diff --git a/src/cmd/5g/gsubr.c b/src/cmd/5g/gsubr.c
index 70ebad6edd0..584a0f62b84 100644
--- a/src/cmd/5g/gsubr.c
+++ b/src/cmd/5g/gsubr.c
@@ -213,6 +213,9 @@ regalloc(Node *n, Type *t, Node *o)
 {
 	int i, et, fixfree, floatfree;
 
+	// guarantee R9 and R10 (m and g) are left alone. BUG.
+	reg[9] = 1;
+	reg[10] = 1;
 	if(debug['r']) {
 		fixfree = 0;
 		for(i=REGALLOC_R0; i<=REGALLOC_RMAX; i++)
diff --git a/src/pkg/compress/flate/huffman_bit_writer.go b/src/pkg/compress/flate/huffman_bit_writer.go
index abff82dd694..294cdf65329 100644
--- a/src/pkg/compress/flate/huffman_bit_writer.go
+++ b/src/pkg/compress/flate/huffman_bit_writer.go
@@ -110,8 +110,14 @@ func newHuffmanBitWriter(w io.Writer) *huffmanBitWriter {
 }
 
 func (err WrongValueError) String() string {
-	return "huffmanBitWriter: " + err.name + " should belong to [" + strconv.Itoa64(int64(err.from)) + ";" +
-		strconv.Itoa64(int64(err.to)) + "] but actual value is " + strconv.Itoa64(int64(err.value))
+	// BUG: work around bug in 5g by simplifying expression.
+	// return "huffmanBitWriter: " + err.name + " should belong to [" + strconv.Itoa64(int64(err.from)) + ";" +
+	// 	strconv.Itoa64(int64(err.to)) + "] but actual value is " + strconv.Itoa64(int64(err.value))
+	str := "huffmanBitWriter: " + err.name + " should belong to ["
+	str += strconv.Itoa64(int64(err.from)) + ";"
+	str += strconv.Itoa64(int64(err.to)) + "] but actual value is "
+	str += strconv.Itoa64(int64(err.value))
+	return str
 }
 
 func (w *huffmanBitWriter) flushBits() {
diff --git a/src/pkg/gob/decode.go b/src/pkg/gob/decode.go
index a70799e9a72..a91a98bf157 100644
--- a/src/pkg/gob/decode.go
+++ b/src/pkg/gob/decode.go
@@ -867,9 +867,14 @@ func (dec *Decoder) compileDec(remoteId typeId, rt reflect.Type) (engine *decEng
 			continue
 		}
 		if !dec.compatibleType(localField.Type, wireField.id) {
-			return nil, os.ErrorString("gob: wrong type (" +
-				localField.Type.String() + ") for received field " +
-				wireStruct.name + "." + wireField.name)
+			// BUG: work around bug in 5g by simplifying expression.
+			// return nil, os.ErrorString("gob: wrong type (" +
+			//	localField.Type.String() + ") for received field " +
+			//	wireStruct.name + "." + wireField.name)
+			str := "gob: wrong type ("
+			str += localField.Type.String() + ") for received field "
+			str += wireStruct.name + "." + wireField.name
+			return nil, os.ErrorString(str)
 		}
 		op, indir, err := dec.decOpFor(wireField.id, localField.Type, localField.Name)
 		if err != nil {
diff --git a/src/pkg/log/log.go b/src/pkg/log/log.go
index 28d6204eb68..cfc1bd7767f 100644
--- a/src/pkg/log/log.go
+++ b/src/pkg/log/log.go
@@ -86,7 +86,11 @@ func (l *Logger) formatHeader(ns int64, calldepth int) string {
 	if l.flag&(Ldate|Ltime|Lmicroseconds) != 0 {
 		t := time.SecondsToLocalTime(ns / 1e9)
 		if l.flag&(Ldate) != 0 {
-			h += itoa(int(t.Year), 4) + "/" + itoa(t.Month, 2) + "/" + itoa(t.Day, 2) + " "
+			// BUG: work around bug in 5g by simplifying expression.
+			// h += itoa(int(t.Year), 4) + "/" + itoa(t.Month, 2) + "/" + itoa(t.Day, 2) + " "
+			h += itoa(int(t.Year), 4)
+			h += "/" + itoa(t.Month, 2)
+			h += "/" + itoa(t.Day, 2) + " "
 		}
 		if l.flag&(Ltime|Lmicroseconds) != 0 {
 			h += itoa(t.Hour, 2) + ":" + itoa(t.Minute, 2) + ":" + itoa(t.Second, 2)
diff --git a/src/pkg/net/ip.go b/src/pkg/net/ip.go
index e82224a2836..29a72971f5a 100644
--- a/src/pkg/net/ip.go
+++ b/src/pkg/net/ip.go
@@ -200,10 +200,16 @@ func (ip IP) String() string {
 
