runtime: avoid append in printint, printuint

Should make cmd/link/internal/ld.TestAbstractOriginSanity happier.

Change-Id: I121927d42e527ff23d996e7387066f149b11cc59
Reviewed-on: https://go-review.googlesource.com/c/go/+/717480
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: rsc

src/internal/strconv/itoa.go

@@ -174,6 +174,14 @@ func small(i int) string {
 	return smalls[i*2 : i*2+2]
 }
 
+// RuntimeFormatBase10 formats u into the tail of a
+// and returns the offset to the first byte written to a.
+// It is only for use by package runtime.
+// Other packages should use AppendUint.
+func RuntimeFormatBase10(a []byte, u uint64) int {
+	return formatBase10(a, u)
+}
+
 // formatBase10 formats the decimal representation of u into the tail of a
 // and returns the offset of the first byte written to a. That is, after
 //

src/runtime/print.go

@@ -140,13 +140,32 @@ func printcomplex64(c complex64) {
 }
 
 func printuint(v uint64) {
+	// Note: Avoiding strconv.AppendUint so that it's clearer
+	// that there are no allocations in this routine.
+	// cmd/link/internal/ld.TestAbstractOriginSanity
+	// sees the append and doesn't realize it doesn't allocate.
 	var buf [20]byte
-	gwrite(strconv.AppendUint(buf[:0], v, 10))
+	i := strconv.RuntimeFormatBase10(buf[:], v)
+	gwrite(buf[i:])
 }
 
 func printint(v int64) {
+	// Note: Avoiding strconv.AppendUint so that it's clearer
+	// that there are no allocations in this routine.
+	// cmd/link/internal/ld.TestAbstractOriginSanity
+	// sees the append and doesn't realize it doesn't allocate.
+	neg := v < 0
+	u := uint64(v)
+	if neg {
+		u = -u
+	}
 	var buf [20]byte
-	gwrite(strconv.AppendInt(buf[:0], v, 10))
+	i := strconv.RuntimeFormatBase10(buf[:], u)
+	if neg {
+		i--
+		buf[i] = '-'
+	}
+	gwrite(buf[i:])
 }
 
 var minhexdigits = 0 // protected by printlock