runtime: use staticuint64s instead of staticbytes for 1-length strings

This was the last remaining use of staticbytes, so we can now
delete it.

The new code appears slightly faster on amd64:

name                   old time/op  new time/op  delta
SliceByteToString/1-4  6.29ns ± 2%  5.89ns ± 1%  -6.46%  (p=0.000 n=14+14)

This may not be the case on the big-endian architectures, since they have
to do an extra addition.

Updates #37612

Change-Id: Icb84c5911ba025f798de152849992a55be99e4f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/221979
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

2 files changed, 6 insertions, 43 deletions

diff --git a/src/runtime/iface.go b/src/runtime/iface.go
index 892e5a400f..e4b0b6d3d3 100644
--- a/src/runtime/iface.go
+++ b/src/runtime/iface.go
@@ -527,42 +527,6 @@ func iterate_itabs(fn func(*itab)) {
 	}
 }
 
-// staticbytes is used to avoid convT2E for byte-sized values.
-var staticbytes = [...]byte{
-	0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
-	0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
-	0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17,
-	0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
-	0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27,
-	0x28, 0x29, 0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f,
-	0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37,
-	0x38, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0x3e, 0x3f,
-	0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47,
-	0x48, 0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f,
-	0x50, 0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57,
-	0x58, 0x59, 0x5a, 0x5b, 0x5c, 0x5d, 0x5e, 0x5f,
-	0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67,
-	0x68, 0x69, 0x6a, 0x6b, 0x6c, 0x6d, 0x6e, 0x6f,
-	0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76, 0x77,
-	0x78, 0x79, 0x7a, 0x7b, 0x7c, 0x7d, 0x7e, 0x7f,
-	0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87,
-	0x88, 0x89, 0x8a, 0x8b, 0x8c, 0x8d, 0x8e, 0x8f,
-	0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97,
-	0x98, 0x99, 0x9a, 0x9b, 0x9c, 0x9d, 0x9e, 0x9f,
-	0xa0, 0xa1, 0xa2, 0xa3, 0xa4, 0xa5, 0xa6, 0xa7,
-	0xa8, 0xa9, 0xaa, 0xab, 0xac, 0xad, 0xae, 0xaf,
-	0xb0, 0xb1, 0xb2, 0xb3, 0xb4, 0xb5, 0xb6, 0xb7,
-	0xb8, 0xb9, 0xba, 0xbb, 0xbc, 0xbd, 0xbe, 0xbf,
-	0xc0, 0xc1, 0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7,
-	0xc8, 0xc9, 0xca, 0xcb, 0xcc, 0xcd, 0xce, 0xcf,
-	0xd0, 0xd1, 0xd2, 0xd3, 0xd4, 0xd5, 0xd6, 0xd7,
-	0xd8, 0xd9, 0xda, 0xdb, 0xdc, 0xdd, 0xde, 0xdf,
-	0xe0, 0xe1, 0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7,
-	0xe8, 0xe9, 0xea, 0xeb, 0xec, 0xed, 0xee, 0xef,
-	0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7,
-	0xf8, 0xf9, 0xfa, 0xfb, 0xfc, 0xfd, 0xfe, 0xff,
-}
-
 // staticuint64s is used to avoid allocating in convTx for small integer values.
 var staticuint64s = [...]uint64{
 	0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
diff --git a/src/runtime/string.go b/src/runtime/string.go
index 184245b105..7dc0bd789f 100644
--- a/src/runtime/string.go
+++ b/src/runtime/string.go
@@ -6,6 +6,7 @@ package runtime
 
 import (
 	"internal/bytealg"
+	"runtime/internal/sys"
 	"unsafe"
 )
 
@@ -90,7 +91,11 @@ func slicebytetostring(buf *tmpBuf, b []byte) (str string) {
 		msanread(unsafe.Pointer(&b[0]), uintptr(l))
 	}
 	if l == 1 {
-		stringStructOf(&str).str = unsafe.Pointer(&staticbytes[b[0]])
+		p := unsafe.Pointer(&staticuint64s[b[0]])
+		if sys.BigEndian {
+			p = add(p, 7)
+		}
+		stringStructOf(&str).str = p
 		stringStructOf(&str).len = 1
 		return
 	}
@@ -231,12 +236,6 @@ func stringStructOf(sp *string) *stringStruct {
 }
 
 func intstring(buf *[4]byte, v int64) (s string) {
-	if v >= 0 && v < runeSelf {
-		stringStructOf(&s).str = unsafe.Pointer(&staticbytes[v])
-		stringStructOf(&s).len = 1
-		return
-	}
-
 	var b []byte
 	if buf != nil {
 		b = buf[:]