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0 pointsmcv2y ago0 comments

I didn't, and frankly, half of the articles I read about it make me think branch prediction is a bug. I mean, I know it's meant to improve performance, which is great, but it has to make assumptions about what's going to happen before it knows it, and those assumptions are going to be wrong. How wrong? How can we con it into making better assumptions? Suddenly programming becomes about second guessing the compiler.

And remember Spectre and Meltdown? Security vulnerabilities caused by branch prediction. If I recall correctly, the pipeline was executing code it wasn't meant to execute because it's executing it before it knows the result of the check that decides if it has to execute it.

Programming is a lot easier if the actual control flow is as linear as I'm writing it.

My broad takeaway of the whole ordeal is that I'm basically avoiding if-statements these days. I feel like I can't trust them anymore.

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layer82y ago

Without branch prediction and speculative execution, performance would always be as bad as with a misprediction. So every correct prediction is a win, and that’s why it’s being done.

> My broad takeaway of the whole ordeal is that I'm basically avoiding if-statements these days.

That seems like an overreaction. For most applications, cache locality and using the right algorithms is more important, if performance is an issue at all.

Galanwe2y ago

> it has to make assumptions about what's going to happen before it knows it, and those assumptions are going to be wrong. How wrong

Speculative/optimistic techniques are not limited to branch prediction, you encounter various forms of it pretty much everywhere, without knowing it.

* Your hard drives have a read ahead buffer, fetching in advance future data, just because most reads are immediately followed by an other one for the data just after.

* Your CPU instructions are pre fetched, because most instructions are _not_ jumps, so you will 99% of the time just execute the instruction right after it.

* Instruction reordering where your compiler will decide that future instructions not affected by previous instructions could be run in advance.

* Overall any kind of caching is some form of optimistic technique. You are preparing future results.

If you think about it, optimistic/speculative techniques are ubiquitous, and used even at _high_ abstraction levels.

The famous python mantra of "better ask for forgiveness than permission" embodies that spirit. It encourages a coding style of "try: do() except: nope()" rather than "if check: do()".

Standing "against optimistic/speculative techniques" is standing against transactions, rollbacks, caches. It's just not a viable line of thinking IMHO.

superb_dev2y ago

You shouldn’t be thinking about the branch predictor this much. The predictor is pretty good at guessing, unless you need to eek every last shred of performance you can ignore it

flohofwoe2y ago

> My broad takeaway of the whole ordeal is that I'm basically avoiding if-statements these days. I feel like I can't trust them anymore.

Such broad "optimization rules" usually don't make much sense on any compiler created in the last 25 years. An optimizing compiler will analyse and optimize your program's control flow at a very high level, not just naively insert a conditional branch for each if (and that's also true in "low level" languages like C)

If you actually care about such things, you really need to look at the compiler output and incrementally tweak your code (but then other compilers or even compile options might destroy such tweaks again), there hardly is a single optimal source code solution across CPU architectures or even just different CPU models of the same architecture (just don't do any obviously stupid things which make the life of the compiler harder than need be)

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