go/src/cmd/compile/internal/ssa/flagalloc.go

// flagalloc allocates the flag register among all the flag-generating
// instructions. Flag values are recomputed if they need to be
// spilled/restored.
func flagalloc(f *Func) { … }

func (v *Value) clobbersFlags() bool { … }

// copyFlags copies v (flag generator) into b, returns the copy.
// If v's arg is also flags, copy recursively.
func copyFlags(v *Value, b *Block) *Value { … }