 	// If IPv4, use dotted notation.
 	if p4 := p.To4(); len(p4) == 4 {
-		return itod(uint(p4[0])) + "." +
-			itod(uint(p4[1])) + "." +
-			itod(uint(p4[2])) + "." +
-			itod(uint(p4[3]))
+		// BUG: work around bug in 5g by simplifying expression.
+		// return itod(uint(p4[0])) + "." +
+		//	itod(uint(p4[1])) + "." +
+		//	itod(uint(p4[2])) + "." +
+		//	itod(uint(p4[3]))
+		str := itod(uint(p4[0])) + "."
+		str += itod(uint(p4[1])) + "."
+		str += itod(uint(p4[2])) + "."
+		str += itod(uint(p4[3]))
+		return str
 	}
 	if len(p) != IPv6len {
 		return "?"
diff --git a/src/pkg/time/format.go b/src/pkg/time/format.go
index 355721e1839..bc94cb78e24 100644
--- a/src/pkg/time/format.go
+++ b/src/pkg/time/format.go
@@ -356,11 +356,17 @@ type ParseError struct {
 // String is the string representation of a ParseError.
 func (e *ParseError) String() string {
 	if e.Message == "" {
-		return "parsing time " +
-			strconv.Quote(e.Value) + " as " +
-			strconv.Quote(e.Layout) + ": cannot parse " +
-			strconv.Quote(e.ValueElem) + " as " +
-			strconv.Quote(e.LayoutElem)
+		// BUG: work around bug in 5g by simplifying expression.
+		// return "parsing time " +
+		//	strconv.Quote(e.Value) + " as " +
+		//	strconv.Quote(e.Layout) + ": cannot parse " +
+		//	strconv.Quote(e.ValueElem) + " as " +
+		//	strconv.Quote(e.LayoutElem)
+		str := "parsing time " + strconv.Quote(e.Value) + " as "
+		str += strconv.Quote(e.Layout) + ": cannot parse "
+		str += strconv.Quote(e.ValueElem) + " as "
+		str += strconv.Quote(e.LayoutElem)
+		return str
 	}
 	return "parsing time " +
 		strconv.Quote(e.Value) + e.Message
diff --git a/src/pkg/xml/xml.go b/src/pkg/xml/xml.go
index cd67f6e2657..c72d726c76e 100644
--- a/src/pkg/xml/xml.go
+++ b/src/pkg/xml/xml.go
@@ -371,8 +371,14 @@ func (p *Parser) popElement(t *EndElement) bool {
 		p.err = p.syntaxError("element <" + s.name.Local + "> closed by </" + name.Local + ">")
 		return false
 	case s.name.Space != name.Space:
-		p.err = p.syntaxError("element <" + s.name.Local + "> in space " + s.name.Space +
-			"closed by </" + name.Local + "> in space " + name.Space)
+		// BUG: work around bug in 5g by simplifying expression.
+		// p.err = p.syntaxError("element <" + s.name.Local + "> in space " + s.name.Space +
+		//	"closed by </" + name.Local + "> in space " + name.Space)
+		str := "element <" + s.name.Local
+		str += "> in space " + s.name.Space
+		str += "closed by </" + name.Local
+		str += "> in space " + name.Space
+		p.err = p.syntaxError(str)
 		return false
 	}