fix #16771 (swap js); improve tsystem_misc.nim; test in js too

[timotheecour]

• fix swap silently broken for objects inside functions (js target) #16771
• fix double evaluation bug for js+rt (js+vm still has this issue)
• improve tsystem_misc.nim; test in js too

future work

• remove swap js workaround in std/lists: Various changes to lists (RFC #303) #16536
• see remaining pre-existing template double evaluation bug affecting swap in vm

(EDIT) links

• https://stackoverflow.com/questions/16201656/how-to-swap-two-variables-in-javascript

[Clyybber]

This is a hack/workaround, instead genSwap should be fixed here: https://github.com/nim-lang/Nim/blob/devel/compiler/jsgen.nim#L1119

[timotheecour]

This is a hack/workaround,

how so? What are benefits of jsgen.genSwap? jsgen.genSwap is more complex and it's better to use regular code than rely on magics, all else being equal. If this PR is accepted, we can remove jsgen.genSwap in a followup PR.

[Clyybber]

Then please use a temporary instead of this approach; related: https://web.archive.org/web/20151229220921/http://jsperf.com/swap-array-vs-variable/34

[Araq]

how so? What are benefits of jsgen.genSwap?

Instead of adding more hacks to poor system.nim we should fix the JS codegen. system.nim bloat is a real problem and causes on-going costs.

[Araq]

Well without a corresponding bugfix for vmgen, nobody knows how many hacks it involves.

[timotheecour]

Well without a corresponding bugfix for vmgen, nobody knows how many hacks it involves.

the remaining vm issue (2nd half of #16779) is a separate pre-existing issue, and fixing jsgen.genSwap instead of the (simpler) approach I took here would not have helped with that VM bug.

This PR fixes 2 issues (and simplifies js code), so IMO is a net improvement; future work can fix the remaining vm issue which will probably involve unrelated code changes.

[Araq]

would not have helped with that VM bug.

So we need to fix both vmgen and jsgen without hacks. I fail to see the disadvantage, still better than a hack inside system.nim that stays for good.

[Araq]

No activity, closing. Sorry.

[ringabout]

Succeeded by #23473