errors: optimize *joinError's Error method for less allocation and faster execution

Handle the case of one error at the beginning.
Use unsafe.String to avoid memory allocation when converting byte slice to string.

Change-Id: Ib23576f72b1d87489e6f17762be483f62ca4998a
GitHub-Last-Rev: ed8003bfbcae8efd42e54895db0554c139b9d3a7
GitHub-Pull-Request: golang/go#60026
Reviewed-on: https://go-review.googlesource.com/c/go/+/493237
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

diff --git a/src/errors/join.go b/src/errors/join.go
index 1c486d591e35bf19e20efabc9ac48f68864e603d..349fc06ed9f75c3323a9d65c2e519713a9b33e7e 100644 (file)
--- a/src/errors/join.go
+++ b/src/errors/join.go
@@ -4,6 +4,10 @@
 
 package errors
 
+import (
+       "unsafe"
+)
+
 // Join returns an error that wraps the given errors.
 // Any nil error values are discarded.
 // Join returns nil if every value in errs is nil.
@@ -38,14 +42,19 @@ type joinError struct {
 }
 
 func (e *joinError) Error() string {
-       var b []byte
-       for i, err := range e.errs {
-               if i > 0 {
-                       b = append(b, '\n')
-               }
+       // Since Join returns nil if every value in errs is nil,
+       // e.errs cannot be empty.
+       if len(e.errs) == 1 {
+               return e.errs[0].Error()
+       }
+
+       b := []byte(e.errs[0].Error())
+       for _, err := range e.errs[1:] {
+               b = append(b, '\n')
                b = append(b, err.Error()...)
        }
-       return string(b)
+       // At this point, b has at least one byte '\n'.
+       return unsafe.String(&b[0], len(b))
 }
 
 func (e *joinError) Unwrap() []error {