runtime: add comment for concatstring2

People always want to remove concatstring{2,3,4,5} for performance,
but we keep them to make the binary smaller. So, add a comment to
document why.

Updates golang#65020

Change-Id: I819976b700d45ce4d0846bf4481b2654b85708da
Reviewed-on: https://go-review.googlesource.com/c/go/+/700095
Auto-Submit: Keith Randall <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: fengyoulin

src/runtime/string.go

@@ -59,6 +59,9 @@ func concatstrings(buf *tmpBuf, a []string) string {
 	return s
 }
 
+// concatstring2 helps make the callsite smaller (compared to concatstrings),
+// and we think this is currently more valuable than omitting one call in the
+// chain, the same goes for concatstring{3,4,5}.
 func concatstring2(buf *tmpBuf, a0, a1 string) string {
 	return concatstrings(buf, []string{a0, a1})
 }
@@ -108,6 +111,9 @@ func concatbytes(buf *tmpBuf, a []string) []byte {
 	return b
 }
 
+// concatbyte2 helps make the callsite smaller (compared to concatbytes),
+// and we think this is currently more valuable than omitting one call in
+// the chain, the same goes for concatbyte{3,4,5}.
 func concatbyte2(buf *tmpBuf, a0, a1 string) []byte {
 	return concatbytes(buf, []string{a0, a1})
 }