Heap allocation found in function prolog

[ju...@sqreen.com]

Hello,

I was surprised to find a heap allocation in the following function prolog:

https://gist.github.com/Julio-Guerra/a3a2088832cd927669c08e1127e4c25a

Under what circumstances is this happening and it is possible to avoid it?

Thanks,

Julio

[Jan Mercl]

On Wed, Sep 23, 2020 at 3:36 PM 'ju...@sqreen.com' via golang-nuts
<golan...@googlegroups.com> wrote:

> I was surprised to find a heap allocation in the following function prolog:
> https://gist.github.com/Julio-Guerra/a3a2088832cd927669c08e1127e4c25a

I think the function prolog consists of the first three assembler
instructions, with the possible additional prolog at 4fedf0 when stack
must be enlarged.

[Keith Randall]

That looks like something escaped to the heap.

If you pass the -S option to the compiler, the assembly output will have the symbolic name of the type being allocated (at the LEAQ 2 instructions before the call to newobject).

If you pass the -m option to the compiler, it will tell you what escaped and why.

[ju...@sqreen.com]

Thanks for the hints, I could finally have a look at it and found that a uint64 value was escaping regardless of the actual function code path. I can reproduce it with this dummy function example: 

func foo(cond bool, v int) *int {
    if cond {
        return &v
    }
    return nil
}

The compiled function prolog allocates the int value regardless of the fact it's not always needed.

I fixed it for now by declaring a new value in the condition block:

func foo(cond bool, v int) *int {
    if cond {

        v := v // new declaration in this scope to avoid using function scope
        return &v
    }
    return nil
}

Is it something expected?

Best

-- 

Julio Guerra