[llvm-dev] [RFC] Simple GVN hoist
Alina Sbirlea via llvm-dev
llvm-dev at lists.llvm.org
Mon Nov 8 14:37:05 PST 2021
Apologies for the delayed reply on this issue.
The status of NewGVN is uncertain at this point due to correctness
issues with the pass.
The idea of adding the GVN Hoist functionality makes sense, but I'd like a
discussion on how to plan this longer term. I added a note about this in
https://reviews.llvm.org/D110822.
Alina
On Tue, Sep 14, 2021 at 12:12 PM Philip Reames <listmail at philipreames.com>
wrote:
> Ok, this response is focused on the proposed transform. See inline.
> On 9/14/21 7:19 AM, Momchil Velikov via llvm-dev wrote:
>
> Looking at a particularly hot function in the SPEC/x264, that LLVM fails to
> vectorise.
>
> typedef short int16_t;
> typedef unsigned short uint16_t;
>
> int q(int16_t d[], uint16_t m[], uint16_t b[]) {
> int n = 0;
> for (int i = 0; i < 16; i++) {
> if (d[i] > 0)
> d[i] = (b[i] + d[i]) * m[i] >> 16;
> else
> d[i] = -((b[i] - d[i]) * m[i] >> 16);
> n |= d[i];
> }
> return n;
> }
>
> As it turns out, LLVM adds runtime alias checks for this function and then it
> considers it legal to vectorise with if-conversion. However, the vectorisation
> cost model effectively bans that particular if-conversion by assigning a
> ridiculous cost to emulated masked loads and stores.
>
> Originally, each branch of the if statement in the loop body contains three
> identical loads. Manually hoisting these allows the loop to be vectorised at
> `-O2` (at `-O3` the loop is fully unrolled and that breaks vectorisation).
> There's a subroutine in `SimplifyCFG` that does a rudimentary hoisting of
> instructions from the two successors of a block, which subroutine does indeed
> hoist two of the three loads, but gives up before hoisting the third one.
>
> We'd need a way to make LLVM hoist all three of the loads by itself. `GVNHoist`
> can do that, but that pass is disabled by default and has been disabled for a
> long, long time.
>
> I haven't looked at this pass recently, but I would strongly recommend
> distrusting the current implementation. The software engineering process
> that went into writing it doesn't give me any confidence. I remember a
> long string of enable-revert-bugfix without obvious justification as to why
> the bugs being found had to be found with such an expensive approach. If
> you do decide to pursue this, expect the need for a) targeted review and b)
> a bunch of dedicated fuzzing.
>
> Having said that, *conceptually* this pass takes the proper approach by
> using MemorySSA. Your current implementation has a bunch of complexity to
> basically do a local renumbering of memory instructions. Some of that can
> be improved (see below), but if you were doing this with MemorySSA, you
> wouldn't need to do anything fancy as GVN would have numbered the loads
> correctly. NewGVN is a MemorySSA version of GVN which I'd nominally say is
> the right place for this, but it has never been productionized. A huge
> blocker was MemorySSA itself which was recently productionized, but I don't
> know what's left for NewGVN itself. (Alina might be able to add context
> here?)
>
> As an alternative, I was thinking of a simpler hoisting transformation, that
> just handles moving instructions from two single-predecessor blocks to their
> common predecessor. That could be made reasonably fast, by pairing instructions
> by their GVN value number. Limiting hoisting to a predecessor block (for the
> most part) would also avoid excessive increase of lifetimes (for the majority of
> the case) and would also simplify correctness checks.
>
> I've written such a transformation as a subroutine to `GVN`, it seemed like a
> good place for it and is an a similar spirit as various PREs the GVN does. The
> Phabricator review is at https://reviews.llvm.org/D109760.
>
> The current structure of algorithm you proposed can probably be
> simplified. Here's what I'd suggest:
>
> Start by handling all of the non-memory instructions. This should be as
> simple as constructing a hash map from value number to instruction for one
> block, and then scanning the other block. If you find the same value
> number in both blocks, the (by assumption scalar) instruction must exist in
> both blocks and can be hoisted. This can be reviewed, tested, submitted,
> etc..
>
> Then add a single pre-pass which "renumbers" all of the memory
> instructions. You can either use a trivial "no clobbers seen" like
> EarlyCSE does, or only handle loads where MD->getDependency() is nonlocal.
> (We already do this query in processLoad, so it should be cached.) The
> same algorithm can be used as above, all that changes is where the
> valuenumber comes from. (The "pre-pass" can probably be implemented as a
> helper function which renumbers memory instructions on demand.)
>
> I would specifically *not* add an extra layer of iteration here. If you
> find you need iteration to handle the cases you care about, I suspect we
> should actually be adjusting the value re-numbering scheme for loads
> described just above.
>
> A couple of assorted notes:
>
> - MemDep should be responsible for handling all aliasing barriers. If
> it returns a non-local dependence, reordering that instruction to the
> beginning of the block should be legal w.r.t. memory. Your use of
> isHoistBarrier is confusing to me. You may be trying to handle speculation
> safety (i.e. avoid introducing faults), but if so, the special casing
> around volatile and atomics seems odd? (Also, why allow hosting of the
> barrier instruction?) I suspect there's a particular case you're trying to
> handle. I would strongly advice dropping that from the initial patch, then
> add back the complexity in a separate change which is well motivated with
> tests, etc..
> - In terms of the actual merge logic, I'd suggest implementing it as
> hoist-one, then CSE. That would allow you to reuse the existing patching
> logic and avoid reimplementing a bunch of complexity.
>
>
> Initial benchmarking on Neoverse N1 looks good (speedup, higher is better):
>
> 500.perlbench_r 1.13%
> 502.gcc_r 0.00%
> 505.mcf_r -1.89%
> 520.omnetpp_r 0.00%
> 523.xalancbmk_r 0.00%
> 525.x264_r 7.67%
> 531.deepsjeng_r 0.60%
> 541.leela_r 0.24%
> 548.exchange2_r 0.00%
> 557.xz_r 0.75%
>
> This looks generally positive. Do you know why MCF regresses? If not,
> taking a glance to make sure you understand the interaction is probably
> warranted.
>
> Comments?
>
> ~chill
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>
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