Add sub, proper negate bound checking, high and low, most significant word mutation

src/private/as_signed_words.nim

@@ -43,7 +43,7 @@ macro most_significant_word*(x: IntImpl): untyped =
   let optim_type = optimInt(x)
   if optim_type.isInt:
     result = quote do:
-      cast[`optim_type`](`x`)
+      (cast[ptr `optim_type`](`x`.unsafeAddr))[]
   else:
     when system.cpuEndian == littleEndian:
       let size = getSize(x)
@@ -51,7 +51,7 @@ macro most_significant_word*(x: IntImpl): untyped =
     else:
       let msw_pos = 0
     result = quote do:
-      cast[`optim_type`](`x`)[`msw_pos`]
+      (cast[ptr `optim_type`](`x`)[`msw_pos`.unsafeAddr])[]
 
 macro asSignedWordsZip*[T](
   x, y: IntImpl[T],

src/private/int_addsubneg.nim

@@ -7,20 +7,43 @@
 #
 # at your option. This file may not be copied, modified, or distributed except according to those terms.
 
-import ./datatypes, ./int_bitwise_ops, ./initialization
+import  ./datatypes, ./int_bitwise_ops,
+        ./initialization, ./as_signed_words, ./int_highlow
 
-func `+=`*(x: var IntImpl, y: IntImpl) {.inline.}=
+func `+`*(x, y: IntImpl): IntImpl {.noInit, inline.}=
-  ## In-place addition for multi-precision signed int
-
-  type SubTy = type x.lo
-  x.lo += y.lo
-  x.hi += (x.lo < y.lo).toSubtype(SubTy) + y.hi
-
-func `+`*(x, y: UintImpl): UintImpl {.noInit, inline.}=
   # Addition for multi-precision signed int
-  result = x
+  type SubTy = type x.lo
-  result += y
+  result.lo = x.lo + y.lo
+  result.hi = (x.lo < y.lo).toSubtype(SubTy) + x.hi + y.hi
+
+  when compileOption("boundChecks"):
+    if unlikely(
+      (result.most_significant_word xor a.most_significant_word >= 0) or
+      (result.most_significant_word xor b.most_significant_word >= 0)
+    ):
+      raise newException(OverflowError, "Addition overflow")
 
 func `-`*[T: IntImpl](x: T): T {.noInit, inline.}=
   result = not x
   result += one(T)
+  when compileOption("boundChecks"):
+    if unlikely(x == low(T)):
+      raise newException(OverflowError, "The lowest negative number cannot be negated")
+
+func `-`*(x, y: IntImpl): IntImpl {.noInit, inline.}=
+  # Substraction for multi-precision signed int
+
+  type SubTy = type x.lo
+  result.lo = x.lo - y.lo
+  result.hi = x.hi - y.hi - (x.lo < y.lo).toSubtype(SubTy)
+
+  when compileOption("boundChecks"):
+    if unlikely(
+      (result.most_significant_word xor a.most_significant_word >= 0) or
+      (result.most_significant_word xor (not b).most_significant_word >= 0)
+    ):
+      raise newException(OverflowError, "Substraction underflow")
+
+func `-=`*(x: var IntImpl, y: IntImpl) {.inline.}=
+  ## In-place substraction for multi-precision signed int
+  x = x - y

src/private/int_highlow.nim

@@ -0,0 +1,30 @@
+# Mpint
+# Copyright 2018 Status Research & Development GmbH
+# Licensed under either of
+#
+#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
+#  * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+#
+# at your option. This file may not be copied, modified, or distributed except according to those terms.
+
+import ./datatypes, ./as_signed_words
+
+func low*(T: typedesc[UintImpl]): T {.inline.}=
+
+  # The lowest signed int has representation
+  # 0b1000_0000_0000_0000 ....
+  # so we only have to set the most significant bit.
+  type Msw = type result.most_significant_word
+  when Msw is uint64:
+    type U = int64
+  else:
+    type U = Msw
+
+  result.most_significant_word = low(U)
+
+func high*(T: typedesc[UintImpl]): T {.inline, noInit.}=
+
+  # The lowest signed int has representation
+  # 0b0111_1111_1111_1111 ....
+  # so we only have to unset the most significant bit.
+  not low(T)

src/private/uint_highlow.nim

@@ -0,0 +1,16 @@
+# Mpint
+# Copyright 2018 Status Research & Development GmbH
+# Licensed under either of
+#
+#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
+#  * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+#
+# at your option. This file may not be copied, modified, or distributed except according to those terms.
+
+import ./datatypes, ./initialization
+
+func low*(T: typedesc[UintImpl]): T {.inline, noInit.}=
+  zero(T)
+
+func high*(T: typedesc[UintImpl]): T {.inline, noInit.}=
+  not zero(T)