Parser: box parser

[MarinPostma]

This PR boxes the yyParser in Parser. The reason for that is that it is huge (141464 bytes!), and I encountered stack overflows because of this.

[gwenn]

I guess that the real culprit is the AST (#8)

[MarinPostma]

can you point me toward where that could be fixed?

[gwenn]

Just run cargo clippy with enum-variant-size-threshold = 50.
There is also this stack:
https://github.com/gwenn/lemon-rs/blob/master/third_party/lemon/lempar.rs#L189
You can check its max size:
https://github.com/gwenn/lemon-rs/blob/master/third_party/lemon/lempar.rs#L184-L185
Or try to reduce its size:
https://github.com/gwenn/lemon-rs/blob/master/third_party/lemon/lemon.c#L4431-L4435

[MarinPostma]

I have investigated this a bit further. Here is the list of the 15 biggest types, along with their size:

141464	parser::parse::yyParser - /Users/mpostma/Documents/code/rust/lemon-rs/target/debug/build/sqlite3-parser-47a895829ff66033/out/parse.rs:2257:1: 2265:2 (#0)
  6480	[&'static str; _] - /Users/mpostma/Documents/code/rust/lemon-rs/target/debug/build/sqlite3-parser-47a895829ff66033/out/parse.rs:2630:1: 3036:3 (#0)
  4688	[&'static str; 293] - /Users/mpostma/Documents/code/rust/lemon-rs/target/debug/build/sqlite3-parser-47a895829ff66033/out/parse.rs:2328:1: 2623:3 (#0)
  4550	[u16; 2275] - /Users/mpostma/Documents/code/rust/lemon-rs/target/debug/build/sqlite3-parser-47a895829ff66033/out/parse.rs:1635:1: 1864:3 (#0)
  4216	[u16; 2108] - /Users/mpostma/Documents/code/rust/lemon-rs/target/debug/build/sqlite3-parser-47a895829ff66033/out/parse.rs:1420:1: 1632:3 (#0)
  1144	parser::Context - src/parser/mod.rs:38:1: 44:2 (#0)
  1114	[u16; 557] - /Users/mpostma/Documents/code/rust/lemon-rs/target/debug/build/sqlite3-parser-47a895829ff66033/out/parse.rs:1981:1: 2038:3 (#0)
  1114	[u16; 557] - /Users/mpostma/Documents/code/rust/lemon-rs/target/debug/build/sqlite3-parser-47a895829ff66033/out/parse.rs:1872:1: 1929:3 (#0)
  1096	parser::parse::yyStackEntry - /Users/mpostma/Documents/code/rust/lemon-rs/target/debug/build/sqlite3-parser-47a895829ff66033/out/parse.rs:2244:1: 2250:2 (#0)
  1096	parser::ast::Cmd - src/parser/ast/mod.rs:117:1: 121:2 (#0)
  1088	parser::parse::YYMINORTYPE - /Users/mpostma/Documents/code/rust/lemon-rs/target/debug/build/sqlite3-parser-47a895829ff66033/out/parse.rs:100:1: 180:2 (#0)
  1088	parser::ast::Stmt - src/parser/ast/mod.rs:157:1: 273:2 (#0)
  1008	parser::ast::TriggerCmd - src/parser/ast/mod.rs:2648:1: 2669:2 (#0)
   928	parser::ast::InsertBody - src/parser/ast/mod.rs:2537:1: 2540:2 (#0)
   810	[u16; _] - /Users/mpostma/Documents/code/rust/lemon-rs/target/debug/build/sqlite3-parser-47a895829ff66033/out/parse.rs:3382:1: 3788:3 (#0)

The main reason yyParser is so large, is because of the SmallVec of yyStackEntry, each of which is 1088 bytes.
Almost all of the size in the ssStackEntry comes from the YYMINORTYPE (1088 bytes) (that's the AST enum). I don't think that 1088 bytes for enum are problematic. There could be performance issues caused by its size of it, but then it would require actual benchmarking to see where to draw the line.

That basically leaves us with two options:

• Stop using a SmallVec<yyStackEntry>, which takes a lot of space on the stack
• Make yyStackEntry leaner. That's made easy by boxing the YYMINORTYPE inside of it.

The former is easier since it doesn't involve editing the grammar file.

[gwenn]

Thanks for your investigation.
In the original C version, you can choose between dynamic vs static stack:
https://github.com/sqlite/sqlite/blob/4e86aa86ea0ab657423f1a715c620c188c6ba879/tool/lempar.c#L215-L222
And SQLite uses a static stack but doesn't generate an AST...

We should try to rollback 8e9925e

[MarinPostma]

@gwenn, I have changed my mind and decided to go with making the AST itself leaner, I'll keep you updated

[MarinPostma]

I'm closing this pr for now, I'll get back to it when I have more time to work on optimizing the AST itself

[gwenn]

@MarinPostma For your information, SmallVec has been removed.
See #57