[RFC][PATCH][LoopVectorize] Teach Loop Vectorizer about interleaved data accesses

[Hal Finkel]

----- Original Message -----

> From: "Hao Liu" <Hao.Liu at arm.com>
> To: aschwaighofer at apple.com, hfinkel at anl.gov, "Nadav Rotem"
> <nrotem at apple.com>, "Elena Demikhovsky"
> <elena.demikhovsky at intel.com>
> Cc: llvm-commits at cs.uiuc.edu, "Jiangning Liu"
> <Jiangning.Liu at arm.com>, "James Molloy" <James.Molloy at arm.com>
> Sent: Friday, March 20, 2015 6:47:52 AM
> Subject: [RFC][PATCH][LoopVectorize] Teach Loop Vectorizer about
> interleaved data accesses

> Hi,

> There are two patches attached can achieve this goal:
> LoopVectorize-InterleaveAccess.patch teaches Loop Vectorizer about
> interleaved data access and generate target independent intrinsics
> for each load/store:
> AArch64Backend-MatchIntrinsics.patch match several target independent
> intrinsics into one AArch64 ldN/stN intrinsics, so that AArch64
> backend can generate ldN/stN instructions.

> Currently, LoopVectorize can vectorize consecutive accesses well. It
> can vectorize loops like
> for (int i = 0; i < n; i++)
> sum += R[i];

> But it doesn't handle strided access well. Interleaved access is a
> subset of strided access. Example for interleaved access:
> for (int i = 0; i < n; i++) {
> int even = A[2*i];
> int odd = A[2*i + 1];
> // do something with odd & even.
> }
> To vectorize such case, we need two vectors: one with even elements,
> another with odd elements. To gather even elements, we need several
> scalar loads for "A[0], A[2], A[4], ...", and several
> INSERT_ELEMENTs to combine them together. The cost is very high and
> will usually prevent loop vectorization on such case.
Perhaps this is a silly question, but why do you need interleaved load/store to support this? If we know that we need to access A[0], A[2], A[4], A[6], can't we generate two vector loads, one for A[0...3], and one for A[4...7], and then shuffle the results together. You need to leave the vector loop one iteration early (so you don't access off the end of the original access range), but that does not seem like a big loss. If I'm right, then I'd love to see this implemented in a way that can take advantage of interleaved load/store on targets that support them, but not require target support. 

Thanks again, 
Hal 

> Some backend like AArch64/ARM support interleaved load/store: ldN/stN
> (N is 2/3/4). And I know X86 can also support similar operations.
> One ld2 can load two vectors: one is with even elements, another is
> with only odd elements. So that this case can be vectorized into
> AArch64 instructions:
> LD2 { V0, V1 } [X0]
> // V0 contains even elements. Do something related to even elements
> with V0.
> // V1 contains odd elements. Do something related to odd elements
> with V1.

> 1. Design
> My design is to follow current Loop Vecotirzer three phases.
> (1) Legality Phase:
> (a). Collect all the constant strided accesses except consecutive
> accesses.
> (b). Collect the load/store accesses with the same Stride, Base
> pointer.
> (c). Fine the consecutive chains in (b). If the number of accesses in
> one chain are equal to the Stride, they are interleaved accesses.
> Example for the case about even and odd. We can find two loads for
> even and odd elements. The strides are both 2. They are also
> consecutive. So they are recorded as interleaved accesses.

> (2) Profitability Phase:
> Add a target hook to calculate the cost. Currently the cost is 1.
> Currently this won't affect to much about the result. So I didn't do
> too much work in this phase.

> (3) Transform Phase:
> As there is no IR for interleaved data, I think we should use
> intrinsics. The problem is that the relationship is "N to one". I.E.
> Several loads/stores to one ldN/stN instructions. There is already
> ldN/stN intrinsics in AArch64/ARM backend such
> llvm.aarch64.neon.ldN, which is like "call { <4 x i32>, <4 x i32>}
> llvm.aarch64.neon.ld2.v4i32()". In the middle end, there are two IR
> loads.

> Need to think a way to match two loads to one target specific
> intrinsic. I think there are two ways:
> (a). Two steps for middle end and backend. 1st step is to match each
> loads/stores to one target independent intrinsic in the loop
> vectorize. 2nd step is to match several intrinsics into one ldN/stN
> intrinsic. This is the choise of my attached patch. For the above
> odd-even example, it will generate two intrinsics in the loop
> vectorization:
> "%even-elements = call <4 x i32> @llvm.interleave.load.v4i32",
> "%odd-elements = call <4 x i32> @llvm.interleave.load.v4i32".
> A backend pass will combine them together into one intrinsic:
> "%even-odd-elements = call { <4 x i32>, <4 x i32> }
> @llvm.aarch64.neon.ld2.v4i32"
> But I think the backend pass is vulnerable and diffecult to
> implement. It will fail to combine if one load is missing, or if one
> load is moved to another basic block. Also I haven't check about
> memory dependency.
> (b). One step only for middle end. We can match several load/stores
> into one ldN/stN like target independent intrinsic. So that the
> AArch64/ARM backend only needs slight modification on replacing the
> current used intrinsic to the new independent intrinsic. This needs
> to introduce a new intrinsic such as "{ <4 x i32>, <4 x i32>}
> llvm.interleaved.load.v4i32()".

> Actually I prefer solution (b), which is easier to be implemented and
> stronger than solution (a). But solution (a) seems more target
> independent. How do you guys think?

> 2. Test
> I've test the attached patch with llvm-test-suit, SPEC2000, SPEC2006,
> EEMBC, Geekbench on AArch64 machine. They all can pass.
> But the performance is not affected. Some specific benchmarks like
> EEMBC rgbcmy and EEMBC rgbyiq are expected to have several times of
> acceleration. The reason is that there are other issues prevent
> vectorization opportunities. Some known issues are:
> (1). Too many unnecessary runtime checkings (The interleaved accesses
> are compared with each other).
> (2). Store-Load Forward checking (Doesn't consider about strided
> accesses).
> (3). Type promotion issue (i8 is illegal but <16 x i8> is legal. i8
> is promoted to i32 so the extend and truncate operations increase
> the total cost).
> (4). The Vector Factor is selected according to the widest type. (If
> there are both i8 and i32, we select a small factor according to i32
> rather than according to i8).
> Anyway. We can fix them in the future and get performance
> improvements.

> What's your oppions on the solution? I'm still hesitating about the
> transform phase.

> Thanks,
> -Hao
-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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