cmd: use 128-bit SHA256 & encode in base64 for content hashes

We used to use SHA1 for content hashes, but CL 402595 changed
all the “don't care” hashes to cmd/internal/notsha256 (negated SHA256).
This made object files a little bit bigger: fmt.a on my Mac laptop grows
from 910678 to 937612 bytes (+3%).

To remove that growth, truncate the hash we use for these purposes
to 128 bits (half a SHA256), and also use base64 instead of hex for
encoding it when a string form is needed. This brings fmt.a down to
901706 bytes (-1% from original, -4% from current).

Change-Id: Id81da1cf3ee85ed130b3cda73aa697d8c0053a62
Reviewed-on: https://go-review.googlesource.com/c/go/+/404294
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/cmd/compile/internal/staticdata/data.go

@@ -5,6 +5,7 @@
 package staticdata
 
 import (
+	"encoding/base64"
 	"fmt"
 	"go/constant"
 	"io"
@@ -61,9 +62,15 @@ func InitSliceBytes(nam *ir.Name, off int64, s string) {
 
 const (
 	stringSymPrefix  = "go.string."
-	stringSymPattern = ".gostring.%d.%x"
+	stringSymPattern = ".gostring.%d.%s"
 )
 
+// shortHashString converts the hash to a string for use with stringSymPattern.
+// We cut it to 16 bytes and then base64-encode to make it even smaller.
+func shortHashString(hash []byte) string {
+	return base64.StdEncoding.EncodeToString(hash[:16])
+}
+
 // StringSym returns a symbol containing the string s.
 // The symbol contains the string data, not a string header.
 func StringSym(pos src.XPos, s string) (data *obj.LSym) {
@@ -75,7 +82,7 @@ func StringSym(pos src.XPos, s string) (data *obj.LSym) {
 		// Same pattern is known to fileStringSym below.
 		h := notsha256.New()
 		io.WriteString(h, s)
-		symname = fmt.Sprintf(stringSymPattern, len(s), h.Sum(nil))
+		symname = fmt.Sprintf(stringSymPattern, len(s), shortHashString(h.Sum(nil)))
 	} else {
 		// Small strings get named directly by their contents.
 		symname = strconv.Quote(s)
@@ -162,7 +169,7 @@ func fileStringSym(pos src.XPos, file string, readonly bool, hash []byte) (*obj.
 
 	var symdata *obj.LSym
 	if readonly {
-		symname := fmt.Sprintf(stringSymPattern, size, sum)
+		symname := fmt.Sprintf(stringSymPattern, size, shortHashString(sum))
 		symdata = base.Ctxt.Lookup(stringSymPrefix + symname)
 		if !symdata.OnList() {
 			info := symdata.NewFileInfo()

src/cmd/internal/goobj/objfile.go

@@ -20,7 +20,6 @@ package goobj
 
 import (
 	"cmd/internal/bio"
-	"cmd/internal/notsha256"
 	"encoding/binary"
 	"errors"
 	"fmt"
@@ -367,7 +366,7 @@ const Hash64Size = 8
 // Hash
 type HashType [HashSize]byte
 
-const HashSize = notsha256.Size
+const HashSize = 16 // truncated SHA256
 
 // Relocation.
 //

src/cmd/internal/obj/sym.go

@@ -35,6 +35,7 @@ import (
 	"cmd/internal/goobj"
 	"cmd/internal/notsha256"
 	"cmd/internal/objabi"
+	"encoding/base64"
 	"fmt"
 	"internal/buildcfg"
 	"log"
@@ -175,7 +176,9 @@ func (ctxt *Link) Int64Sym(i int64) *LSym {
 
 // GCLocalsSym generates a content-addressable sym containing data.
 func (ctxt *Link) GCLocalsSym(data []byte) *LSym {
-	return ctxt.LookupInit(fmt.Sprintf("gclocals·%x", notsha256.Sum256(data)), func(lsym *LSym) {
+	sum := notsha256.Sum256(data)
+	str := base64.StdEncoding.EncodeToString(sum[:16])
+	return ctxt.LookupInit(fmt.Sprintf("gclocals·%s", str), func(lsym *LSym) {
 		lsym.P = data
 		lsym.Set(AttrContentAddressable, true)
 	})