go.tools/ssa: eliminate (non-nil) slice constants.

go/types no longer reports a constant value for a slice,
e.g. []byte("foo").

ssa.Const no longer support slice constants, only
bool/string/numeric and nil.

emitConv emits a Convert instruction for string->[]byte
conversions.

Added regression test for bug 6949.

R=gri
CC=golang-codereviews
https://golang.org/cl/50590043

pointer/testdata/conv.go

@@ -14,10 +14,6 @@ func conv1() {
 }
 
 func conv2() {
-	// []byte/[]rune literal
-	print([]byte("foo")) // @pointsto "foo":[]byte
-	print([]rune("bar")) // @pointsto "bar":[]rune
-
 	// string -> []byte/[]rune conversion
 	s := "foo"
 	ba := []byte(s) // @line c2ba

ssa/emit.go

@@ -218,13 +218,19 @@ func emitConv(f *Function, val Value, typ types.Type) Value {
 		return f.emit(mi)
 	}
 
-	// Conversion of a constant to a non-interface type results in
-	// a new constant of the destination type and (initially) the
-	// same abstract value.  We don't compute the representation
-	// change yet; this defers the point at which the number of
-	// possible representations explodes.
+	// Conversion of a compile-time constant value?
 	if c, ok := val.(*Const); ok {
-		return NewConst(c.Value, typ)
+		if _, ok := ut_dst.(*types.Basic); ok || c.IsNil() {
+			// Conversion of a compile-time constant to
+			// another constant type results in a new
+			// constant of the destination type and
+			// (initially) the same abstract value.
+			// We don't truncate the value yet.
+			return NewConst(c.Value, typ)
+		}
+
+		// We're converting from constant to non-constant type,
+		// e.g. string -> []byte/[]rune.
 	}
 
 	// A representation-changing conversion.

ssa/interp/ops.go

@@ -37,9 +37,7 @@ func constValue(c *ssa.Const) value {
 		return zero(c.Type()) // typed nil
 	}
 
-	// By destination type:
-	switch t := c.Type().Underlying().(type) {
-	case *types.Basic:
+	if t, ok := c.Type().Underlying().(*types.Basic); ok {
 		// TODO(adonovan): eliminate untyped constants from SSA form.
 		switch t.Kind() {
 		case types.Bool, types.UntypedBool:
@@ -82,29 +80,6 @@ func constValue(c *ssa.Const) value {
 				return exact.StringVal(c.Value)
 			}
 			return string(rune(c.Int64()))
-		case types.UnsafePointer:
-			panic("unsafe.Pointer constant") // not possible
-		case types.UntypedNil:
-			// nil was handled above.
-		}
-
-	case *types.Slice:
-		switch et := t.Elem().Underlying().(type) {
-		case *types.Basic:
-			switch et.Kind() {
-			case types.Byte: // string -> []byte
-				var v []value
-				for _, b := range []byte(exact.StringVal(c.Value)) {
-					v = append(v, b)
-				}
-				return v
-			case types.Rune: // string -> []rune
-				var v []value
-				for _, r := range []rune(exact.StringVal(c.Value)) {
-					v = append(v, r)
-				}
-				return v
-			}
 		}
 	}
 

ssa/interp/testdata/coverage.go

@@ -610,3 +610,17 @@ func init() {
 	_ = x
 	_ = y
 }
+
+// Regression test for issue 6949:
+// []byte("foo") is not a constant since it allocates memory.
+func init() {
+	var r string
+	for i, b := range "ABC" {
+		x := []byte("abc")
+		x[i] = byte(b)
+		r += string(x)
+	}
+	if r != "AbcaBcabC" {
+		panic(r)
+	}
+}

ssa/ssa.go

@@ -95,7 +95,7 @@ type NamedConst struct {
 	pkg    *Package
 }
 
-// An SSA value that can be referenced by an instruction.
+// A Value is an SSA value that can be referenced by an instruction.
 type Value interface {
 	// Name returns the name of this value, and determines how
 	// this Value appears when used as an operand of an
@@ -232,7 +232,7 @@ type Instruction interface {
 // or method.
 //
 // If Blocks is nil, this indicates an external function for which no
-// Go source code is available.  In this case, Captures and Locals
+// Go source code is available.  In this case, FreeVars and Locals
 // will be nil too.  Clients performing whole-program analysis must
 // handle external functions specially.
 //
@@ -368,15 +368,10 @@ type Parameter struct {
 
 // A Const represents the value of a constant expression.
 //
-// It may have a nil, boolean, string or numeric (integer, fraction or
-// complex) value, or a []byte or []rune conversion of a string
-// constant.
-//
-// Consts may be of named types.  A constant's underlying type can be
-// a basic type, possibly one of the "untyped" types, or a slice type
-// whose elements' underlying type is byte or rune.  A nil constant can
-// have any reference type: interface, map, channel, pointer, slice,
-// or function---but not "untyped nil".
+// The underlying type of a constant may be any boolean, numeric, or
+// string type.  In addition, a Const may represent the nil value of
+// any reference type: interface, map, channel, pointer, slice, or
+// function---but not "untyped nil".
 //
 // All source-level constant expressions are represented by a Const
 // of equal type and value.