[davecheney]

When interp cannot run a store at compile time, the store goes into runtime.initAll. Runtime code can then read the object that the stored pointer points to. Interp did not know this, so a later store to that object at compile time changed the value that runtime code read.

This program prints before: 2 with TinyGo 0.43.0 and dev, but before: 1 with Go.

package main

var value = 1
var pointer *int

func observe() {
	defer func() {}()
	println("before:", *pointer)
}

func init() {
	println("pointer:", &pointer)
	pointer = &value
	observe()
	value = 2
}

func main() {
	println("after:", value)
}

println(&pointer) runs at runtime, so interp marks pointer as external. Thus pointer = &value also runs at runtime, but interp did not mark value. Then value = 2 runs at compile time and changes the initializer of value to 2.

Why the fix is small

The general rule is that each object in the operands of a runtime instruction must be marked as external. Other runtime instructions, for example insertvalue and integer arithmetic on a ptrtoint constant, have the same problem. This PR applies the rule only to stores, so that the fix for the reported bug is small and easy to review. A stacked follow up PR applies the rule to all instructions that runAtRuntime makes. That PR has its own failing tests.

Notes for reviewers

• interp/testdata/escape.ll stores a pointer at runtime and then stores to the object at compile time. Without the fix, the initializer of store.value changes to 2.
• testdata/init.go does the same with atomic.Pointer.Store, because an atomic store runs at runtime. Without the fix TinyGo prints escape: 2. With the fix it prints escape: 1, the same as Go. The test does not print an address, so the output is deterministic.
• go test ./interp and go test -run TestBuild/Host -only-current-os . pass on macOS arm64 with LLVM 22.
• Code size on -target=cortex-m-qemu does not change for alias, atomic, calls, channel, float, gc, goroutines, interface, map, math, print, reflect, slice, sort, stdlib, string, structs, zeroalloc and init.