[dev.unified] cmd/compile/internal/noder: add optExpr for optional expressions

Previously, {writer,reader}.expr would allow for nil
expressions (i.e., no expression at all, not a "nil" identifier). But
only a few contexts allow this, and it simplifies some logic if we can
assume the expression is non-nil.

So this CL introduces optExpr as a wrapper method for handling nil
expressions specially.

Change-Id: I438bae7a3191126f7790ec0bf5b77320fe855514
Reviewed-on: https://go-review.googlesource.com/c/go/+/410099
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

diff --git a/src/cmd/compile/internal/noder/codes.go b/src/cmd/compile/internal/noder/codes.go
index 28991e7b9c887729dc674cd8eaab71a16f988d1b..7fe6e39c15cf2355aa17e06e19cc10650ced6915 100644 (file)
--- a/src/cmd/compile/internal/noder/codes.go
+++ b/src/cmd/compile/internal/noder/codes.go
@@ -38,11 +38,10 @@ func (c codeExpr) Value() int                 { return int(c) }
 
 // TODO(mdempsky): Split expr into addr, for lvalues.
 const (
-       exprNone codeExpr = iota
-       exprConst
-       exprType   // type expression
-       exprLocal  // local variable
-       exprGlobal // global variable or function
+       exprConst  codeExpr = iota
+       exprType            // type expression
+       exprLocal           // local variable
+       exprGlobal          // global variable or function
        exprBlank
        exprCompLit
        exprFuncLit

diff --git a/src/cmd/compile/internal/noder/reader.go b/src/cmd/compile/internal/noder/reader.go
index a231fe5d503cf890dce9774bff2ee6335e8691b8..d4ab6a975f12dcde49462109ab5076e4f221d2f0 100644 (file)
--- a/src/cmd/compile/internal/noder/reader.go
+++ b/src/cmd/compile/internal/noder/reader.go
@@ -1380,7 +1380,7 @@ func (r *reader) forStmt(label *types.Sym) ir.Node {
 
        pos := r.pos()
        init := r.stmt()
-       cond := r.expr()
+       cond := r.optExpr()
        post := r.stmt()
        body := r.blockStmt()
        r.closeAnotherScope()
@@ -1450,7 +1450,7 @@ func (r *reader) switchStmt(label *types.Sym) ir.Node {
                iface = x.Type()
                tag = ir.NewTypeSwitchGuard(pos, ident, x)
        } else {
-               tag = r.expr()
+               tag = r.optExpr()
        }
 
        clauses := make([]*ir.CaseClause, r.Len())
@@ -1551,9 +1551,6 @@ func (r *reader) expr() (res ir.Node) {
        default:
                panic("unhandled expression")
 
-       case exprNone:
-               return nil
-
        case exprBlank:
                // blank only allowed in LHS of assignments
                // TODO(mdempsky): Handle directly in assignList instead?
@@ -1622,7 +1619,7 @@ func (r *reader) expr() (res ir.Node) {
                pos := r.pos()
                var index [3]ir.Node
                for i := range index {
-                       index[i] = r.expr()
+                       index[i] = r.optExpr()
                }
                op := ir.OSLICE
                if index[2] != nil {
@@ -1701,6 +1698,13 @@ func (r *reader) expr() (res ir.Node) {
        }
 }
 
+func (r *reader) optExpr() ir.Node {
+       if r.Bool() {
+               return r.expr()
+       }
+       return nil
+}
+
 func (r *reader) compLit() ir.Node {
        r.Sync(pkgbits.SyncCompLit)
        pos := r.pos()

diff --git a/src/cmd/compile/internal/noder/writer.go b/src/cmd/compile/internal/noder/writer.go
index 3bf67171170ad389558cb76a184499d28b874b9c..ac1ec972859f51ba97816fe0bc8f20d3d4925d4d 100644 (file)
--- a/src/cmd/compile/internal/noder/writer.go
+++ b/src/cmd/compile/internal/noder/writer.go
@@ -1077,7 +1077,7 @@ func (w *writer) forStmt(stmt *syntax.ForStmt) {
        } else {
                w.pos(stmt)
                w.stmt(stmt.Init)
-               w.expr(stmt.Cond)
+               w.optExpr(stmt.Cond)
                w.stmt(stmt.Post)
        }
 
@@ -1136,7 +1136,7 @@ func (w *writer) switchStmt(stmt *syntax.SwitchStmt) {
                }
                w.expr(guard.X)
        } else {
-               w.expr(stmt.Tag)
+               w.optExpr(stmt.Tag)
        }
 
        w.Len(len(stmt.Body))
@@ -1201,6 +1201,8 @@ func (w *writer) optLabel(label *syntax.Name) {
 // @@@ Expressions
 
 func (w *writer) expr(expr syntax.Expr) {
+       base.Assertf(expr != nil, "missing expression")
+
        expr = unparen(expr) // skip parens; unneeded after typecheck
 
        obj, inst := lookupObj(w.p.info, expr)
@@ -1254,9 +1256,6 @@ func (w *writer) expr(expr syntax.Expr) {
        default:
                w.p.unexpected("expression", expr)
 
-       case nil: // absent slice index, for condition, or switch tag
-               w.Code(exprNone)
-
        case *syntax.Name:
                assert(expr.Value == "_")
                w.Code(exprBlank)
@@ -1292,7 +1291,7 @@ func (w *writer) expr(expr syntax.Expr) {
                w.expr(expr.X)
                w.pos(expr)
                for _, n := range &expr.Index {
-                       w.expr(n)
+                       w.optExpr(n)
                }
 
        case *syntax.AssertExpr:
@@ -1356,6 +1355,12 @@ func (w *writer) expr(expr syntax.Expr) {
        }
 }
 
+func (w *writer) optExpr(expr syntax.Expr) {
+       if w.Bool(expr != nil) {
+               w.expr(expr)
+       }
+}
+
 func (w *writer) compLit(lit *syntax.CompositeLit) {
        tv, ok := w.p.info.Types[lit]
        assert(ok)