cmd/gc: make qsort comparisons totally ordered

Otherwise different qsort implementations might result
in different sort orders and therefore different compiled
object files.

Change-Id: Ie783ba55a55af06941307e150b0c406e0a8128b0
Reviewed-on: https://go-review.googlesource.com/4590
Reviewed-by: Austin Clements <[email protected]>

Author: rsc

src/cmd/gc/popt.c

@@ -532,6 +532,14 @@ startcmp(const void *va, const void *vb)
 		return -1;
 	if(a->start > b->start)
 		return +1;
+	// Order what's left by id or symbol name,
+	// just so that sort is forced into a specific ordering,
+	// so that the result of the sort does not depend on
+	// the sort implementation.
+	if(a->def != b->def)
+		return a->def->id - b->def->id;
+	if(a->node != b->node)
+		return strcmp(a->node->sym->name, b->node->sym->name);
 	return 0;
 }
 

src/cmd/gc/reg.c

@@ -50,15 +50,16 @@ static int
 rcmp(const void *a1, const void *a2)
 {
 	Rgn *p1, *p2;
-	int c1, c2;
 
 	p1 = (Rgn*)a1;
 	p2 = (Rgn*)a2;
-	c1 = p2->cost;
-	c2 = p1->cost;
-	if(c1 -= c2)
-		return c1;
-	return p2->varno - p1->varno;
+	if(p1->cost != p2->cost)
+		return p2->cost - p1->cost;
+	if(p1->varno != p2->varno)
+		return p2->varno - p1->varno;
+	if(p1->enter != p2->enter)
+		return p2->enter->id - p1->enter->id;
+	return 0;
 }
 
 static void