🍣

1. Hacking Go Compiler
Internals
Moriyoshi Koizumi <mozo@mozo.jp>

2. Intended Audience
• An eccentric Go programmer who happens to want
to add feture XX to the language, knowing her
patch will never be merged.
• A keen-minded programmer who wants to know
how the compiler works.

3. Overall Architecture
Lexer
Parser
Escape Analysis
Typegen GCproggen
Codegen

4. Phase 1. Lexer

5. Lexer
• A lexer scans over the source code and cut it into a
bunch of meaningful chunks (the first abstraction).
• Example:
a := b + c()
LNAME LASOP LNAME + LNAME ( )

6. Lexer
src/go/cmd/gc/lexer.c
static int32
_yylex(void)
{
...
l0:
c = getc();
if(yy_isspace(c)) {
if(c == 'n' && curio.nlsemi) {
ungetc(c);
DBG("lex: implicit semin");
return ';';
}
goto l0;
}
...

7. Lexer
...
switch(c) {
...
case '+':
c1 = getc();
if(c1 == '+') {
c = LINC;
goto lx;
}
if(c1 == '=') {
c = OADD;
goto asop;
}
break;
....
}

8. When do you want to hack the
lexer
• Modify the keyword such as func and make.
• Modify the operator only cosmetically (e.g. != →
~=)
• Modify how literals and identifiers are represented.
• Add a new keyword or operator to the language to
later use in the parser.

9. Example: Emojis for identifiers
• http://moriyoshi.hatenablog.com/entry/2014/06/0
3/121728
• Go doesn’t treat emojis as part of identifiers.
./sushi.go:8: invalid identifier character U+1f363
• But I wanted to have 寿司(in the source)

10. Example: Emojis for identifiers
• Patched the following place to let it accept emojis:
if(c >= Runeself) {
ungetc(c);
rune = getr();
// 0xb7 · is used for internal names
if(!isalpharune(rune) && !isdigitrune(rune) &&
(importpkg == nil || rune != 0xb7))
yyerror("invalid identifier character U+%04x",
rune);
cp += runetochar(cp, &rune);
} else if(!yy_isalnum(c) && c != '_')
break;

11. Phase 2. Parser

12. Parser
• Parser repeatedly calls the lexer to fetch the tokens
and builds an abstract syntax tree (AST) that
represents the source code.
• The AST is retouched (“typecheck”and “walk” sub-phase)
during type inference and assersion phase
so it would be less verbose and contain information
helpful for the later stages.
• src/cmd/gc/go.y, src/cmd/gc/dcl.c
src/cmd/gc/typecheck.c,
src/cmd/gc/walk.c,
src/cmd/gc/reflect.c

13. Parser
LNAME LASOP LNAME + LNAME ( )
OAS
ONAME OADD
ONAME
OCALL
ONAME ∅
Tokens
AST

14. Parser
• src/cmd/gc/go.y
…
/*
* expressions
*/
expr:
uexpr
| expr LOROR expr
{
$$ = nod(OOROR, $1, $3);
}
| expr LANDAND expr
{
$$ = nod(OANDAND, $1, $3);
}
…

15. Example: Bracket operator
overload!
• Let the following code (A) expand to (B)
• https://gist.github.com/moriyoshi/c0e2b2f9be688
3e33251
(A)
(B)
a := &struct{}{}
fmt.Println(a[1])
a[1] = "test2"
fmt.Println(a.__getindex(1))
a.__setindex(1, "test2")

16. Example: Bracket operator
overload!
• Things to do:
• Introduce a new AST node type (e.g. OINDEXINTER)
• Add a branch point in “typecheck” to handle the case
where the indexed target is neither a string, array, slice
nor map type.
• Supply a code in “walk” to specially treat the assignment
and dereference that involves that kind of node. The
code synthesizes the node to invoke the special
functions, then typecheck and walk over themselves in a
recursive manner.
• Don’t forget to take care of evaluation order corrector.

17. Helpful functions to debug your
hack
• print(const char *, …)
• This is actually printf() of standard libc.
• Accepts the following extra format specifiers:
• %N (node)
• %T (type)
• %E, %J, %H, %L, %O, %S, %V, %Z, %B, %F

18. Roll-up
• Go’s compiler internals should look complex at first
glance, but it would turn out pretty straightforward
and hacker-friendly ;)