move mask to common

constantine/config/common.nim

@@ -31,12 +31,15 @@ const
 
   Zero* = Word(0)
   One* = Word(1)
-  MaxWord* = (not Zero) shr 1
+  MaxWord* = (not Zero) shr (WordPhysBitSize - WordBitSize)
     ## This represents 0x7F_FF_FF_FF__FF_FF_FF_FF
     ## also 0b0111...1111
     ## This biggest representable number in our limbs.
     ## i.e. The most significant bit is never set at the end of each function
 
+template mask*(w: Word): Word =
+  w and MaxWord
+
 # ############################################################
 #
 #                  Instrumentation

constantine/io/io_bigints.nim

@@ -51,7 +51,7 @@ func fromRawUintLE(
 
     # if full, dump
     if acc_len >= WordBitSize:
-      dst.limbs[dst_idx] = acc and MaxWord
+      dst.limbs[dst_idx] = mask(acc)
       inc dst_idx
       acc_len -= WordBitSize
       acc = src_byte shr (8 - acc_len)
@@ -88,7 +88,7 @@ func fromRawUintBE(
 
     # if full, dump
     if acc_len >= WordBitSize:
-      dst.limbs[dst_idx] = acc and MaxWord
+      dst.limbs[dst_idx] = mask(acc)
       inc dst_idx
       acc_len -= WordBitSize
       acc = src_byte shr (8 - acc_len)

constantine/math/precomputed.nim

@@ -39,14 +39,14 @@ func double(a: var BigInt): bool =
   for i in 0 ..< a.limbs.len:
     var z = BaseType(a.limbs[i]) * 2 + BaseType(result)
     result = z.isMsbSet()
-    a.limbs[i] = Word(z) and MaxWord
+    a.limbs[i] = mask(Word(z))
 
 func sub(a: var BigInt, b: BigInt, ctl: bool): bool =
   ## In-place optional substraction
   for i in 0 ..< a.limbs.len:
     let new_a = BaseType(a.limbs[i]) - BaseType(b.limbs[i]) - BaseType(result)
     result = new_a.isMsbSet()
-    a.limbs[i] = if ctl: new_a.Word and MaxWord
+    a.limbs[i] = if ctl: new_a.Word.mask()
                  else: a.limbs[i]
 
 func doubleMod(a: var BigInt, M: BigInt) =