cmd/link/internal/ld: add support for GOARCH=mips{,le}

Change-Id: Ida214ccc5858969ea60abb0787f4d98bab4336d6
Reviewed-on: https://go-review.googlesource.com/31480
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/link/internal/ld/elf.go

@@ -946,18 +946,22 @@ func Elfinit(ctxt *Link) {
 		ehdr.shentsize = ELF64SHDRSIZE /* Must be ELF64SHDRSIZE */
 
 	// 32-bit architectures
-	case sys.ARM:
-		// we use EABI on linux/arm, freebsd/arm, netbsd/arm.
-		if Headtype == obj.Hlinux || Headtype == obj.Hfreebsd || Headtype == obj.Hnetbsd {
-			// We set a value here that makes no indication of which
-			// float ABI the object uses, because this is information
-			// used by the dynamic linker to compare executables and
-			// shared libraries -- so it only matters for cgo calls, and
-			// the information properly comes from the object files
-			// produced by the host C compiler. parseArmAttributes in
-			// ldelf.go reads that information and updates this field as
-			// appropriate.
-			ehdr.flags = 0x5000002 // has entry point, Version5 EABI
+	case sys.ARM, sys.MIPS:
+		if SysArch.Family == sys.ARM {
+			// we use EABI on linux/arm, freebsd/arm, netbsd/arm.
+			if Headtype == obj.Hlinux || Headtype == obj.Hfreebsd || Headtype == obj.Hnetbsd {
+				// We set a value here that makes no indication of which
+				// float ABI the object uses, because this is information
+				// used by the dynamic linker to compare executables and
+				// shared libraries -- so it only matters for cgo calls, and
+				// the information properly comes from the object files
+				// produced by the host C compiler. parseArmAttributes in
+				// ldelf.go reads that information and updates this field as
+				// appropriate.
+				ehdr.flags = 0x5000002 // has entry point, Version5 EABI
+			}
+		} else if SysArch.Family == sys.MIPS {
+			ehdr.flags = 0x50000000 /* MIPS 32 */
 		}
 		fallthrough
 	default:

src/cmd/link/internal/ld/ldelf.go

@@ -546,6 +546,12 @@ func ldelf(ctxt *Link, f *bio.Reader, pkg string, length int64, pn string) {
 		Errorf(nil, "%s: elf %s unimplemented", pn, SysArch.Name)
 		return
 
+	case sys.MIPS:
+		if elfobj.machine != ElfMachMips || hdr.Ident[4] != ElfClass32 {
+			Errorf(nil, "%s: elf object but not mips", pn)
+			return
+		}
+
 	case sys.MIPS64:
 		if elfobj.machine != ElfMachMips || hdr.Ident[4] != ElfClass64 {
 			Errorf(nil, "%s: elf object but not mips64", pn)