Quadrupling code performance with a "useless" if

https://purplesyringa.moe/blog/quadrupling-code-performance-with-a-useless-if/

by birdculture • 10 days ago

112 29 10 days ago

anematode• 10 days ago

Brilliant! Hadn't seen this technique before.

latency optimization is a skill. I liked how you went till CSE pass. I myself wrote several passes to go to lowest latency possible

akoboldfrying• 10 days ago

This is really surprising! I've never considered the possibility that using an equality test to skip a write that would be a no-op could break a dependency and thus lead to higher perf overall if the "equal" outcome occurs often enough. This might be applicable in many situations where you "edit" some data in-place, but most of the time there are few or no changes.

anthonj• 10 days ago

I think this call for something similar to "__builtin_expect" or linux' likely()/unlikely().

Not very clean, but better than inserting obscure optimisations in the source.

gblargg• 10 days ago

Assuming compilers are smart enough to insert an unnecessary branch to break the dependency.

vitally3643• 9 days ago

That's what the hints are for. Expect/likely/unlikely are the programmer informing the compiler what it should expect and how it should optimize

purplesyringa6• 10 days ago

That would be great, if only it worked as intended! From the perspective of an optimizing compiler, `a == b ? a : b` is worse than `b` regardless of the probability you assign to `a == b`.

ETA: someone on Lobsters (https://lobste.rs/s/1an425/quadrupling_code_performance_with...) noticed that `[[unlikely]]` actually works on LLVM (not on GCC, and with worse codegen on LLVM, but it's still good to know) -- updated the post.

MaxBarraclough• 10 days ago

I was wondering the same thing. Also, could profile-guided optimisation help here?

mcv2• 10 days ago

Interesting. I thought modern CPU optimisation required avoiding branches, but here adding the branch allows the branch pediction to parallelise what it otherwise couldn't.

zipy124• 10 days ago

It does and the key here is that adding the if is akin to avoiding a branch, since getting data then doing something with it is a hidden branch if you already have the data. All this code does is formalise the hidden branch so that it can be avoided when possible.

summarybot• 9 days ago

That's pretty cool. Is there something obcluding the compiler from noticing this parallelization opportunity without the new `if` ?

My understanding is the assignment and the evaluation are somehow coupled in this case based on the essay, but I could use an explanation.

nextaccountic• 9 days ago

since getting data then doing something with it is a hidden branch if you already have the data

You mean that the naive, simpler code, despite not having an if, has a "branch" on the microarchitectural state? (which is like.. if we have this already in cache, do something. if not, do something else)

    // Find the optimal encoding for each symbol.
    // Chunk boundaries are located where encodings change.
    uint8_t encoding[n_symbols];
    uint8_t j = 0; // always start with encoding 0 for simplicity
    for (int i = 0; i < n_symbols; i++) {
        j = next_j[i][j];
        encoding[i] = j;
    }

throawayonthe• 10 days ago

lobsters comment points out [[unlikely]] works here for clang

https://clang.godbolt.org/z/r4xYWfPfe

edit: oh the article also mentions it now :)

6510• 10 days ago

my js brain keeps thinking encoding[i] = next_j[i][j];

pillmillipedes• 9 days ago

wow, what an interesting optimization! how would you even figure out that that if is what's needed to make it faster?

purplesyringa6• 9 days ago

I knew the loop was latency-bound and I couldn't easily decrease the latency, so I knew I had to somehow avoid the dependency chain at all. I remembered that CPUs predict some properties of memory accesses (e.g. they might predict that a store and then a load from different addresses likely don't intersect), but not addresses, so I thought about another way to force it to predict `j` well. Branch prediction turned out to be the simplest way to do so.

Actually, since then I've found out that I could reduce latency by replacing a load on the critical chain with a vector shuffle instruction (`pshufb`, takes just 1 cycle on x86). Ironically, if I realized that sooner, I probably wouldn't have tried to use branch prediction at all!

nonadhocproblem• 9 days ago

I've also been working on a compressor recently, and the same general idea (letting the compiler know that a data dependency occurs very infrequently, and it should therefore assume none exists to exploit ILP) has allowed me to speed up my decompression by a lot:

https://github.com/welcome-to-the-sunny-side/misa77/blob/777...

patrulek3• 9 days ago

You can utilize MLP (memory-level parallelism) and reduce memory latency by reading whole 8 bytes of next_j at every iteration. Just do something like

`uint64_t packed_next_j = *(uint64_t*)(&next_j[i])`

and then just shift right to find proper j. This way you remove dependency on previous iteration. Did you tried that?

purplesyringa6• 9 days ago

The right-shift is the problem. You need to shift right by `j * 8`, which itself requires a shift to compute (`j << 3`), so you have two shifts on the critical path, resulting in a latency of 2 cycles. It's better than a load, but it's still noticeable.

patrulek3• 9 days ago

2 cycles vs ~5 cycles is still nice improvement plus its more deterministic (but not that fast in optimistic case; better in pessimistic case) than `if` appraoch and probably easier to "invent" and reason about. Definitely not that cool as your `if` solution though :)

OptionOfT• 9 days ago

Don't you need the previous iteration to find out how much you need to shift right?

patrulek3• 9 days ago

Yes you need, but the initial problem here were sequential loads from L1 into registers.

exmadscientist3• 9 days ago

A huge part of the problem here is that you're playing with the 8-bit registers. They work, but they're like travelling the back roads: the CPU designers put all their effort into the big highway next door, and not so much into keeping the back roads clear. They're infamous for random weird stalls and such. I wouldn't be surprised at all if there's dependency chain tracking weirdness using them.

