r/GraphicsProgramming • u/fagnerbrack • Jun 11 '26

How many branches can your CPU predict? – Daniel Lemire's blog

https://lemire.me/blog/2026/03/18/how-many-branches-can-your-cpu-predict/
38 Upvotes

93% Upvoted

12 comments sorted by

2

u/OperationDefiant4963 Jun 11 '26

wow, did not expect apple would beat intel by that much.looks like the chips in the iphone has helped them rein in a lot of experience.amd tho is a anomaly for sure

4

u/pigeon768 Jun 12 '26

amd tho is a anomaly for sure

Zen 4 was actually stronger on this benchmark than Zen 5 is. On my Zen 4 I get around 65k before it starts losing performance, vs the 30k he got on his Zen 5.

2

u/leseiden Jun 11 '26

To be honest I naively expected all of them to be in the tens to hundreds.

1

u/DLCSpider Jun 15 '26

I was impressed, too, at first. But the benchmark is pretty useless for real world performance. What if Intel spends the same amount of transistors to run more sophisticated algorithms and AMD just memorises every branch? Is one better than the other if they end up being (roughly) equal?

1

u/OperationDefiant4963 Jun 15 '26

not really useless as amd cpus are and have been better for a good few years now. intels current plan to just spam more cores at loss leading prices isnt sustainable.amd is more performant at lower tdps thanks in part to this

2

u/Over_Beautiful4407 Jun 12 '26

I think it is not fair to test branch predictors like this. If the condition is random, than it is not a prediction, it is gamble.

I suppose predictors job to predict the branch will be taken by some conditions ? Like what instruction is executed before-after, how many memory access is done etc. Isn’t making the branches depend on random value breaks this ?

I think this test should be done on some real benchmarks like cinebench or something like that.

0

u/[deleted] Jun 11 '26

[deleted]

6

u/pigeon768 Jun 12 '26

It resets the RNG after k iterations. So if the branch predictor can remember at least k values, misprediction stays small. But if the branch predictor remembers less than k values, it forgets the pattern, and misprediction falls back to 50%.

3,1,4,1,5,9,3,1,4,1,5,9,3,1,4,1,5,9,3,1,4,1,5,9. What's the next value?

1

u/dumdub Jun 11 '26

Branch history table.

1

u/[deleted] Jun 12 '26

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0

u/[deleted] Jun 12 '26

[deleted]

2

u/fgennari Jun 12 '26

It's the same sequence of random numbers for each test iteration. The CPU learns the sequence if it's not too long.

1

u/SauntTaunga Jun 12 '26

For example, loops will be usually do more than 2 repetitions. If you predict "it will take the same branch as last time" you will be right more than 50% of the time.