Make new stack size arithmetic match the old one exactly.

Author: duvenaud

lib/fft.dx

@@ -20,11 +20,6 @@ def odd_sized_palindrome(mid:a, seq:n=>a) -> ((n `Either` () `Either` n)=>a) giv
         Right () -> mid
       Right i -> seq[i]
 
-def nextpow2(x:Nat) -> Nat =
-  case is_power_of_2 x of
-    True -> natlog2 x
-    False -> 1 + natlog2 x
-
 '## Inner FFT functions
 
 data FTDirection =

lib/prelude.dx

@@ -2241,6 +2241,11 @@ def natlog2(x:Nat) -> Nat =
           False
   unsafe_nat_diff(tmp, 1)  -- TODO: something less horrible
 
+def nextpow2(x:Nat) -> Nat =
+  case is_power_of_2 x of
+    True -> natlog2 x
+    False -> 1 + natlog2 x
+
 def general_integer_power(
     times:(a,a)->a,
     one:a, base:a,
@@ -2477,7 +2482,7 @@ struct Stack(h:Heap, a|Data) =
 
   def ensure_size_at_least(req_size:Nat) -> {State h} () =
     if req_size > self.buf_size() then
-      new_buf_size = intpow2 (1 + natlog2 req_size)
+      new_buf_size = intpow2 $ nextpow2 req_size
       buf = self.unsafe_get_buffer()
       logical_size = self.size()
       cur_data = get $ unsafe_coerce(to=Ref(h, Fin logical_size => a), buf)