build: implement GOEXPERIMENT again in runtime, and add to liblink

For Austin's framepointer experiment.

Change-Id: I81b6f414943b3578924f853300b9193684f79bf4
Reviewed-on: https://go-review.googlesource.com/2994
Reviewed-by: Austin Clements <austin@google.com>

2 files changed, 61 insertions, 0 deletions

diff --git a/src/liblink/go.c b/src/liblink/go.c
index 9f5a423d38..e31c71ab92 100644
--- a/src/liblink/go.c
+++ b/src/liblink/go.c
@@ -9,6 +9,66 @@
 #include <bio.h>
 #include <link.h>
 
+int framepointer_enabled;
+int fieldtrack_enabled;
+
+// Toolchain experiments.
+// These are controlled by the GOEXPERIMENT environment
+// variable recorded when the toolchain is built.
+// This list is also known to cmd/gc.
+static struct {
+	char *name;
+	int *val;
+} exper[] = {
+	{"fieldtrack", &fieldtrack_enabled},
+	{"basepointer", &framepointer_enabled}, 
+};
+
+static void
+addexp(char *s)
+{
+	int i;
+
+	for(i=0; i < nelem(exper); i++ ) {
+		if(strcmp(exper[i].name, s) == 0) {
+			if(exper[i].val != nil)
+				*exper[i].val = 1;
+			return;
+		}
+	}
+	
+	print("unknown experiment %s\n", s);
+	exits("unknown experiment");
+}
+
+void
+linksetexp(void)
+{
+	char *f[20];
+	int i, nf;
+
+	// cmd/dist #defines GOEXPERIMENT for us.
+	nf = getfields(GOEXPERIMENT, f, nelem(f), 1, ",");
+	for(i=0; i<nf; i++)
+		addexp(f[i]);
+}
+
+char*
+expstring(void)
+{
+	int i;
+	static char buf[512];
+
+	strcpy(buf, "X");
+	for(i=0; i<nelem(exper); i++)
+		if(*exper[i].val)
+			seprint(buf+strlen(buf), buf+sizeof buf, ",%s", exper[i].name);
+	if(strlen(buf) == 1)
+		strcpy(buf, "X,none");
+	buf[1] = ':';
+	return buf;
+}
+
 // replace all "". with pkg.
 char*
 expandpkg(char *t0, char *pkg)
diff --git a/src/liblink/sym.c b/src/liblink/sym.c
index e06837118a..35b88f44a2 100644
--- a/src/liblink/sym.c
+++ b/src/liblink/sym.c
@@ -90,6 +90,7 @@ linknew(LinkArch *arch)
 	char *p;
 	char buf[1024];
 
+	linksetexp();
 	nuxiinit(arch);
 	
 	ctxt = emallocz(sizeof *ctxt);