proc: simplify logic of (*stackIterator).newStackframe (go-delve#1513)

The lookup at it.pc-1 is only ever done if fn is not nil and pc !=
fn.entry. Also when it happens only the File and Line fields are
allowed to change.

pkg/proc/stack.go

@@ -375,36 +375,21 @@ func (it *stackIterator) newStackframe(ret, retaddr uint64) Stackframe {
 		it.regs.FrameBase = it.frameBase(fn)
 	}
 	r := Stackframe{Current: Location{PC: it.pc, File: f, Line: l, Fn: fn}, Regs: it.regs, Ret: ret, addrret: retaddr, stackHi: it.stackhi, SystemStack: it.systemstack, lastpc: it.pc}
-	if !it.top {
-		fnname := ""
-		if r.Current.Fn != nil {
-			fnname = r.Current.Fn.Name
-		}
-		switch fnname {
+	r.Call = r.Current
+	if !it.top && r.Current.Fn != nil && it.pc != r.Current.Fn.Entry {
+		// if the return address is the entry point of the function that
+		// contains it then this is some kind of fake return frame (for example
+		// runtime.sigreturn) that didn't actually call the current frame,
+		// attempting to get the location of the CALL instruction would just
+		// obfuscate what's going on, since there is no CALL instruction.
+		switch r.Current.Fn.Name {
 		case "runtime.mstart", "runtime.systemstack_switch":
 			// these frames are inserted by runtime.systemstack and there is no CALL
 			// instruction to look for at pc - 1
-			r.Call = r.Current
 		default:
-			if r.Current.Fn != nil && it.pc == r.Current.Fn.Entry {
-				// if the return address is the entry point of the function that
-				// contains it then this is some kind of fake return frame (for example
-				// runtime.sigreturn) that didn't actually call the current frame,
-				// attempting to get the location of the CALL instruction would just
-				// obfuscate what's going on, since there is no CALL instruction.
-				r.Call = r.Current
-			} else {
-				r.lastpc = it.pc - 1
-				r.Call.File, r.Call.Line, r.Call.Fn = it.bi.PCToLine(it.pc - 1)
-				if r.Call.Fn == nil {
-					r.Call.File = "?"
-					r.Call.Line = -1
-				}
-				r.Call.PC = r.Current.PC
-			}
+			r.lastpc = it.pc - 1
+			r.Call.File, r.Call.Line = r.Current.Fn.cu.lineInfo.PCToLine(r.Current.Fn.Entry, it.pc-1)
 		}
-	} else {
-		r.Call = r.Current
 	}
 	return r
 }