cmd/cc, cmd/ld, runtime: disallow conservative data/bss objects

In linker, refuse to write conservative (array of pointers) as the
garbage collection type for any variable in the data/bss GC program.

In the linker, attach the Go type to an already-read C declaration
during dedup. This gives us Go types for C globals for free as long
as the cmd/dist-generated Go code contains the declaration.
(Most runtime C declarations have a corresponding Go declaration.
Both are bss declarations and so the linker dedups them.)

In cmd/dist, add a few more C files to the auto-Go-declaration list
in order to get Go type information for the C declarations into the linker.

In C compiler, mark all non-pointer-containing global declarations
and all string data as NOPTR. This allows them to exist in C files
without any corresponding Go declaration. Count C function pointers
as "non-pointer-containing", since we have no heap-allocated C functions.

In runtime, add NOPTR to the remaining pointer-containing declarations,
none of which refer to Go heap objects.

In runtime, also move os.Args and syscall.envs data into runtime-owned
variables. Otherwise, in programs that do not import os or syscall, the
runtime variables named os.Args and syscall.envs will be missing type
information.

I believe that this CL eliminates the final source of conservative GC scanning
in non-SWIG Go programs, and therefore...

Fixes #909.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/149770043

1 files changed, 4 insertions, 2 deletions

diff --git a/src/liblink/objfile.c b/src/liblink/objfile.c
index 9b1e1b7a8f..15d602df92 100644
--- a/src/liblink/objfile.c
+++ b/src/liblink/objfile.c
@@ -550,7 +550,7 @@ readsym(Link *ctxt, Biobuf *f, char *pkg, char *pn)
 	static int ndup;
 	char *name;
 	Reloc *r;
-	LSym *s, *dup;
+	LSym *s, *dup, *typ;
 	Pcln *pc;
 	Auto *a;
 	
@@ -586,7 +586,9 @@ readsym(Link *ctxt, Biobuf *f, char *pkg, char *pn)
 	s->type = t;
 	if(s->size < size)
 		s->size = size;
-	s->gotype = rdsym(ctxt, f, pkg);
+	typ = rdsym(ctxt, f, pkg);
+	if(typ != nil) // if bss sym defined multiple times, take type from any one def
+		s->gotype = typ;
 	rddata(f, &s->p, &s->np);
 	s->maxp = s->np;
 	n = rdint(f);