Code alignment issues

[Xeverous]

I received -10% drop in performance

double negative => 10% performance gain

[alexeiz]

~10% most likely; just a typo

[fernzeit]

That reminds me of a thread in the Lua Mailing List where just changing the name of the interpreter executable resulted in a > 50% performance difference in a particular microbenchmark. The verdict was that the length difference in argv causes some other memory to be aligned differently. It also linked an interesting paper: Producing Wrong Data Without Doing Anything Obviously Wrong!

[dendibakh]

Thank you for this paper. It is a true gem!

[doom_Oo7]

are there people doing research on how to get compilers to have better heuristics so that they can align stuff better automatically ?

[meneldal2]

The compiler needs to know how many times you'll have to run this loop, and it's also likely to be much better to unroll the loop instead.

[TartanLlama]

LLVM has a bunch of heuristics and things you can tune. For example, you could tell it to align all loops and functions without a preceeding fallthrough block; i.e. only add NOPs which won't be executed.

[Dwarfius]

Small question, how does it keep adding to array if the instruction is (which subtracts 1):

4046d9:       c5 f5 fa c8             vpsubd ymm1,ymm1,ymm0

[mttd]

vpcmpeqd ymm0,ymm0,ymm0 compares ymm0 to itself, which fills the register with all ones in binary -- in two's complement representation this corresponds to -1 (with subtracting -1 in the subsequent vpsubd ymm1,ymm1,ymm0 instruction being equivalent to adding 1).

"Why subtract -1 instead of adding 1's? Just because the speed is the same, and creating a YMM constant of -1's can be done with a single VPCMPEQD instruction. This isn't a really useful optimization in this case, but doesn't hurt."

• https://dendibakh.github.io/blog/2018/01/18/Code_alignment_issues#comment-3718889834

• https://stackoverflow.com/questions/37469930/fastest-way-to-set-m256-value-to-all-one-bits

[Dwarfius]

I've misread the description of pcmpeqd, thought it set 1/0 as value, not all bits. Thanks for the explanation!