cmd/compile/internal/ssa: recognize constant pointer comparison

Teach the backend to recognize that the address of a symbol
is equal with itself, and that the addresses of two different
symbols are different.

Some examples of where this rule hits in the standard library:

 - inlined uses of (*time.Time).setLoc (e.g. time.UTC)
 - inlined uses of bufio.NewReader (via type assertion)

Change-Id: I23dcb068c2ec333655c1292917bec13bbd908c24
Reviewed-on: https://go-review.googlesource.com/38338Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -1366,7 +1366,7 @@ func (s *state) expr(n *Node) *ssa.Value {
 		if n.Class == PFUNC {
 			// "value" of a function is the address of the function's closure
 			sym := Linksym(funcsym(n.Sym))
-			aux := &ssa.ExternSymbol{Typ: n.Type, Sym: sym}
+			aux := s.lookupSymbol(n, &ssa.ExternSymbol{Typ: n.Type, Sym: sym})
 			return s.entryNewValue1A(ssa.OpAddr, typPtr(n.Type), aux, s.sb)
 		}
 		if s.canSSA(n) {
@@ -2137,7 +2137,8 @@ func (s *state) append(n *Node, inplace bool) *ssa.Value {
 
 	// Call growslice
 	s.startBlock(grow)
-	taddr := s.newValue1A(ssa.OpAddr, Types[TUINTPTR], &ssa.ExternSymbol{Typ: Types[TUINTPTR], Sym: Linksym(typenamesym(n.Type.Elem()))}, s.sb)
+	sym := s.lookupSymbol(n, &ssa.ExternSymbol{Typ: Types[TUINTPTR], Sym: Linksym(typenamesym(n.Type.Elem()))})
+	taddr := s.newValue1A(ssa.OpAddr, Types[TUINTPTR], sym, s.sb)
 
 	r := s.rtcall(growslice, true, []*Type{pt, Types[TINT], Types[TINT]}, taddr, p, l, c, nl)
 
@@ -4087,7 +4088,8 @@ func (s *state) dottype(n *Node, commaok bool) (res, resok *ssa.Value) {
 	if !commaok {
 		// on failure, panic by calling panicdottype
 		s.startBlock(bFail)
-		taddr := s.newValue1A(ssa.OpAddr, byteptr, &ssa.ExternSymbol{Typ: byteptr, Sym: Linksym(typenamesym(n.Left.Type))}, s.sb)
+		sym := s.lookupSymbol(n, &ssa.ExternSymbol{Typ: byteptr, Sym: Linksym(typenamesym(n.Left.Type))})
+		taddr := s.newValue1A(ssa.OpAddr, byteptr, sym, s.sb)
 		if n.Left.Type.IsEmptyInterface() {
 			s.rtcall(panicdottypeE, false, nil, itab, target, taddr)
 		} else {

src/cmd/compile/internal/ssa/gen/generic.rules

@@ -1432,6 +1432,10 @@
 	&& warnRule(fe.Debug_checknil() && v.Pos.Line() > 1, v, "removed nil check")
 	-> (Invalid)
 
+// Address comparison shows up in type assertions.
+(EqPtr x x) -> (ConstBool [1])
+(EqPtr (Addr {a} x) (Addr {b} x)) -> (ConstBool [b2i(a == b)])
+
 // De-virtualize interface calls into static calls.
 // Note that (ITab (IMake)) doesn't get
 // rewritten until after the first opt pass,

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

@@ -5332,6 +5332,40 @@ func rewriteValuegeneric_OpEqPtr(v *Value) bool {
 		v.AddArg(v0)
 		return true
 	}
+	// match: (EqPtr x x)
+	// cond:
+	// result: (ConstBool [1])
+	for {
+		x := v.Args[0]
+		if x != v.Args[1] {
+			break
+		}
+		v.reset(OpConstBool)
+		v.AuxInt = 1
+		return true
+	}
+	// match: (EqPtr (Addr {a} x) (Addr {b} x))
+	// cond:
+	// result: (ConstBool [b2i(a == b)])
+	for {
+		v_0 := v.Args[0]
+		if v_0.Op != OpAddr {
+			break
+		}
+		a := v_0.Aux
+		x := v_0.Args[0]
+		v_1 := v.Args[1]
+		if v_1.Op != OpAddr {
+			break
+		}
+		b := v_1.Aux
+		if x != v_1.Args[0] {
+			break
+		}
+		v.reset(OpConstBool)
+		v.AuxInt = b2i(a == b)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpEqSlice(v *Value) bool {