gcWriteBarrier currently promises to preserve all general purpose registers. It does this by saving the general purpose registers to the stack. However it saves its arguments into the argument slots for gcBufFlush and then restores them from the same slots. This isn't technically correct. As @randall77 mentioned in this post:

The Go calling convention allows the callee to modify the argument slots on the stack. The caller must not assume that they are unmodified across a call.
The compiler does in fact use the argument slots for spill locations. For example:

func f(x int) int {
	x++
	runtime.GC()
	return x
}

The x+1 value will be spilled to x's argument slot across the runtime.GC() call.

This should probably be fixed for correctness. We could either mark gcWriteBarrier as clobbering its argument registers and then not bothering to restore them at all or we can save them to the stack somewhere else.

Examples of unsafe restores:

amd64:

s390x: