Go: How Are Loops Translated to Assembly?

Illustration created for “A Journey With Go”, made from the original Go Gopher, created by Renee French.

ℹ️ This article is based on Go 1.13.

Loops are powerful concepts in programming and quite easy to handle. However, it has to be translated into basic instructions the computer can understand. The way it is compiled could also impact other components in the standard library. Let’s start by analyzing the range loop.

Loop assembly

A range loop iterates an array, slice, or channel. Here is an example of a function that adds up numbers with looping on the slice:

func main() {
   l := []int{9, 45, 23, 67, 78}
   t := 0

   for _, v := range l {
      t += v
   }

   println(t)
}

The command go tool compile -S main.go dumps the generated assembly code, and here is the output related to the range loop:

0x0041 00065 (main.go:4)   XORL   AX, AX
0x0043 00067 (main.go:4)   XORL   CX, CX

0x0045 00069 (main.go:7)   JMP    82
0x0047 00071 (main.go:7)   MOVQ   ""..autotmp_5+16(SP)(AX*8), DX
0x004c 00076 (main.go:7)   INCQ   AX
0x004f 00079 (main.go:8)   ADDQ   DX, CX
0x0052 00082 (main.go:7)   CMPQ   AX, $5
0x0056 00086 (main.go:7)   JLT    71
0x0058 00088 (main.go:11)  MOVQ   CX, "".t+8(SP)

I split the instructions in two parts: the initialization and the loop itself. The first two instructions…