hici is a decent example. The branches at the end can be eliminated by
unconditionally hashing the last two words of the input (as by v[k-2] and
v[k-1], if v and k were never modified). In the event that the number of
words to be hashed is a multiple of 3 or one more than a multiple of 3,
some words will be hashed more than once, but this is harmless. At a
rough estimate, this costs 10 cycles for random inputs. (For batch
inputs, such as i. on skinny matrices, the branch wins, but a
transposed-ish traversal order will do better still, so it doesn't matter
much.)
On Sat, 9 Apr 2022, Henry Rich wrote:
If you show me an example I'd be obliged.
Henry Rich
On 4/9/2022 10:19 PM, Elijah Stone wrote:
Duff's device is a technique for loop unrolling which involves jumping
into the middle of the unrolled loop. Overlapping access is an
alternate technique also useful for loop unrolling (as well as
arranging for aligned access), which involves doing a small amount of
redundant work, but saves branches. Though it is slightly less
general, this tends to be more performant on modern hardware.
On Sat, 9 Apr 2022, Henry Rich wrote:
I don't understand the last sentence. 'duff'? 'overlapping access'?
Henry Rich
On 4/9/2022 9:37 PM, Elijah Stone wrote:
On Sat, 9 Apr 2022, Henry Rich wrote:
JE generally does few data-dependent branches, and I expect it
would not be a good idea to use two hyperthreads in one core; but
you'll have to make that decision.
Branch miss is tens of cycles; cache miss is hundreds.
Re branches: from what I've seen, there is too much duff and not
enough overlapping access. Though I guess those are
metadata-dependent branches, not data-dependent branches :)
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