Enabling scalarized conditional stores in the loop vectorizer

Hi,

I’d like to enable the scalarized conditional stores feature in the loop vectorizer (-enable-cond-stores-vec=true). The feature allows us to vectorize loops containing conditional stores that must be scalarized and predicated in the vectorized loop.

Note that this flag does not affect the decision to generate masked vector stores. That is a separate feature and is guarded by a TTI hook. Currently, we give up on loops containing conditional stores that must be scalarized (i.e., conditional stores that can’t be represented with masked vector stores). If the feature is enabled, we attempt to vectorize those loops if profitable, while scalarizing and predicating the conditional stores.

I think these stores are fairly well modeled in the cost model at this point using the static estimates. They’re modeled similar to the way we model other non-store conditional instructions that must be scalarized and predicated (e.g., instructions that may divide by zero); however, only the conditional stores are currently disabled by default.

I’d appreciate any opinions on how/if we can enable this feature. For example, can we enable it for all targets or would a target-by-target opt-in mechanism using a TTI hook be preferable? If you’d like to test the feature on your target, please report any significant regressions and improvements you find.

Thanks!

– Matt

Conceptually speaking, I think we really ought to enable this.

Practically, I’m going to test it on our benchmarks (on x86), and see if we have any regressions - this seems like a fairly major change.
Re targets - let’s see where we stand w.r.t regressions first. What kind of performance testing have you already run on this? Do you know of specific targets where the cost model is known to be good enough, so it’s clearly beneficial?

(+Arnold, who probably knows why this is disabled by default. :slight_smile: )

Thanks,
Michael

Hi Michael,

Thanks for testing this on your benchmarks and target. I think the results will help guide the direction we go. I tested the feature with spec2k/2k6 on AArch64/Kryo and saw minor performance swings, aside from a large (30%) improvement in spec2k6/libquantum. The primary loop in that benchmark has a conditional store, so I expected it to benefit.

Regarding the cost model, I think the vectorizer’s modeling of the conditional stores is good. We could potentially improve it by using profile information if available. But I’m not sure of the quality of the individual TTI implementations other than AArch64. I assume they are adequate.

Since the conditional stores remain scalar in the vector loop, their cost is essentially the same as it is in the scalar loop (aside from scalarization overhead, which we account for). So when we compare the cost of the scalar and vector loops when deciding to vectorize, we’re basically comparing the cost of everything else.

– Matt

I added this feature for libquantum (http://llvm.org/viewvc/llvm-project?view=revision&revision=200270) waiting for an update to the cost model modeling the scalarization of stores which you recently added.

Assuming no serious regressions this SGTM.

From: "Arnold Schwaighofer via llvm-dev" <llvm-dev@lists.llvm.org>
To: "Matthew Simpson" <mssimpso@codeaurora.org>
Cc: "llvm-dev" <llvm-dev@lists.llvm.org>
Sent: Tuesday, December 13, 2016 9:17:08 AM
Subject: Re: [llvm-dev] Enabling scalarized conditional stores in the loop vectorizer

I added this feature for libquantum
(http://llvm.org/viewvc/llvm-project?view=revision&revision=200270)
waiting for an update to the cost model modeling the scalarization
of stores which you recently added.

Assuming no serious regressions this SGTM.

Great!

-Hal

Performance looks more or less flat for me.

Michael

Hi Michael-

Since you bring up libquantum performance can you let me know what the IR will look like for this small code snippet (libquantum-like) with –enable-cond-stores-vec ? I ask because I don’t see vectorization kicking in unless -force-vector-width=<> is specified. Let me know if I am missing something.

Hi Dibyendu,

Are you using a recent compiler? What architecture are you targeting? The target will determine whether the vectorizer thinks vectorization is profitable without having to manually force the vector width.

For example, top-of-trunk vectorizes your snippet with “clang -O2 -mllvm -enable-cond-stores-vec” and “–target=aarch64-unknown-linux-gnu”. However, with “–target=x86_64-unknown-linux-gnu” the vectorizer doesn’t find the snippet profitable to vectorize.

This is probably due to the interleaved load in the loop. When targeting AArch64, the cost model reports the interleaved load as inexpensive (AArch64 has dedicated instructions for interleaved memory accesses), but when targeting X86 it doesn’t. You can take a look at the costs with “-mllvm -debug-only=loop-vectorize”

Hope that helps.

– Matt

Hi Matt-

Yeah I used a pretty recent llvm (post 3.9) on an x86-64 ( both AMD and Intel ).

-dibyendu

I haven’t verified what Matt described is what actually happens, but assuming it is - that is a known issue in the x86 cost model.

Vectorizing interleaved memory accesses on x86 was, until recently, disabled by default. It’s been enabled since r284779, but the cost model is very conservative, and basically assumes we’re going to scalarize interleaved ops.

I believe Farhana is working on improving that.

Michael

If there are no objections, I’ll submit a patch for review that sets the default value of “-enable-cond-stores-vec” to “true”. Thanks!

– Matt

Thanks Michael and Dibyendu for doing the experimentation and bringing this up to our attention. It might be the case what Matt described here. I will take a look at it.

Farhana

I haven’t done any experimentation, it’s all Matt. :slight_smile:

SGTM.