runtime: init GC later

Init GC later as it needs to read GOGC env var.
Fixes #8562.

LGTM=daniel.morsing, rsc
R=golang-codereviews, daniel.morsing, rsc
CC=golang-codereviews, khr, rlh
https://golang.org/cl/130990043

src/pkg/runtime/env_posix.c

@@ -22,6 +22,8 @@ runtime·getenv(int8 *s)
 	bs = (byte*)s;
 	len = runtime·findnull(bs);
 	envv = (String*)syscall·envs.array;
+	if(envv == nil)
+		runtime·throw("getenv before env init");
 	envc = syscall·envs.len;
 	for(i=0; i<envc; i++){
 		if(envv[i].len <= len)

src/pkg/runtime/proc.c

@@ -158,7 +158,6 @@ runtime·schedinit(void)
 	runtime·symtabinit();
 	runtime·stackinit();
 	runtime·mallocinit();
-	runtime·gcinit();
 	runtime·chaninit();
 	mcommoninit(g->m);
 	
@@ -168,13 +167,10 @@ runtime·schedinit(void)
 	// need to allocated memory.
 	runtime·newErrorCString(0, &i);
 	
-	// Initialize the cached gotraceback value, since
-	// gotraceback calls getenv, which mallocs on Plan 9.
-	runtime·gotraceback(nil);
-
 	runtime·goargs();
 	runtime·goenvs();
 	runtime·parsedebugvars();
+	runtime·gcinit();
 
 	runtime·sched.lastpoll = runtime·nanotime();
 	procs = 1;

src/pkg/runtime/runtime.c

@@ -12,7 +12,7 @@
 // The cached value is a uint32 in which the low bit
 // is the "crash" setting and the top 31 bits are the
 // gotraceback value.
-static uint32 traceback_cache = ~(uint32)0;
+static uint32 traceback_cache = 2<<1;
 
 // The GOTRACEBACK environment variable controls the
 // behavior of a Go program that is crashing and exiting.
@@ -23,29 +23,13 @@ static uint32 traceback_cache = ~(uint32)0;
 int32
 runtime·gotraceback(bool *crash)
 {
-	byte *p;
-	uint32 x;
-
 	if(crash != nil)
 		*crash = false;
 	if(g->m->traceback != 0)
 		return g->m->traceback;
-	x = runtime·atomicload(&traceback_cache);
-	if(x == ~(uint32)0) {
-		p = runtime·getenv("GOTRACEBACK");
-		if(p == nil)
-			p = (byte*)"";
-		if(p[0] == '\0')
-			x = 1<<1;
-		else if(runtime·strcmp(p, (byte*)"crash") == 0)
-			x = (2<<1) | 1;
-		else
-			x = runtime·atoi(p)<<1;	
-		runtime·atomicstore(&traceback_cache, x);
-	}
 	if(crash != nil)
-		*crash = x&1;
-	return x>>1;
+		*crash = traceback_cache&1;
+	return traceback_cache>>1;
 }
 
 int32
@@ -134,8 +118,6 @@ runtime·goenvs_unix(void)
 	syscall·envs.array = (byte*)s;
 	syscall·envs.len = n;
 	syscall·envs.cap = n;
-
-	traceback_cache = ~(uint32)0;
 }
 
 int32
@@ -354,6 +336,16 @@ runtime·parsedebugvars(void)
 			break;
 		p++;
 	}
+
+	p = runtime·getenv("GOTRACEBACK");
+	if(p == nil)
+		p = (byte*)"";
+	if(p[0] == '\0')
+		traceback_cache = 1<<1;
+	else if(runtime·strcmp(p, (byte*)"crash") == 0)
+		traceback_cache = (2<<1) | 1;
+	else
+		traceback_cache = runtime·atoi(p)<<1;	
 }
 
 // Poor mans 64-bit division.