close #8269: grow arrays incrementally by factors of 1.5 rather than 2

Author: stevengj

src/array.c

@@ -633,7 +633,7 @@ JL_DLLEXPORT void jl_array_grow_end(jl_array_t *a, size_t inc)
     if ((alen + inc) > a->maxsize - a->offset) {
         size_t newlen = a->maxsize==0 ? (inc<4?4:inc) : a->maxsize*2;
         while ((alen + inc) > newlen - a->offset)
-            newlen *= 2;
+            newlen = (newlen * 3) >> 1;
 
         newlen = limit_overallocation(a, alen, newlen, inc);
         array_resize_buffer(a, newlen, alen, a->offset);
@@ -694,7 +694,7 @@ JL_DLLEXPORT void jl_array_grow_beg(jl_array_t *a, size_t inc)
         if (inc > (a->maxsize-alen)/2 - (a->maxsize-alen)/20) {
             size_t newlen = a->maxsize==0 ? inc*2 : a->maxsize*2;
             while (alen+2*inc > newlen-a->offset)
-                newlen *= 2;
+                newlen = (newlen * 3) >> 1;
 
             newlen = limit_overallocation(a, alen, newlen, 2*inc);
             size_t center = (newlen - (alen + inc))/2;