Unexpected aliasing of union parameters in "odin" convention

[hitchh1k3r]

Context

I am storing a level id union into a globally loaded level, but when reloading the level (loading the current level's id), I found this bug...

Odin: dev-2025-05:4d7876bdb
OS: Windows 10 Professional (version: 22H2), build 19045.5737
CPU: Intel(R) Core(TM) i7-4771 CPU @ 3.50GHz
RAM: 16311 MiB
Backend: LLVM 20.1.0

Expected Behavior

Non-pointer parameters should always act as if they are copied by value, not reference.

Failure Information

It seems like the "odin" calling convention is optimizing large (over 128 bit) union values to pointers, even when the value is modified in the procedure.

Here is a simple test case, that additionally shows that this issue is not present in "cdecl" procedures:

package main

import "base:runtime"

import "core:fmt"

global : UnionType

UnionType :: union {
  i128,
}

main :: proc() {
  global = 42
  set_global_cdecl(global, context)
  set_global_odin(global)
}

set_global_cdecl :: proc "cdecl" (new_value : UnionType, ctx : runtime.Context) {
  context = ctx

  fmt.println("C BEFORE:", new_value) // C BEFORE: 42
  global = {}
  fmt.println("C AFTER:", new_value)  // C AFTER: 42

  global = new_value
}

set_global_odin :: proc "odin" (new_value : UnionType) {
  fmt.println("O BEFORE:", new_value) // O BEFORE: 42
  global = {}
  fmt.println("O AFTER:", new_value)  // O AFTER: nil

  global = new_value
}

[laytan]

This is intended behaviour.
https://odin-lang.org/docs/overview/#parameters
https://odin-lang.org/docs/faq/#when-are-procedure-parameters-passed-by-value