go中的ASM函数调用的开销

Question

I currently play around with go, it's assembly, performance of floating point operations ( float32 ) and optimizations in the nano-seconds-scale. I was a bit confused by the overhead of a simple function call:

func BenchmarkEmpty(b *testing.B) {
    for i := 0; i < b.N; i++ {
    }
}
func BenchmarkNop(b *testing.B) {
    for i := 0; i < b.N; i++ {
        doNop()
    }
}

The implementation of doNop:

TEXT ·doNop(SB),0,$0-0
    RET

The result ( go test -bench . ):

BenchmarkEmpty        2000000000               0.30 ns/op
BenchmarkNop  2000000000               1.73 ns/op

Im not used to assembly and/ or the internals of go. It is possible fo the go compiler/ linker to inline a function defined in assembly? Can I give the linker a hint somehow? For some simple functions like 'add two R3-vectors' this eats up all possible performance gain.

(go 1.4.2, amd64)

Answer 1

Assembly functions are not inlined. Here are 3 things you could try:

1. Move your loop into assembly. For example with this function:

    func Sum(xs []int64) int64 

   You can do this:

    #include "textflag.h" TEXT ·Sum(SB),NOSPLIT,$0-24 MOVQ xs+0(FP),DI MOVQ xs+8(FP),SI MOVQ $0,CX MOVQ $0,AX L1: CMPQ AX,SI // i < len(xs) JGE Z1 LEAQ (DI)(AX*8),BX // BX = &xs[i] MOVQ (BX),BX // BX = *BX ADDQ BX,CX // CX += BX INCQ AX // i++ JMP L1 Z1: MOVQ CX,ret+24(FP) RET 

   If you look in the standard libraries you will see examples of this.

2. Write some of your code in c, leverage the support it has for intrinsics or inline assembly, and use cgo to call it from go.

3. Use gccgo to do the same thing as #2, except you can do it directly:

    //extern open func c_open(name *byte, mode int, perm int) int 

   https://golang.org/doc/install/gccgo#Function_names