Zen 5's 2-ahead branch predictor: how a 30 year old idea allows for new tricks
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Re: Zen 5's 2-ahead branch predictor: how a 30 year old idea allows for new tricks
#2As a novice in this area, it's not clear to me after reading this what exactly the 2-ahead branch predictor is.
Re: Zen 5's 2-ahead branch predictor: how a 30 year old idea allows for new tricks
#3that's probably bad idea but I would like to learn why:
why when we have a conditional branch we cannot just fetch and prepare instructions for both possible branches and then discard the incorrect one?
is this that much harder or there are other reasons that makes this not worth it
Re: Zen 5's 2-ahead branch predictor: how a 30 year old idea allows for new tricks
#4that's probably bad idea but I would like to learn why: why when we have a conditional branch we cannot just fetch and prepare instructions for both possible branches and then discard the incorrect one? is this that much harder or there are other reasons that makes this not worth it
Transistor count; now you have to duplicate all the decode and speculative execution circuitry for both possible branches
Re: Zen 5's 2-ahead branch predictor: how a 30 year old idea allows for new tricks
#5It's always interesting to see decades old papers, sometimes published with little to no fanfares, suddenly becomes "state of the art" because hardware have become powerful enough.
For example Z-buffers[1]. It's used by 3d video games. When it's first published on paper, it's not even the main topic of the paper, just some side notes because it requires expensive amount of memory to run.
Turn out megabytes is quite cheap few decades latter, and every realtime 3d renderer ended up using it.
Re: Zen 5's 2-ahead branch predictor: how a 30 year old idea allows for new tricks
#6As a novice in this area, it's not clear to me after reading this what exactly the 2-ahead branch predictor is.
You can check out the seminal paper linked in the article. Or start by summarizing the paper with Gemini, Claude, ChatGPT, etc. to get a high level overview (and then confirm the answer by reading the paper).
Re: Zen 5's 2-ahead branch predictor: how a 30 year old idea allows for new tricks
#7that's probably bad idea but I would like to learn why: why when we have a conditional branch we cannot just fetch and prepare instructions for both possible branches and then discard the incorrect one? is this that much harder or there are other reasons that makes this not worth it
It's a huge waste of energy and in some cases it would even be slower because you'd execute more instructions overall. If the branch mispredict rate is around 1% it's simply not worth paying a penalty 99% of the time to get a gain 1% of the time. Maybe it would be worth doing on low-confidence branches.
Re: Zen 5's 2-ahead branch predictor: how a 30 year old idea allows for new tricks
#8It's always interesting to see decades old papers, sometimes published with little to no fanfares, suddenly becomes "state of the art" because hardware have become powerful enough. For example Z-buffers[1]. It's used by 3d video games. When it's first published on paper, it's not even the main topic of the paper, just some side notes because it requires expensive amount of memory to run. Turn out megabytes is quite c…
I sometimes wonder if there’s an academic career hidden in there for an engineer: go to the library and read what the CS folks were publishing on physical papers, maybe there are some ideas that can actually be implemented now that weren’t practical back then.
Re: Zen 5's 2-ahead branch predictor: how a 30 year old idea allows for new tricks
#9that's probably bad idea but I would like to learn why: why when we have a conditional branch we cannot just fetch and prepare instructions for both possible branches and then discard the incorrect one? is this that much harder or there are other reasons that makes this not worth it
Disclaimer: I don't work in this field, just an enthusiast.
As far as I can tell, branch predictors have always been too good for it to be worth it. Moderns CPUs have instruction reorder buffers that are hundreds of instructions deep, so even if only 8 of those instructions are conditional jumps, there's 256 different paths your program could take. If your branch predictor predicts all 8 correctly >50% of the time (It does), doing 256x the work to cover your ass is not worth it.
Re: Zen 5's 2-ahead branch predictor: how a 30 year old idea allows for new tricks
#10It's always interesting to see decades old papers, sometimes published with little to no fanfares, suddenly becomes "state of the art" because hardware have become powerful enough. For example Z-buffers[1]. It's used by 3d video games. When it's first published on paper, it's not even the main topic of the paper, just some side notes because it requires expensive amount of memory to run. Turn out megabytes is quite c…
I sometimes wonder if there’s an academic career hidden in there for an engineer: go to the library and read what the CS folks were publishing on physical papers, maybe there are some ideas that can actually be implemented now that weren’t practical back then.
Yes, "read 10 year old papers as a source of ideas ripe for commercialization" IS common advice in universities.