2102922286

3년 전discuss

Here's a video made 53 years ago about how we need to tackle pollution: https://www.youtube.com/watch?v=MA45iKHjoGg

A lot of it still rings true today. The sad thing is that we _knew_ that _something_ had to be done, but our political and economic structures provide no incentive for long-term thinking, and so we didn't change anything.

2102922286

3년 전discuss

https://uops.info/uiCA.html it's really cool!!

2102922286

3년 전discuss

This is one area where Profile-Guided Optimization (PGO) can help a lot! With PGO, you run your program on some sample input and it logs info like how many times each side of a branch was taken. From there, you can recompile your code. If the compiler sees that one side of the branch dominates, it can emit code to prioritize that branch. However if the branch counts are approximately even and the branch is hard to predict (n.b. this is technically distinct from having even branch counts), then the compiler can know that the CPU would have trouble predicting the branch, and can emit a cmov instruction instead.

2102922286

3년 전discuss

For the given assembly from the blog post

    loop:
        lea (%rdx,%rax,4),%rcx
        cmp (%rcx),%esi
        cmovg %rcx,%rdx
        shr %rax
        jne loop

Here's a simulated CPU trace on Intel skylake: https://uica.uops.info/tmp/2de9d862d05d482ebed576d7e3923b93_...

Note that this tracer makes the assumption that all memory loads are in cache (otherwise the memory lookup will dominate). So bear that in mind for this code, especially since memory reads will likely dominate the cost of a binary search.

Regardless, it appears that the cost of conditional move is not the source of bottleneck.

2102922286

3년 전discuss

If what you're saying is (roughly)

        cmovne  rax, rdx
        jmp     rax

that is, a cmov followed by an indirect jump to the address contained in rax, "jmp rax" is _always_ an indirect jump. It doesn't matter whether rax was set via a conditional move instruction or not.

2102922286

3년 전discuss

When implementing cryptographic primitives, you want to avoid branching on secret values. The reason why is that the CPU's branch predictor will attempt to predict the value that you're branching on, and thus something about the values that you're branching on gets revealed if you can see how long a CPU takes to run a task/perform a function.

This is more than just a theoretical issue. These channel attacks have been demonstrated in practice, even if the victim CPU is running across the internet.

2102922286

3년 전discuss

Cmov doesn't branch. A branch refers specifically to the program counter ending up in more than one possible place after an instruction has executed. It is this behavior that mucks with the CPU state and slows everything down.

It's true that the cmov instruction uses the CPU flags register (which I'm sure the CPU designers at Intel hate), but that doesn't mean that it branches.

You can achieve the same effect as cmov by using bitwise operations and a subtraction, though it'd just be a few cycles slower--but it would be even more clear that it doesn't branch.

2102922286

3년 전discuss

A cool related algorithm is https://algorithmica.org/en/eytzinger

In addition to being branchless, it also has better cache properties than a standard binary search tree.

If you're doing a binary search in an array, you start in the middle, and then jump all the way to the midway point of one half, and so on. As a result, there's a lot of distance between each read that you do in the array. Thus, each read is putting extra strain on the cache (or the reads won't be cached).

The CPU cache performs much better if the values you want to read are close to each other. Enter the Eytzinger Binary Search. The idea is that, if you're always going to be accessing the root of the tree, and then one of its two children--you should just put those children physically close to the root, so they'll all be on the same cache line!

2102922286

3년 전discuss

cmov (i.e. conditional move) doesn't branch. By branch here, we mean "at point at which the successor program counter value might be one of two locations." It's true that this _does_ have security implications, however this is frequently used as a term of art by performance engineers (see https://www.youtube.com/watch?v=g-WPhYREFjk for example).

> And it doesn't really matter whether a branch can be well predicted.

I'm not so sure this is true. See https://lemire.me/blog/2019/10/15/mispredicted-branches-can-... as an example.

A properly predicted branch can be _faster_ than a cmov instruction, however the important point is avoiding the branch mispredict. The fact that we're using cmov is beside the point. We could achieve a similar effect by performing a subtraction and doing bitwise arithmetic to extract the sign bit (which, in effect, performs a comparison).

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