Pas2js optimizations
From Lazarus wiki
Overview
This page describes existing optimizations for the pas2js transpiler as well as feature requests.
Generated JavaScript
Existing optimizations
- -O-: Disable all optimizations
- -O1 : Level 1 optimizations (quick and debugger friendly)
- -OoEnumNumbers[-]: write enum value as number instead of name. Default in -O1.
- -OoRemoveNotUsedPrivates[-]: Default is enabled
- -OoRemoveNotUsedDeclarations[-]: Default enabled for programs with -Jc
- omit unneeded brackets in associative operations (a||b)||(c||d), (a&&b)&&(c&&d), (a|b)|c, (a&b)&c, (a^b)^c, (a+b)+c, (a-b)-c, (a*b)*c
- Use {$modeswitch OmitRTTI} to treat published sections as public and thus remove them by WPO. Published section provide RTTI, so the compiler cannot know at compile time if a published identifier is needed, so it keeps it.
Feature requests
- -O1 : Level 1 otpimizations, fast and debugger friendly
- -O2 : Level 2 optimizations, slow and not debugger friendly
- -O3 : Level 3 optimizations, slow and might break special code
- -O4 : Level 4 optimizations, very slow and/or only for rare cases
- move rtl.js functions to system.pp and let the WPO decide if they are needed
- add $mod only if needed
- add Self only if needed
- use a number for small sets instead of JS objects
- shortcut for test if a set is empty a=[] a<>[]
- SetLength(scope.a,l) -> read scope only once, same for Include, Exclude, Inc, Dec, +=, -=, *=, /=
- ECMAScript6: use const for local const
- -O1: shortcut: a in [b] -> a=b
- -O1: shortcut: [a]*b=[] -> not (a in b)
- -O1: shortcut: [a]*b<>[] -> a in b
- -O1: combine multiple var a=0,b=0
- -O1: init a local var with the first assignment
- -O1: skip clone array for new array and arraysetlength
- -O1/-O2: autoinline
- -O1: replace constant expression with result, e.g. b:=1+2 -> b:=3
- -O1: create constants for set literals in expressions [lit,lit]
- -O1: no function Result var when assigned only once, e.g. Result = 3; return Result; -> return 3;
- -O1: by reference: create getter/setter object only once
- -O1: pass array element by reference: when index is constant, use that directly (i.e. no temporary variable)
- -O1: var/out argument: create accessor as var outside loops and combine multiple to one
- -O2: inline -Si
- -O2: case-of with 6+ elements as binary tree
- -O2: insert local/unit vars for global type references:
- at start of intf var $r1=null;
- at end of impl: $r1=path;
- -O2 removeemptyprocs
- -O2 skip dead code If(false){...}
- -O2 CSE
- -O3 DFA
Compiler itself
The compiler was deliberately written
- in a modular fashion. Each part has its own test suite and is used by other projects.
- can be used in a library, i.e. no restart required to compile another program.
- can be used without a filesystem, i.e. in a browser
- threadsafe, i.e. the compilation can run in a thread. Compiling a program with multiple threads is planned.
- easy maintenance has higher priority than high speed and low memory