D registers by using vld1.8/vst1.8 to load/store.

It looks to me that a target that supports NEON could support unaligned
access for D registers by using vld1.8/vst1.8 regardless of whether it
supports unaligned access for other types. Currently these two are tied
together into a single flag. The new flag was proposed as a way to make a
distinction between these.

I guess one way to keep the current behavior would be to look back through
the store instruction to find the actual vector type (everything gets
legalized to f64 or v2f64) and allow it for real vector types but disallow
it for a real f64 type. It seems that it would be good to support the
unaligned f64 access if we have NEON though.

> The -arm-strict-align would set both of the fields to false. This 
> would retain the behavior that seems to be desired from the 
> test/CodeGen/ARM/unaligned_load_store.ll test case.
> A bit of a grey area is if we have an unaligned f64 store and 
> AllowsElementAlignedNEON is true. We can actually generate a vstr1.8 
> to support this store directly instead of using the target-independent 
> method and I think it would be good to do so.

Don't we already do this as of svn r161962?

No, the vstr1 will only be generated if the target supports unaligned
access. Setting AllowsUnAlignedMem to true will then also allow unaligned
access using smaller types (e.g. word) which may not be desirable.


> I have some code to do this that I will likely be able to upstream.
> -Dave
> -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, 
> hosted by The Linux Foundation
> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] 
> On Behalf Of Peter Couperus
> Sent: Thursday, September 06, 2012 8:14 AM
> To: Jim Grosbach
> Cc: Jakob Olesen; llvmdev at cs.uiuc.edu (LLVMdev at cs.uiuc.edu)
> Subject: Re: [LLVMdev] Unaligned vector memory access for ARM/NEON.
> Hello,
> Thanks again.  We did try overestimating the alignment, and saw the 
> vldr you reference here.
> It looks like a recent change (r161962?) did enable vld1 generation 
> for this case (great!) on darwin, but not linux.
> I'm not sure if the effect of lowering load <4 x i16>* align 2 to
> vld1.16 this was intentional in this change or not.
> If so, my question is what is the preferable way to inform the 
> Subtarget that it is allowed to use unaligned vector loads/stores when 
> NEON is available, but can't use unaligned accesses generally speaking?
> A new field in ARMSubtarget?
> Should the -arm-strict-align flag force expansion even on unaligned 
> vector loads/stores?
> We got this working by adding a field to ARMSubtarget and changing 
> logic in ARMTargetLowering::allowsUnalignedMemoryAccesses, but I am 
> admittedly not entirely sure of the downstream consequences of this, 
> as we don't allow unaligned access generally.
> Pete
> On 09/05/2012 04:58 PM, Jim Grosbach wrote:
>> Hmmm. Well, it's entirely possible that it's LLVM that's confused 
>> about the alignment requirements here. :)
>> I think I see, in general, where. I twiddled the IR to give it higher
> alignment (16 bytes) and get:
>> extend:                                 @ @extend
>> @ BB#0:
>> 	vldr	d16, [r0]
>> 	vmovl.s16	q8, d16
>> 	vstmia	r1, {d16, d17}
>> 	vldr	d16, [r0, #8]
>> 	add	r0, r1, #16
>> 	vmovl.s16	q8, d16
>> 	vstmia	r0, {d16, d17}
>> 	bx	lr
>> Note that we're using a plain vldr instruction here to load the d
> register, not a vld1 instruction. Similarly for the stores. According 
> to the ARM ARM (DDI 0406C),  you're correct about the element size 
> alignment requirement for VLD1, but our isel isn't attempting to use 
> that instruction, but rather VLDR, which has word alignment required, so
it falls over.
>> Given that, it seems that the answer to your original question is 
>> that to
> improve codegen for this case, the proper place to look is in 
> instruction selection for loads and stores to the VFP/NEON registers. 
> That code can be made smarter to better use the NEON instructions. I 
> know Jakob has done some work related to better utilization of those for
other constructs.
>> -Jim
>> On Sep 5, 2012, at 4:25 PM, Peter Couperus<peter.couperus at st.com>  wrote:
>>> Hello Jim,
>>> Thank you for the response.  I may be confused about the alignment 
>>> rules
> here.
>>> I had been looking at the ARM RVCT Assembler Guide, which seems to 
>>> indicate vld1.16 operates on 16-bit aligned data, unless I am
> misinterpreting their table (Table 5-11 in ARM DUI 0204H, pg 5-70,5-71).
>>> Prior to the table, It does mention the accesses need to be "element"
> aligned, where I took element in this case to mean i16.
>>> Anyhow, to make this a little more concrete:
>>> void extend(short* a, int* b) {
>>>  for(int i = 0; i<  8; i++)
>>>    b[i] = (int)a[i];
>>> }
>>> When I compile this program with clang -O3 -ccc-host-triple 
>>> armv7-none-linux-gnueabi -mfpu=neon -mllvm -vectorize, the 
>>> intermediate
> LLVM assembly looks OK (and it has an align 2 vector load), but the 
> generated ARM assembly has the scalar loads.
>>> When I compile with (4.6) gcc -std=c99 -ftree-vectorize -marm 
>>> -mfpu=neon
> -O3, it uses vld1.16 and vst1.32 regardless of the parameter alignment.
> This is on armv7a.
>>> The gcc version (and the clang version with our modified backend) 
>>> runs
> fine, even on 2-byte aligned data.  Is this not a guarantee across 
> armv7/armv7a generally?
>>> Pete
>>> On 09/05/2012 03:15 PM, Jim Grosbach wrote:
>>>> VLD1 expects a 64-bit aligned address unless the target explicitly 
>>>> days
> that unaligned loads are OK.
>>>> For your situation, either the subtarget should set 
>>>> AllowsUnalignedMem
> to true (if that's accurate), or the load address should be made 
> 64-bit aligned.
>>>> -Jim
>>>> On Sep 5, 2012, at 2:42 PM, Peter Couperus<peter.couperus at st.com>
> wrote:
>>>>> Hello all,
>>>>> I am a first time writer here, but am a happy LLVM tinkerer.  It 
>>>>> is a
> pleasure to use :).
>>>>> We have come across some sub-optimal behavior when LLVM lowers 
>>>>> loads for vectors with small integers, i.e. load<4 x i16>* %a, 
>>>>> align 2,
> using a sequence of scalar loads rather than a single vld1 on armv7 
> linux with NEON.
>>>>> Looking at the code in svn, it appears the ARM backend is capable 
>>>>> of
> lowering these loads as desired, and will if we use an appropriate 
> darwin triple.
>>>>> It appears this was actually enabled relatively recently.
>>>>> Seemingly, the case where the Subtarget has NEON available should 
>>>>> be
> handled the same on Darwin and Linux.
>>>>> Is this true, or am I missing something?
>>>>> Do the regulars have an opinion on the best way to handle this?
>>>>> Thanks!
>>>>> Pete
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>> <extend.c>
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