How does Go do string comparison?

Question

http://golang.org/ref/spec#Comparison_operators

Go supports string comparison without any special functions. Does the Go runtime do work behind the scenes to make comparisons of string literals?

Answer 1

As you can see in the following assembly dump, string comparison is delegated to a runtime.eqstring function from the runtime (line 17) after a short circuit check to see if the two operands are the same in-memory string (line 11):

$ cat foo.go package main  func main() {         a := "hello"         b := "world"         _ = a == b }  $ go tool 6g -S foo.go --- prog list "main" --- 0000 (foo.go:3) TEXT    main+0(SB),$40-0 0001 (foo.go:3) LOCALS  ,$0 0002 (foo.go:4) LEAQ    go.string."hello"+0(SB),BX 0003 (foo.go:4) MOVQ    (BX),SI 0004 (foo.go:4) MOVQ    8(BX),CX 0005 (foo.go:5) LEAQ    go.string."world"+0(SB),BX 0006 (foo.go:5) MOVQ    (BX),DX 0007 (foo.go:5) MOVQ    8(BX),AX 0008 (foo.go:6) JMP     ,11 0009 (foo.go:6) MOVQ    $1,AX 0010 (foo.go:6) JMP     ,23 0011 (foo.go:6) CMPQ    CX,AX 0012 (foo.go:6) JNE     ,22 0013 (foo.go:6) MOVQ    SI,(SP) 0014 (foo.go:6) MOVQ    CX,8(SP) 0015 (foo.go:6) MOVQ    DX,16(SP) 0016 (foo.go:6) MOVQ    AX,24(SP) 0017 (foo.go:6) CALL    ,runtime.eqstring+0(SB) 0018 (foo.go:6) MOVBQZX 32(SP),BX 0019 (foo.go:6) CMPB    BX,$0 0020 (foo.go:6) JEQ     ,22 0021 (foo.go:6) JMP     ,9 0022 (foo.go:6) MOVQ    $0,AX 0023 (foo.go:7) RET     ,  --- prog list "init" --- 0024 (foo.go:7) TEXT    init+0(SB),$0-0 0025 (foo.go:7) MOVBQZX initdone·+0(SB),AX 0026 (foo.go:7) LOCALS  ,$0 0027 (foo.go:7) CMPB    AX,$0 0028 (foo.go:7) JEQ     ,34 0029 (foo.go:7) CMPB    AX,$2 0030 (foo.go:7) JNE     ,32 0031 (foo.go:7) RET     , 0032 (foo.go:7) CALL    ,runtime.throwinit+0(SB) 0033 (foo.go:7) UNDEF   , 0034 (foo.go:7) MOVB    $2,initdone·+0(SB) 0035 (foo.go:7) RET     , 

Unless you are working on the compiler or runtime, this shouldn't concern you too much: just use the operators as the spec defines, and expect that the comparison to be O(n) with the length of the string.

Answer 2

runtime/string.goc (go1.3):

func eqstring(s1 String, s2 String) (v bool) {     if(s1.len != s2.len) {         v = false;         return;     }     if(s1.str == s2.str) {         v = true;         return;     }     v = runtime·memeq(s1.str, s2.str, s1.len); }  int32 runtime·strcmp(byte *s1, byte *s2) {     uintptr i;     byte c1, c2;      for(i=0;; i++) {         c1 = s1[i];         c2 = s2[i];         if(c1 < c2)             return -1;         if(c1 > c2)             return +1;         if(c1 == 0)             return 0;     } } 

Note: The runtime· separator is a Unicode middle-dot, not a period.