Fix exp-time parse backtrack by saving if '[' starts Array or Dictionary

[modelorganism]

Save the results of discovering whether a given '[' is the start of an Array or a Dictionary. Remembering this prevents parseExprCollection() from having to parse the first sub-expr in square brackets {2^n} times, where n is # of nested square brackets.

Swift in parseExprCollection parses the first expr in a [x..], a square-bracketed expr-list, twice. Once to see if there is a colon after x or not, [x : v ..] vs, [x , y ..], then a second parse 'for real'. If there is a [..] inside of the first expr of another [..], [[..]...], the first expr of the inner list is parsed 4-times, or more generally 2^n, where n is the depth of nesting.

This PR adds a structure to class Parser, ArrayOrDictStartLocCache, which remembers whether each [ starts a Array or Dict for as long any surrounding [...] is being parsed. For the duration of a [...] being parsed this cached value will be used in place a performing a subsequent speculative parse. There are still 2 parses of each head of a [h ...], but not 2^n. Once the outermost ] is encountered, the information is discarded as the parser can longer backtrack over the subexprs.

Tests ran successfully on Mac at this rev (that is with this change after Merge pull request #20500 from xedin/additional-tests-for-autoclosure…)

Resolves SR-9220.

[dan-zheng]

I wonder how quickly does this parse now?

[modelorganism]

let l = [[[[[[[[[[[[[[[ ]]]]]]]]]]]]]]]
0.09 real 0.06 user 0.02 sys
let l = [[[[[[[[[[[[[[[[ ]]]]]]]]]]]]]]]]
0.11 real 0.07 user 0.02 sys
let l = [[[[[[[[[[[[[[[[[ ]]]]]]]]]]]]]]]]]
0.13 real 0.10 user 0.02 sys
let l = [[[[[[[[[[[[[[[[[[ ]]]]]]]]]]]]]]]]]]
0.18 real 0.14 user 0.03 sys
let l = [[[[[[[[[[[[[[[[[[[ ]]]]]]]]]]]]]]]]]]]
0.27 real 0.23 user 0.03 sys
let l = [[[[[[[[[[[[[[[[[[[[ ]]]]]]]]]]]]]]]]]]]]
0.47 real 0.42 user 0.04 sys
let l = [[[[[[[[[[[[[[[[[[[[[ ]]]]]]]]]]]]]]]]]]]]]
0.86 real 0.78 user 0.06 sys
let l = [[[[[[[[[[[[[[[[[[[[[[ ]]]]]]]]]]]]]]]]]]]]]]
1.69 real 1.50 user 0.11 sys
let l = [[[[[[[[[[[[[[[[[[[[[[[ ]]]]]]]]]]]]]]]]]]]]]]]
3.30 real 2.94 user 0.21 sys
let l = [[[[[[[[[[[[[[[[[[[[[[[[ ]]]]]]]]]]]]]]]]]]]]]]]]
6.47 real 5.85 user 0.39 sys
let l = [[[[[[[[[[[[[[[[[[[[[[[[[ ]]]]]]]]]]]]]]]]]]]]]]]]]
13.47 real 12.44 user 0.80 sys
let l = [[[[[[[[[[[[[[[[[[[[[[[[[[ ]]]]]]]]]]]]]]]]]]]]]]]]]]
26.47 real 24.59 user 1.61 sys
let l = [[[[[[[[[[[[[[[[[[[[[[[[[[[ ]]]]]]]]]]]]]]]]]]]]]]]]]]]
51.32 real 47.20 user 3.63 sys
let l = [[[[[[[[[[[[[[[[[[[[[[[[[[[[ ]]]]]]]]]]]]]]]]]]]]]]]]]]]]
103.45 real 95.47 user 7.31 sys
let l = [[[[[[[[[[[[[[[[[[[[[[[[[[[[[ ]]]]]]]]]]]]]]]]]]]]]]]]]]]]]
280.94 real 195.70 user 14.23 sys
let l = [[[[[[[[[[[[[[[[[[[[[[[[[[[[[[ ]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]
463.93 real 386.87 user 27.48 sys

[dan-zheng]

Huh, 463.93 is still pretty long. Maybe it's the best we can do.

[rintaro]

Thanks for tackling this!
This is clever idea. But, we want to eliminate backtracking here. Because parsing expression might have some side effects, and backtracking might not be able to cancel them. Not sure how it's bad, but still.

Previously, we weren't using backtracking:
https://github.com/apple/swift/blob/bb157a070ec6534e4b534456d208b03adc07704b/lib/Parse/ParseExpr.cpp#L3042-L3058
However, when we introduced libSyntax parsing, we started to use backtracking here because parseList() which is used in parseExprDictionary()/parseExprArray() could not insert pre-parsed expression to the list. But using SyntaxParsingContext::popIf() in parseExprCollection() and SyntaxParsingContext::addSyntax() in parseExprDictionary()/parseExprArray(), we can inject pre-parsed expression to the list.

@modelorganism do you want to try that?

[modelorganism]

@modelorganism do you want to try that?

Yes let me give that a shot.

[rintaro]

@modelorganism Sorry, I'm taking over this in #21487