If you can change the type on `next_j` over to `unsigned` (or `size_t` or ...) then a lot of weird stuff goes away. The dependency chains in address calculation at the beginning of the loop are gone too, which seems a win. (Or I'm sure there's some other, more complicated, way to get the compiler to get rid of them.) But if the space hit from using `unsigned` is allowable, I'd expect it to be both faster and less brittle.

The processor doesn't really know what you're up to, it only sees registers. When `j` is stored in `ecx`/`rcx` and `ecx` isn't changing, it knows it's free to push ahead pretty far on speculation. If the update to `ecx` is rare, putting it behind a well-predicted-not-taken branch will make it go away, as far as the speculation engine is concerned, and let the processor go far and fast. (The rollback on mispredict can be pretty painful, but, well, that's out-of-order execution for you.) That's what we're really trying to accomplish here: stuff that single register write behind a rare branch, somehow.

Here's what that can look like: https://clang.godbolt.org/z/1ss6hsEKb

Note that I disabled unrolling because some approaches got unrolled more than others. (I saw no unrolling, 2x, and more!) Comparing them without unrolling is fairest, I think, though it's also interesting to see how far they can unroll without assistance.

I also found that clang didn't need any additional fencing (or care if it was present), but gcc did, so there's an `asm volatile` style fence, which I think is a lot more clear than the other tricks. (Your mileage may vary; people who've done a lot of low-level work won't blink at these, while others will just stare agape. Perhaps also not blinking, but for opposite reasons!)

(Mind you, I didn't actually run any of this, so, well, you get what you pay for.)

purplesyringa6• 9 days ago

A huge part of the problem here is that you're playing with the 8-bit registers. [...]

That's an interesting thought, though in this particular case I think it's a little misguided. 8-bit arithmetic (or, god forbid, 16-bit arithmetic) can quickly cause unexpected stalls, but there is no arithmetic here: the only inherent difference between 8-bit and 32-bit code is the use of `movzx` versus `mov` in a memory load, which to the best of my knowledge are equivalent in performance (modulo store-to-load forwarding and such).

It does look like LLVM generates ugly code for address calculation, though. I don't think it's necessarily going to cause a large effect, since `movzx r8d, cl; lea rdx, [rdi + r8]` doesn't depend on the previous iteration, and the critical latency chain here is just `inc rax`, but it might affect throughput a little. It's quite unfortunate that the induction heuristics didn't do their best here.

so there's an `asm volatile` style fence

I've seen a couple people suggest the compiler fence, and all of them did the same thing -- they put `asm volatile` after the assignment to `j`, not before. Could you explain your thought process here? I thought putting it before the second load would make more sense, because I can imagine

    if (j != next_j[i][j]) {
        j = next_j[i][j];
        asm volatile("" : "+r"(j)); 
    }

being (de)optimized to

    unsigned char value = next_j[i][j];
    _Bool flag = j != value;
    j = value;
    if (flag) {
        asm volatile("" : "+r"(j)); 
    }

which in turn could be compiled to `mov reg, [mem]; cmp reg, reg; mov reg, reg; je`, whereas

    if (j != next_j[i][j]) {
        asm volatile("" : "+r"(j)); 
        j = next_j[i][j];
    }

doesn't permit such an optimization because it forces a conditional load, which you can't avoid putting behind a branch.

I have no doubt that putting the fence after the assignment works in this scenario (clearly it does, since LLVM and GCC recognize it), but I don't intuitively see why.

exmadscientist3• 9 days ago

in this particular case I think it's a little misguided ... only inherent difference between 8-bit and 32-bit code is the use of `movzx` versus `mov` ... generates ugly code for address calculation ... the induction heuristics didn't do their best here

OK, it seems I've been a bit confused myself. I think my final conclusion is actually just that clang is generating kind of crap code here, and this change gets good codegen out of clang. Since I was mostly looking at clang and was leaving gcc as an afterthought, I didn't pick up on this. (Obviously I'm a little rusty on my x86, I shouldn't have fallen for that!) As gcc shows, it doesn't matter much either way. But when clang is happy, its code is quite nice, so there's that.

That little address dependency chain is nastier than it looks, though, the way clang is writing it. Best done away with. There's really no reason for clang to be emitting that crud. gcc handles the addressing in a much cleaner manner whether it's byte- or dword-wide. But gcc also does need that fence when clang does not. I'm sure that's not a coincidence.

Could you explain your thought process [on placing the compiler fence]?

I ended up dumping this in kind of carelessly and noting that it worked before moving on (it was a last-minute add and I was kind of done by this point). But GCC does care where it goes, since it doesn't work if placed before. Let's try this explanation: the fence basically means "resolve all funny business with this variable before proceeding". Before the write, there's no funny business to resolve: nothing has happened to that variable. No writes, no outstanding reads (we're well into the loop at this point, in theory). So it's a fancy nop. But if it's after the write, we've now got a consequence: `next_j[i][j]` is different now. The fence forces us to resolve that now and not let it linger until the next loop iteration. Where it can be handled perfectly well, but not particularly quickly (which we do care about), and by the way that choice puts the update in the fast path. Forcing it to be dealt with before we re-enter the fast path is how we force the stupid compiler to keep it out of the fast path.

ugly code for address calculation... might affect throughput a little

What I'd actually worry most about is poisoning the prefetchers, and speculation more generally. They're very very good at handling things like "increasing stride off this base". They struggle more with things like "double indirect register access to determine next address", though there is so much compiler crud out there that they have some surprising capabilities. Basically: simple stupid loop stride is going to do well with them. Complicated four-instruction chains to generate an address that could have been `[base + 8*stride]` or whatever is just... not helping. gcc does perfectly well here. It has multiple offsets floating around, but all of those instructions to update them can update superscalar, in a single cycle. The clang chain is a four-cycle delay. And to top it all off the branch can probably fuse in the simple addressing case!

Quadrupling code performance with a "useless" if

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