Go’s structured logging library (log/slog) is quite the clever piece of standard library code. This is thanks to the processing of log attributes {key:value} with zero heap allocations. And despite the slog.Logger interface (inspired by packages uber-go/zap and rs/zerolog) now long being the go-to idiom for any generic logging use-cases in Go; it is additionally highly-optimized for use in application hot-paths.

Wanting to know exactly how Go achieves this with the the compiler optimizing away unnecessary overhead, I wrote a quick mock logger following the log/slog implementation and analyzed the output assembly via compiler explorer (go.godbolt.org) (thank you Matt Godbolt, sire).

In the following sections, I am going to walk through how the Go compiler avoids common “interface boxing” (allocating variables to the heap when converting to interfaces), fully inlines function calls to save CPU cycles, and passes data between stack frames without copying entire arrays.

Code Setup

Here is a stripped-down look at how a zero-allocation logger takes in attributes. Instead of using a variadic type T set (...any) (which forces Go to box primitives into interfaces on the heap), we define a concrete intAttr struct.

package p

import "fmt"

func myFunction(x int) int {
	log("my msg", newIntAttr("k1", x), newIntAttr("k2", 2))
	return 0
}

//go:noinline
func log(msg string, attrs ...intAttr) {
	fmt.Print(msg)
	for _, attr := range attrs {
		fmt.Print(attr)
	}
}

type intAttr struct {
	key   string
	value int
}

func newIntAttr(k string, v int) intAttr {
	return intAttr{
		key:   k,
		value: v,
	}
}

(I suggest you follow along using the same code above to fully understand this process).

Your prize for compiling this is a fat block of Plan 9 Assembly. But before we decode the output, let’s first address the weird symbol naming.

Checking Out command-line-arguments

If you look at the raw assembly, you’ll see memory locations and function calls prefixed with command-line-arguments, like this:

CALL command-line-arguments.log(SB) or MOVQ DX, command-line-arguments..autotmp_8+40(SP).

Why? In Go’s assembly dialect, the assembler attaches human-readable symbols to memory locations to help the debugger and the Garbage Collector map machine code back to your source file. When you compile a standalone Go file without a properly initialized Go module (like in a compiler explorer sandbox), the compiler assigns it a default package name of command-line-arguments.

The ..autotmp_8 simply means the compiler created a hidden, “automatic temporary” variable on the stack to hold our data. The CPU never sees these names; it only sees the raw memory offsets.

Optimization 1: Function Inlining & Struct Initialization

In our Go code, we call newIntAttr("k1", x) and newIntAttr("k2", 2). You might expect the assembly to set up a stack frame, push arguments, jump to the newIntAttr function, and return a struct.

The Go compiler is much smarter than that. It completely inlines the calls to save CPU cycles.

A Go string is a 16-byte struct (an 8-byte pointer to the text, and an 8-byte length). Our int is 8 bytes. That means each intAttr struct is exactly 24 bytes. Because we pass two of them in our variadic ...intAttr slice, the compiler needs 48 bytes of contiguous memory.

Instead of calling newIntAttr, the compiler manually writes the data directly to the stack, byte by byte:

        // --- Building the first intAttr: newIntAttr("k1", x) ---
        // Grab the pointer to the "k1" string bytes and put it in DX
        LEAQ    go:string."k1"(SB), DX 
        // Write the Pointer to SP+40
        MOVQ    DX, command-line-arguments..autotmp_8+40(SP) 
        // Write the string Length (2) to SP+48
        MOVQ    $2, command-line-arguments..autotmp_8+48(SP) 
        // Write the integer value (x, held in AX) to SP+56
        MOVQ    AX, command-line-arguments..autotmp_8+56(SP) 

        // --- Building the second intAttr: newIntAttr("k2", 2) ---
        // Grab the pointer to the "k2" string bytes and put it in DX
        LEAQ    go:string."k2"(SB), DX
        // Write the Pointer to SP+64
        MOVQ    DX, command-line-arguments..autotmp_8+64(SP)
        // Write the string Length (2) to SP+72
        MOVQ    $2, command-line-arguments..autotmp_8+72(SP)
        // Write the integer value (2) to SP+80
        MOVQ    $2, command-line-arguments..autotmp_8+80(SP)

Optimization 2: Escape Analysis and the Stack

Notice where it wrote those 48 bytes: +40(SP) up through +80(SP). SP is our Stack Pointer.

If we had passed our attributes as ...any (the standard fmt.Printf approach), the compiler wouldn’t know the size or type of the arguments at compile time. It would be forced to allocate them dynamically on the heap (a process called “boxing”), which requires the Garbage Collector to clean them up later.

Because we used a concrete type (intAttr), the compiler’s Escape Analysis kicked in. It realized this variadic slice never “escapes” the lifecycle of myFunction and log. Therefore, it entirely skips the runtime.newobject heap allocation and safely builds the array directly within myFunction’s local stack space. This is the zero allocation we are looking for!

Optimization 3: The Register-Based Calling Convention

Now that we have a 48-byte backing array on myFunction’s stack, how do we pass it to log? A common misconception is that the whole array is copied into the new function’s stack frame.

Thanks to Go’s modern register-based ABI (Application Binary Interface), arguments are passed to functions via CPU registers whenever possible. Our log function expects two arguments: a string (Pointer, Length) and a slice (Pointer, Length, Capacity).

Right before the CALL to log, the compiler packs these 5 words of data into 5 CPU registers:

        CALL    command-line-arguments.log(SB)

When execution jumps into log, the 48 bytes of array data are left untouched, sitting safely back in myFunction’s stack frame. The log function simply uses the pointer passed in the CX register to reach “up” the stack and read the structs one by one.

Takeaway

By passing a 24-byte slice header in registers rather than copying a whole 48-byte array across stack frames, and by avoiding the heap completely, Go keeps function calls blazingly fast. This combination of inlining, stack allocation, and register-passing is that which makes our log/slog so performant. Lovely, isn’t it?