Improve scalar quoting performance

[juergenhoetzel]

Results on my system:

Current:

(benchmark 100000 '(emacsql-quote-scalar "I'm learning Python."))
"Elapsed time: 2.657825s (1.080004s in 23 GCs)"

PR:

(benchmark 100000 '(emacsql-quote-scalar "I'm learning Python."))
"Elapsed time: 1.067836s (0.787702s in 17 GCs)"

[skeeto]

The original implementation was based on replace-regexp-in-string: 3b70e8f. I switched to the temp buffer version because whatever throwaway benchmark I used at the time showed it was faster, particularly because replace-regexp-in-string generates tons of garbage.

Revisiting it now with Emacs 27.1, there's a crossover point between 3 and 4 apostrophes. The temp buffer has a high startup cost, but is overall more efficient (the gap buffer avoids generating garbage).

;;; -*- lexical-binding: t; -*-
(dotimes (i 25)
  (let ((s (format "abc%sdef" (make-string i ?'))))
    (benchmark 100000 (list 'emacsql-quote-scalar s))))

The regexp-replace version runs the garbage collector much more often:

No matter the number of apostrophes, it spends most of its time in the garbage collector:

Since most string scalars are expected to have fewer than 4 apostrophes, it probably makes sense just to use the replace-regexp version, but I'm still wary about the worst case and generating so much garbage.

[juergenhoetzel]

Thank you for the detailed insights and explanation of the historical reasons that led to the current implementation.

Interestingly, the replace-regex implementation on my system is always faster. Anyway, I have looked at the implementation of replace-regex and now I have implemented a specialized version, which is much faster:

regexp-replace                                   temp-buffer                                      own implementation

Elapsed time: 0.521850s (0.524323s in 8 GCs)     Elapsed time: 2.559998s (1.298774s in 28 GCs)    Elapsed time: 0.443598s (0.333841s in 5 GCs)
Elapsed time: 0.767311s (0.703203s in 13 GCs)    Elapsed time: 2.969729s (1.616639s in 33 GCs)    Elapsed time: 0.686606s (0.538863s in 10 GCs)
Elapsed time: 1.061757s (0.913401s in 17 GCs)    Elapsed time: 3.216180s (1.788128s in 38 GCs)    Elapsed time: 0.812525s (0.611451s in 11 GCs)
Elapsed time: 1.413832s (1.185098s in 22 GCs)    Elapsed time: 3.816427s (2.116909s in 43 GCs)    Elapsed time: 0.842065s (0.695212s in 13 GCs)
Elapsed time: 1.704233s (1.399007s in 26 GCs)    Elapsed time: 4.111683s (2.391927s in 49 GCs)    Elapsed time: 0.948747s (0.748723s in 14 GCs)
Elapsed time: 2.054108s (1.668513s in 31 GCs)    Elapsed time: 4.198231s (2.453352s in 54 GCs)    Elapsed time: 1.053291s (0.800285s in 15 GCs)
Elapsed time: 2.357662s (1.882876s in 35 GCs)    Elapsed time: 4.572700s (2.619197s in 58 GCs)    Elapsed time: 1.267134s (0.854599s in 16 GCs)
Elapsed time: 2.651335s (2.098092s in 39 GCs)    Elapsed time: 5.015285s (2.917136s in 64 GCs)    Elapsed time: 1.318649s (0.960398s in 18 GCs)
Elapsed time: 3.002761s (2.367188s in 44 GCs)    Elapsed time: 5.384789s (3.157256s in 69 GCs)    Elapsed time: 1.426605s (1.015898s in 19 GCs)
Elapsed time: 3.298794s (2.583839s in 48 GCs)    Elapsed time: 6.189858s (3.695950s in 75 GCs)    Elapsed time: 1.638877s (1.068143s in 20 GCs)
Elapsed time: 3.653449s (2.860977s in 53 GCs)    Elapsed time: 6.193920s (3.765986s in 79 GCs)    Elapsed time: 1.690573s (1.175211s in 22 GCs)
Elapsed time: 3.985965s (3.077324s in 57 GCs)    Elapsed time: 6.444607s (3.869025s in 84 GCs)    Elapsed time: 1.796182s (1.227437s in 23 GCs)
Elapsed time: 4.228551s (3.292603s in 61 GCs)    Elapsed time: 6.861403s (4.213131s in 90 GCs)    Elapsed time: 2.010033s (1.280468s in 24 GCs)
Elapsed time: 4.579165s (3.552230s in 66 GCs)    Elapsed time: 7.236405s (4.441292s in 95 GCs)    Elapsed time: 2.068538s (1.388774s in 26 GCs)
Elapsed time: 4.988655s (3.771820s in 70 GCs)    Elapsed time: 7.431056s (4.595113s in 100 GCs)   Elapsed time: 2.215706s (1.444392s in 27 GCs)
Elapsed time: 5.239030s (4.038972s in 75 GCs)    Elapsed time: 7.802493s (4.873736s in 105 GCs)   Elapsed time: 2.393896s (1.497034s in 28 GCs)
Elapsed time: 5.526718s (4.253166s in 79 GCs)    Elapsed time: 8.137921s (5.080095s in 110 GCs)   Elapsed time: 2.440634s (1.600902s in 30 GCs)
Elapsed time: 5.929200s (4.468238s in 83 GCs)    Elapsed time: 8.626883s (5.356165s in 115 GCs)   Elapsed time: 2.547778s (1.655972s in 31 GCs)
Elapsed time: 6.272569s (4.738194s in 88 GCs)    Elapsed time: 8.847913s (5.546497s in 121 GCs)   Elapsed time: 2.758850s (1.706297s in 32 GCs)
Elapsed time: 6.551936s (5.034335s in 93 GCs)    Elapsed time: 9.078585s (5.633488s in 125 GCs)   Elapsed time: 2.815046s (1.816697s in 34 GCs)
Elapsed time: 6.862757s (5.165854s in 96 GCs)    Elapsed time: 9.448582s (5.894076s in 131 GCs)   Elapsed time: 2.919532s (1.867242s in 35 GCs)
Elapsed time: 7.156676s (5.486435s in 102 GCs)   Elapsed time: 9.682056s (6.110939s in 136 GCs)   Elapsed time: 3.025861s (1.920943s in 36 GCs)
Elapsed time: 7.398661s (5.650340s in 105 GCs)   Elapsed time: 10.027343s (6.341478s in 141 GCs)  Elapsed time: 3.238696s (1.975818s in 37 GCs)
Elapsed time: 7.751980s (5.920265s in 110 GCs)   Elapsed time: 10.654161s (6.716559s in 146 GCs)  Elapsed time: 3.290497s (2.083083s in 39 GCs)
Elapsed time: 8.152714s (6.133111s in 114 GCs)   Elapsed time: 12.392045s (7.733312s in 152 GCs)  Elapsed time: 3.501953s (2.137579s in 40 GCs)