使用gdb调试禁用优化后，golang仍进行了优化

Question

My main.go:

package main

import "fmt"
// main
func main() {
    fmt.Println("hello")
}

Build it:

go build -o main -gcflags '-N -l'

Hit breakpoint:

gdb main    
b runtime.(*mheap).sysAlloc
r

At line 540, i print v, it gives me "optimized out".
At line 549 print hint, it gives me: No symbol "hint" in current context.
What strange is that i can print *h.arenaHints value which infers hint.down is false, but line 550 still be steped into.

535 func (h *mheap) sysAlloc(n uintptr) (v unsafe.Pointer, size uintptr) {
536     n = round(n, heapArenaBytes)
537 
538     // First, try the arena pre-reservation.
539     v = h.arena.alloc(n, heapArenaBytes, &memstats.heap_sys)
540     if v != nil {
541         size = n
542         goto mapped
543     }
544 
545     // Try to grow the heap at a hint address.
546     for h.arenaHints != nil {
547         hint := h.arenaHints
548         p := hint.addr
549         if hint.down {
550             p -= n
551         }

So why disable optimization failed? Why line 550 can be steped into?

Env:
golang 1.12
ubuntu 18.04