Why are function arguments in Go stack traces sometimes displayed as seemingly unrelated values?

Unveiling the Mystery of "Unknown Field" in Panic Stack Traces

In the realm of debugging, encountering enigmatic entries within panic stack traces can be perplexing. One such instance arises when analyzing the output of a simple Go program:

<code class="go">package main

func F(a int) {
    panic(nil)
}

func main() {
    F(1)
}</code>

When executed, the program produces an unanticipated stack trace:

panic: nil

goroutine 1 [running]:
main.F(0x1, 0x10436000)
    /tmp/sandbox090887108/main.go:4 +0x20
main.main()
    /tmp/sandbox090887108/main.go:8 +0x20

The second number in the stack frame, 0x10436000, remains elusive in its purpose. To decipher this enigma, we must delve into the intricacies of Go's memory representation and stack trace generation.

Decoding the "Unknown Field": Unraveling the Argument Puzzle

The data displayed in the stack trace originates from the function arguments, but their values deviate from those explicitly passed in. The reason lies in how arguments are stored and printed in certain memory architectures.

Specifically, in the case of the playground environment being used, a 64-bit word architecture with 32-bit pointers (GOARCH=amd64p32) comes into play. This peculiar combination results in the arguments being printed as pointer-sized values, which typically coincides with the native word size. However, in this instance, the word size is twice the size of the pointer size.

Consequently, each 64-bit word accommodates two arguments, leading to an even number of values being printed in the frame arguments. The data presented is essentially the raw argument values stored in pointer-sized chunks.

Further Examples: Exploring the Variability in Data Representation

To illustrate this phenomenon, consider the following function:

<code class="go">func F(a uint8) {
    panic(nil)
}</code>

When called with the argument 1, the stack trace reveals:

main.F(0x97301, 0x10436000)

Here, only the first 8 bits of the 64-bit word are utilized, representing the value 1. The remaining bits are simply unused portions of the 64-bit word.

Similarly, on amd64 systems with multiple arguments, each 32-bit argument consumes a 64-bit word. For instance:

<code class="go">func F(a, b, c uint32)</code>

When invoked with F(1, 1, 1), the stack trace displays:

main.F(0x100000001, 0xc400000001)

indicating two words allocated for the arguments.

Return Values: Unveiling the Hidden Presence in Stack Traces

Stack frames also incorporate return values, which are allocated on the stack. For example:

<code class="go">func F(a int64) (int, int)</code>

On amd64, the stack frame arguments would appear as:

main.F(0xa, 0x1054d60, 0xc420078058)

The first word represents the input argument, while the remaining two words hold the return values.

Conclusion

Understanding the intricacies of memory representation and stack trace generation in Go empowers developers to decipher even the most enigmatic entries in panic stack traces. By unraveling the puzzle of the "unknown field," programmers can effectively debug and resolve issues, gaining invaluable insights into the inner workings of their code.

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