Got rid of floating-point division in table init.

Ideally we would get rid of all floating-point operations
in table.c, but that's a job for another day.

tests/test_table.cc

@@ -618,6 +618,16 @@ void test_delete() {
   upb_inttable_uninit(&t);
 }
 
+void test_init() {
+  for (int i = 0; i < 2048; i++) {
+    /* Tests that the size calculations in init() (lg2 size for target load)
+     * work for all expected sizes. */
+    upb_strtable t;
+    upb_strtable_init2(&t, UPB_CTYPE_BOOL, i, &upb_alloc_global);
+    upb_strtable_uninit(&t);
+  }
+}
+
 extern "C" {
 
 int run_tests(int argc, char *argv[]) {

upb/table.c

@@ -294,11 +294,9 @@ static bool streql(upb_tabkey k1, lookupkey_t k2) {
 bool upb_strtable_init2(upb_strtable *t, upb_ctype_t ctype,
                         size_t expected_size, upb_alloc *a) {
   UPB_UNUSED(ctype);  /* TODO(haberman): rm */
-  size_t need_entries = (expected_size + 1) / MAX_LOAD;
-  int size_lg2 = 2;
-  while (1 << size_lg2 < need_entries) {
-    size_lg2++;
-  }
+  size_t need_entries = (expected_size + 1) * 1204 / 1024;
+  UPB_ASSERT(need_entries >= expected_size * 0.85);
+  int size_lg2 = _upb_lg2ceil(need_entries);
   return init(&t->t, size_lg2, a);
 }
 

upb/upb.h

@@ -313,6 +313,17 @@ UPB_INLINE uint64_t _upb_be_swap64(uint64_t val) {
   }
 }
 
+UPB_INLINE int _upb_lg2ceil(int x) {
+  if (x == 0) return 0;
+#ifdef __GNUC__
+  return 32 - __builtin_clz(x - 1);
+#else
+  int lg2 = 0;
+  while (1 << lg2 < x) lg2++;
+  return lg2;
+#endif
+}
+
 #include "upb/port_undef.inc"
 
 #ifdef __cplusplus