Failure in fusion leads to very inefficient code about vector

[cartazio]

whats the performance delta one way vs the other?

does this happen if you use vector head? (the bundle rep sometimes fuses better i'm told )

[takano-akio]

Here are some benchmark results:

https://gist.github.com/takano-akio/e3c4c3c56ecc72b60b3d

In summary, the bad version is about 4.9x slower than the good one, with both versions of vector.

[cartazio]

how does the code optimize with vector HEAD/master?

[takano-akio]

@cartazio I'm not sure that I understand the question. Would you like to see the optimized core, or something else?

[glguy]

@cartazio The "Stream" constructors optimize away in GHC 7.10.1-rc1 and GHC 7.11.20150120

[cartazio]

@glguy in which case, its not really a bug on the vector side, but variability in how ghc optimizes, right?

[cartazio]

i've tried running your example code "good" vs "bad" benchmark locally with the most recent release of vector 0.11 on ghc 8.0.1 on os x with O2 optimization, and the timings seem to be essentially the same (only a ~ 10% difference but the total range of the times all lies in 80-100 microseconds)

so pending a code example that reproduces it with recent vector and ghc, i'm closing this and marking it invalid for now

[cartazio]

(that said, I really do appreciate that you provided code for the purposes of this bug)