Well, the assembler looks the same to me too, but I can confirm different runtimes on two different machines (fast is always faster here). $ ./main.test -tes

Well, the assembler looks the same to me too, but I can confirm different runtimes on two different machines (fast is always faster here). $ ./main.test -test.bench . -test.cpuprofile=test.profile goos: linux goarch: amd64 BenchmarkInLoop-8 2000000000 0.42 ns/op BenchmarkOutLoop-8 2000000000 0.47 ns/op PASS (pprof) disasm fast Total: 1.86s ROUTINE ======================== command-line-arguments.fast 990ms 990ms (flat, cum) 53.23% of Total . . 4ed7b0: MOVQ 0x8(SP), AX ;mem_test.go:16 . . 4ed7b5: XORL CX, CX . . 4ed7b7: MOVQ CX, DX . . 4ed7ba: JMP 0x4ed7c6 ;mem_test.go:19 650ms 650ms 4ed7bc: LEAQ 0x1(CX), BX ;command-line-arguments.fast mem_test.go:19 . . 4ed7c0: MOVQ CX, DX ;mem_test.go:19 10ms 10ms 4ed7c3: MOVQ BX, CX ;command-line-arguments.fast mem_test.go:19 330ms 330ms 4ed7c6: CMPQ AX, CX . . 4ed7c9: JL 0x4ed7bc ;mem_test.go:19 . . 4ed7cb: MOVQ DX, 0x10(SP) ;mem_test.go:23 (pprof) disasm slow Total: 1.86s ROUTINE ======================== command-line-arguments.slow 870ms 870ms (flat, cum) 46.77% of Total . . 4ed780: MOVQ 0x8(SP), AX ;mem_test.go:7 . . 4ed785: XORL CX, CX . . 4ed787: MOVQ CX, DX . . 4ed78a: JMP 0x4ed796 ;mem_test.go:9 560ms 560ms 4ed78c: LEAQ 0x1(CX), BX ;command-line-arguments.slow mem_test.go:9 . . 4ed790: MOVQ CX, DX ;mem_test.go:9 20ms 20ms 4ed793: MOVQ BX, CX ;command-line-arguments.slow mem_test.go:9 290ms 290ms 4ed796: CMPQ AX, CX . . 4ed799: JL 0x4ed78c ;mem_test.go:9 . . 4ed79b: MOVQ DX, 0x10(SP) ;mem_test.go:13