Limit number of flags to 32

Resolves #370

Author: lukewagner

design/mvp/Binary.md

@@ -186,7 +186,7 @@ defvaltype    ::= pvt:<primvaltype>                       => pvt
                 | 0x71 case*:vec(<case>)                  => (variant case+) (if |case*| > 0)
                 | 0x70 t:<valtype>                        => (list t)
                 | 0x6f t*:vec(<valtype>)                  => (tuple t+)    (if |t*| > 0)
-                | 0x6e l*:vec(<label'>)                   => (flags l+)    (if |l*| > 0)
+                | 0x6e l*:vec(<label'>)                   => (flags l+)    (if 0 < |l*| <= 32)
                 | 0x6d l*:vec(<label'>)                   => (enum l+)     (if |l*| > 0)
                 | 0x6b t:<valtype>                        => (option t)
                 | 0x6a t?:<valtype>? u?:<valtype>?        => (result t? (error u)?)

design/mvp/CanonicalABI.md

@@ -158,6 +158,7 @@ back to sequences of `i32`s when there are more than 32 flags.
 ```python
 def alignment_flags(labels):
   n = len(labels)
+  assert(0 < n <= 32)
   if n <= 8: return 1
   if n <= 16: return 2
   return 4
@@ -216,13 +217,10 @@ def elem_size_variant(cases):
 
 def elem_size_flags(labels):
   n = len(labels)
-  assert(n > 0)
+  assert(0 < n <= 32)
   if n <= 8: return 1
   if n <= 16: return 2
-  return 4 * num_i32_flags(labels)
-
-def num_i32_flags(labels):
-  return math.ceil(len(labels) / 32)
+  return 4
 ```
 
 ### Call Context
@@ -1584,7 +1582,7 @@ def flatten_type(t):
     case String() | List(_)   : return ['i32', 'i32']
     case Record(fields)       : return flatten_record(fields)
     case Variant(cases)       : return flatten_variant(cases)
-    case Flags(labels)        : return ['i32'] * num_i32_flags(labels)
+    case Flags(labels)        : return ['i32']
     case Own(_) | Borrow(_)   : return ['i32']
 ```
 
@@ -1755,16 +1753,12 @@ def wrap_i64_to_i32(i):
   return i % (1 << 32)
 ```
 
-Finally, flags are lifted by OR-ing together all the flattened `i32` values
-and then lifting to a record the same way as when loading flags from linear
-memory.
+Finally, flags are lifted by lifting to a record the same way as when loading
+flags from linear memory.
 ```python
 def lift_flat_flags(vi, labels):
-  i = 0
-  shift = 0
-  for _ in range(num_i32_flags(labels)):
-    i |= (vi.next('i32') << shift)
-    shift += 32
+  assert(0 < len(labels) <= 32)
+  i = vi.next('i32')
   return unpack_flags_from_int(i, labels)
 ```
 
@@ -1857,16 +1851,11 @@ def lower_flat_variant(cx, v, cases):
   return [case_index] + payload
 ```
 
-Finally, flags are lowered by slicing the bit vector into `i32` chunks:
+Finally, flags are lowered by packing the flags into on `i32` bitvector.
 ```python
 def lower_flat_flags(v, labels):
-  i = pack_flags_into_int(v, labels)
-  flat = []
-  for _ in range(num_i32_flags(labels)):
-    flat.append(i & 0xffffffff)
-    i >>= 32
-  assert(i == 0)
-  return flat
+  assert(0 < len(labels) <= 32)
+  return [pack_flags_into_int(v, labels)]
 ```
 
 ### Lifting and Lowering Values

design/mvp/canonical-abi/definitions.py

@@ -221,6 +221,7 @@ def max_case_alignment(cases):
 
 def alignment_flags(labels):
   n = len(labels)
+  assert(0 < n <= 32)
   if n <= 8: return 1
   if n <= 16: return 2
   return 4
@@ -266,13 +267,10 @@ def elem_size_variant(cases):
 
 def elem_size_flags(labels):
   n = len(labels)
-  assert(n > 0)
+  assert(0 < n <= 32)
   if n <= 8: return 1
   if n <= 16: return 2
-  return 4 * num_i32_flags(labels)
-
-def num_i32_flags(labels):
-  return math.ceil(len(labels) / 32)
+  return 4
 
 ### Call Context
 
@@ -1100,7 +1098,7 @@ def flatten_type(t):
     case String() | List(_)   : return ['i32', 'i32']
     case Record(fields)       : return flatten_record(fields)
     case Variant(cases)       : return flatten_variant(cases)
-    case Flags(labels)        : return ['i32'] * num_i32_flags(labels)
+    case Flags(labels)        : return ['i32']
     case Own(_) | Borrow(_)   : return ['i32']
 
 def flatten_record(fields):
@@ -1222,11 +1220,8 @@ def wrap_i64_to_i32(i):
   return i % (1 << 32)
 
 def lift_flat_flags(vi, labels):
-  i = 0
-  shift = 0
-  for _ in range(num_i32_flags(labels)):
-    i |= (vi.next('i32') << shift)
-    shift += 32
+  assert(0 < len(labels) <= 32)
+  i = vi.next('i32')
   return unpack_flags_from_int(i, labels)
 
 ### Flat Lowering
@@ -1294,13 +1289,8 @@ def lower_flat_variant(cx, v, cases):
   return [case_index] + payload
 
 def lower_flat_flags(v, labels):
-  i = pack_flags_into_int(v, labels)
-  flat = []
-  for _ in range(num_i32_flags(labels)):
-    flat.append(i & 0xffffffff)
-    i >>= 32
-  assert(i == 0)
-  return flat
+  assert(0 < len(labels) <= 32)
+  return [pack_flags_into_int(v, labels)]
 
 ### Lifting and Lowering Values
 

design/mvp/canonical-abi/run_tests.py

@@ -114,7 +114,7 @@ def test_name():
 test(t, [2], {'a':False,'b':True})
 test(t, [3], {'a':True,'b':True})
 test(t, [4], {'a':False,'b':False})
-test(Flags([str(i) for i in range(33)]), [0xffffffff,0x1], { str(i):True for i in range(33) })
+test(Flags([str(i) for i in range(32)]), [0xffffffff], { str(i):True for i in range(32) })
 t = Variant([Case('x',U8()),Case('y',F32()),Case('z',None)])
 test(t, [0,42], {'x': 42})
 test(t, [0,256], {'x': 0})
@@ -304,12 +304,10 @@ def test_heap(t, expect, args, byte_array):
           [0xff,0xff,0x1,0, 0,0,0,0])
 test_heap(t, v, [0,2],
           [0xff,0xff,0x3,0, 0,0,0,0])
-t = List(Flags([str(i) for i in range(33)]))
-v = [{ str(i):b for i in range(33) } for b in [True,False]]
+t = List(Flags([str(i) for i in range(32)]))
+v = [{ str(i):b for i in range(32) } for b in [True,False]]
 test_heap(t, v, [0,2],
-          [0xff,0xff,0xff,0xff,0x1,0,0,0, 0,0,0,0,0,0,0,0])
-test_heap(t, v, [0,2],
-          [0xff,0xff,0xff,0xff,0x3,0,0,0, 0,0,0,0,0,0,0,0])
+          [0xff,0xff,0xff,0xff, 0,0,0,0])
 
 def test_flatten(t, params, results):
   expect = CoreFuncType(params, results)