cmd/gc: implement -pack flag

The -pack flag causes 5g, 6g, 8g to write a Go archive directly,
instead of requiring the use of 'go tool pack' to convert the .5/.6/.8
to .a format.

Writing directly avoids the copy and also avoids having the
export data stored twice in the archive (once in __.PKGDEF,
once in .5/.6/.8).

A separate CL will enable the use of this flag by cmd/go.

Other build systems that do not know about -pack will be unaffected.

The changes to cmd/ld handle a minor simplification to the format:
an unused section is removed.

R=iant, r
CC=golang-dev
https://golang.org/cl/42880043

2 files changed, 11 insertions, 31 deletions

diff --git a/src/cmd/ld/go.c b/src/cmd/ld/go.c
index 900ecb0ae9..a5a0fa5433 100644
--- a/src/cmd/ld/go.c
+++ b/src/cmd/ld/go.c
@@ -151,28 +151,11 @@ ldpkg(Biobuf *f, char *pkg, int64 len, char *filename, int whence)
 		}
 		loadpkgdata(filename, pkg, p0, p1 - p0);
 	}
-
-	// The __.PKGDEF archive summary has no local types.
+	
+	// __.PKGDEF has no cgo section - those are in the C compiler-generated object files.
 	if(whence == Pkgdef)
 		return;
 
-	// local types begin where exports end.
-	// skip rest of line after $$ we found above
-	p0 = p1 + 3;
-	while(*p0 != '\n' && *p0 != '\0')
-		p0++;
-
-	// local types end at next \n$$.
-	p1 = strstr(p0, "\n$$");
-	if(p1 == nil) {
-		fprint(2, "%s: cannot find end of local types in %s\n", argv0, filename);
-		if(debug['u'])
-			errorexit();
-		return;
-	}
-
-	loadpkgdata(filename, pkg, p0, p1 - p0);
-
 	// look for cgo section
 	p0 = strstr(p1, "\n$$  // cgo");
 	if(p0 != nil) {
diff --git a/src/cmd/ld/lib.c b/src/cmd/ld/lib.c
index 76a3a1393a..f4ac30a571 100644
--- a/src/cmd/ld/lib.c
+++ b/src/cmd/ld/lib.c
@@ -327,25 +327,22 @@ objfile(char *file, char *pkg)
 		return;
 	}
 	
-	/* skip over __.GOSYMDEF */
+	/* skip over optional __.GOSYMDEF and process __.PKGDEF */
 	off = Boffset(f);
 	if((l = nextar(f, off, &arhdr)) <= 0) {
 		diag("%s: short read on archive file symbol header", file);
 		goto out;
 	}
-	if(strncmp(arhdr.name, symname, strlen(symname))) {
-		diag("%s: first entry not symbol header", file);
-		goto out;
-	}
-	off += l;
-	
-	/* skip over (or process) __.PKGDEF */
-	if((l = nextar(f, off, &arhdr)) <= 0) {
-		diag("%s: short read on archive file symbol header", file);
-		goto out;
+	if(strncmp(arhdr.name, symname, strlen(symname)) == 0) {
+		off += l;
+		if((l = nextar(f, off, &arhdr)) <= 0) {
+			diag("%s: short read on archive file symbol header", file);
+			goto out;
+		}
 	}
+
 	if(strncmp(arhdr.name, pkgname, strlen(pkgname))) {
-		diag("%s: second entry not package header", file);
+		diag("%s: cannot find package header", file);
 		goto out;
 	}
 	off += l;