Replace dimension array access with simple index

[chrispcampbell]

SDEverywhere generates arrays of index numbers to help support subdimensions:

  function arrayDimensionsSection() {
    // Emit a declaration for each array dimension's index numbers.
    // These index number arrays will be used to indirectly reference array elements.
    // The indirection is required to support subdimensions that are a non-contiguous subset of the array elements.
    let a = R.map(dim => `const size_t ${dim.name}[${dim.size}] = { ${indexNumberList(sub(dim.name).value)} };`)
    let arrayDims = R.pipe(a, asort, lines)
    return arrayDims(allDimensions())
  }

These get declared at the top of the generated c file, for example:

// Array dimensions
const size_t _all_things[5] = { 0, 1, 2, 3, 4 };
const size_t _all_things_except_foo_and_bar[3] = { 1, 2, 4 };

When accessing values, these arrays are used as a form of indirection, for example:

for (i = 0; ...) {
  _something[_all_things[i]] = _another[_all_things[i]];
}

In En-ROADS, some subsets are non-contiguous, but in a majority of cases, these dimension index arrays are "trivial", meaning their elements are identical to their index/position. For those cases, we can generate more optimal code that skips the array access; the example above would become:

for (i = 0; ...) {
  _something[i] = _another[i];
}

Avoiding unnecessary array accesses leads to some more modest performance gains.

[chrispcampbell]

Here are some performance numbers relative to other recent optimizations.

Performance

MacBook Pro (2019) | 2.4 GHz 8-core i9, 32 GB RAM, macOS 10.15

Issue	C run (ms)	Wasm run (ms)	Wasm init (ms)	JS mem (MB)	Page mem (MB)
baseline	45.8	87.5	38.0	94	685
SDE 18	46.0	85.6	18.0	39	672
SDE 19	42.8	49.4	15.0	38	25
SDE 22	34.8	44.8	15.0	38	21
SDE 23	32.7	42.8	13.0	38	32

iPhone 8 | A11, iOS 13

Issue	C run (ms)	Wasm run (ms)	Wasm init (ms)	JS mem (MB)	Page mem (MB)
baseline	39.9	187.0	165.0	39	645
SDE 18	40.3	219.0	86.0	38	724
SDE 19	40.1	81.6	83.0	38	41
SDE 22	35.5	74.6	86.0	40	40
SDE 23	31.1	73.6	82.0	41	39

iPad Air (2013) | A7, iOS 12

Issue	C run (ms)	Wasm run (ms)	Wasm init (ms)	JS mem (MB)	Page mem (MB)
baseline	151.0	1372.2	30146.0	77	331
SDE 18	166.0	1408.0	4416.0	42	395
SDE 19	151.0	837.6	1291.0	45	41
SDE 22	137.0	771.6	1484.0	44	40
SDE 23	110.1	642.2	1148.0	44	41

Size

Issue	Wasm size (bytes)
baseline	1,084,036
SDE 18	773,968
SDE 19	776,851
SDE 22	737,028
SDE 23	741,668

Legend

Issue	Date	Notes
baseline	2020/07/08	baseline prior to performance work
SDE 18	2020/07/09	change lookup init to use static arrays
SDE 19	2020/07/09	break large functions into chunked subfunctions
SDE 22	2020/07/10	replace wrapper functions with macros
SDE 23	2020/07/10	replace dimension array access with simple index

[chrispcampbell]

Merged to develop in 767a1c8.