cmd/compile: keep closure pointer live for range body closures

For range-over-function, the compiler generates a hidden closure
for the range body, and call the iterator function with the hidden
closure as the yield parameter. For debuggers, if it stops inside
the range body (hidden closure), it needs some way to find the
outer function (that contains the range statement), to access the
variables that are in scope. To do this, we keep the closure
pointer live on stack with a special name ".closureptr", so the
debugger can look for this name and find the closure pointer. In
the usual case, the closure is a struct defined in the outer
frame, so following the pointer it will find the frame. We do this
in SSA generation, so if the range func is inlined and there is no
actual closure, we don't generate any extra code. In the case that
there is an actual closure, it's just a single store to the stack,
so the overhead is still small.

TODO: add some test

Change-Id: I0e8219b895733f8943a13c67b03ca776bdc02bc9
Reviewed-on: https://go-review.googlesource.com/c/go/+/586975
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: David Chase <[email protected]>

Author: cherrymui

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

@@ -515,6 +515,21 @@ func buildssa(fn *ir.Func, worker int, isPgoHot bool) *ssa.Func {
 	// Populate closure variables.
 	if fn.Needctxt() {
 		clo := s.entryNewValue0(ssa.OpGetClosurePtr, s.f.Config.Types.BytePtr)
+		if fn.RangeParent != nil {
+			// For a range body closure, keep its closure pointer live on the
+			// stack with a special name, so the debugger can look for it and
+			// find the parent frame.
+			sym := &types.Sym{Name: ".closureptr", Pkg: types.LocalPkg}
+			cloSlot := s.curfn.NewLocal(src.NoXPos, sym, s.f.Config.Types.BytePtr)
+			cloSlot.SetUsed(true)
+			cloSlot.SetEsc(ir.EscNever)
+			cloSlot.SetAddrtaken(true)
+			s.vars[memVar] = s.newValue1Apos(ssa.OpVarDef, types.TypeMem, cloSlot, s.mem(), false)
+			addr := s.addr(cloSlot)
+			s.store(s.f.Config.Types.BytePtr, addr, clo)
+			// Keep it from being dead-store eliminated.
+			s.vars[memVar] = s.newValue1Apos(ssa.OpVarLive, types.TypeMem, cloSlot, s.mem(), false)
+		}
 		csiter := typecheck.NewClosureStructIter(fn.ClosureVars)
 		for {
 			n, typ, offset := csiter.Next()