Add volatile load/store

Adds volatile load/store capability to the impure part of the standard
library.  This makes use of two new C macros stored in the nimbase.h
file and some macro trickery to perform type checking on the nim side.
Note that this currently only supports numerical types.

Should resolve nim-lang#3382

lib/impure/volatile.nim

@@ -0,0 +1,49 @@
+#
+#
+#            Nim's Runtime Library
+#        (c) Copyright 2017 Jeff Ciesielski
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+import strutils
+import macros
+
+# Keeps track of the total number of casts in the module (used to
+# generate unique function names used for aliasing)
+var totalCasts {.compiletime.} = 0
+
+const
+  vStoreDecl = """
+proc volatileStore_$1(dest: ptr $2, val: $2) {.importc: "VSTORE", nodecl.}
+volatileStore_$1
+"""
+  vLoadDecl = """
+proc volatileLoad_$1(s: ptr $2): $2 {.importc: "VLOAD", nodecl.}
+volatileLoad_$1
+"""
+
+macro genVolatileStore(varType: string): untyped =
+  result = parseStmt(vStoreDecl.format(`totalCasts`, varType))
+  totalCasts += 1
+
+template volatileStore*[T: SomeNumber](dest: ptr T, val: T) =
+  ## Generates a volatile store into the container `dest` of the value
+  ## `val`. Note that this only effects code generation on `C` like
+  ## backends
+  when defined(js):
+    dest[] = val
+  else:
+    genVolatileStore(T.name)(dest, val)
+
+macro genVolatileLoad(varType: string): untyped =
+  result = parseStmt(vLoadDecl.format(`totalCasts`, varType))
+  totalCasts += 1
+
+template volatileLoad*[T: SomeNumber](src: ptr T): T =
+  ## Generates a volatile load of the value stored in the container `src`.
+  ## Note that this only effects code generation on `C` like backends
+  when defined(js):
+    src[]
+  else:
+    genVolatileLoad(T.name)(src)

lib/nimbase.h

@@ -463,3 +463,7 @@ typedef int Nim_and_C_compiler_disagree_on_target_architecture[sizeof(NI) == siz
 /* Compile with -d:checkAbi and a sufficiently C11:ish compiler to enable */
 #define NIM_CHECK_SIZE(typ, sz) \
   _Static_assert(sizeof(typ) == sz, "Nim & C disagree on type size")
+
+/* ---------------- Volatile Load/Store casts ----------------------- */
+#define VLOAD(x) *(typeof(*x) volatile *)(x)
+#define VSTORE(x, y) *(typeof(*x) *)(x) = (y)