cmd/internal/obj: reuse the varint encoding buffer

This reduces the number of allocations in the compiler
while building the stdlib by 15.66%.

No functional changes. Passes toolstash -cmp.

Change-Id: Ia21b37134a8906a4e23d53fdc15235b4aa7bbb34
Reviewed-on: https://go-review.googlesource.com/9085Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/obj/objfile.go

@@ -463,18 +463,21 @@ func writesym(ctxt *Link, b *Biobuf, s *LSym) {
 	}
 }
 
+// Reusable buffer to avoid allocations.
+// This buffer was responsible for 15% of gc's allocations.
+var varintbuf [10]uint8
+
 func wrint(b *Biobuf, sval int64) {
 	var v uint64
-	var buf [10]uint8
 	uv := (uint64(sval) << 1) ^ uint64(int64(sval>>63))
-	p := buf[:]
+	p := varintbuf[:]
 	for v = uv; v >= 0x80; v >>= 7 {
 		p[0] = uint8(v | 0x80)
 		p = p[1:]
 	}
 	p[0] = uint8(v)
 	p = p[1:]
-	Bwrite(b, buf[:len(buf)-len(p)])
+	Bwrite(b, varintbuf[:len(varintbuf)-len(p)])
 }
 
 func wrstring(b *Biobuf, s string) {