Go's Hidden Pragmas

1 .Go’s Hidden #pragmas GopherChina 2017 

2 . @davecheney Go programmer from Sydney, Australia 

3 .But first, a history lesson. 

4 . Perl use strict;
 use strict "vars";
 use strict "refs";
 use strict "subs";
 use strict;
 no strict "vars"; 

5 . Javascript "use strict"; 

6 . Rust #[inline(always)]
 fn super_fast_fn() { #[cfg(target_os = "macos")]
 mod macos_only { 

7 .Warning: History time 

8 .ALGOL 68 pragmat 

9 . C #pragma pack(2) 
 struct T {
 int i; 
 short j;
 double k;
 }; 

10 .Does Go have pragmas? 

11 .Yes. Go has pragmas // 

12 . They're actually called pragmas in the source 

13 . syscall/syscall_linux_amd64.go //go:noescape
 func gettimeofday(tv *Timeval) (err Errno) 

14 .cmd/compile/internal/gc/testdata/arith.go //go:noinline
 func lshNop1(x uint64) uint64 {
 // two outer shifts should be removed
 return (((x << 5) >> 2) << 2)
 } 

15 . runtime/atomic_pointer.go //go:nosplit
 func atomicstorep(ptr unsafe.Pointer, new unsafe.Pointer) {
 writebarrierptr_prewrite((*uintptr)(ptr), uintptr(new))
 atomic.StorepNoWB(noescape(ptr), new)
 } 

16 .A word of caution
 🚨 

17 . "Useful" is always true for a feature request. The question is, does the usefulness justify the cost? The cost here is continued proliferation of magic comments, which are becoming too numerous already. –Rob Pike 

18 .//go:noescape 

19 . Escape Analysis func NewBook() *Book {
 b := Book{Mice: 12, Men: 9}
 return &b
 } 

20 . Escape Analysis (cont.) func BuildLibrary() {
 b := Book{Mice: 99: Men: 3}
 AddToCollection(&b)
 } 

21 .Answer: it depends
 🤔 

22 . b does not escape func AddToCollection(b *Book) {
 b.Classification = "fiction"
 } 

23 . b escapes var AvailableForLoan []*Book func AddToCollection(b *Book) {
 AvailableForLoan = append(AvailableForLoan, b)
 } 

24 . os.File.Read f, _ := os.Open("/tmp/foo")
 buf := make([]byte, 4096)
 n, _ := f.Read(buf) 

25 . os.File.Read // Read reads up to len(b) bytes from the File.
 // It returns the number of bytes read and any error encountered.
 // At end of file, Read returns 0, io.EOF.
 func (f *File) Read(b []byte) (n int, err error) {
 if err := f.checkValid("read"); err != nil {
 return 0, err
 }
 n, e := f.read(b)
 if e != nil {
 if e == io.EOF {
 err = e
 } else {
 err = &PathError{"read", f.name, e}
 }
 }
 return n, err
 } 

26 . golang.org/issue/4099 commit fd178d6a7e62796c71258ba155b957616be86ff4
 Author: Russ Cox <rsc@golang.org>
 Date: Tue Feb 5 07:00:38 2013 -0500 cmd/gc: add way to specify 'noescape' for extern funcs A new comment directive //go:noescape instructs the compiler
 that the following external (no body) func declaration should be
 treated as if none of its arguments escape to the heap. Fixes #4099. R=golang-dev, dave, minux.ma, daniel.morsing, remyoudompheng, adg, agl, iant
 CC=golang-dev
 https://golang.org/cl/7289048 

27 . bytes.IndexByte (circa Go 1.5) package bytes //go:noescape
 // IndexByte returns the index of the first instance of c in s,
 // or -1 if c is not present in s.
 func IndexByte(s []byte, c byte) int // ../runtime/asm_$GOARCH.s 

28 .Can you use //go:noescape in your code? 

29 .//go:norace