add support cast[integer] in VM

compiler/vmgen.nim

@@ -30,7 +30,7 @@
 import
   strutils, ast, astalgo, types, msgs, renderer, vmdef,
   trees, intsets, rodread, magicsys, options, lowerings
-
+import platform
 from os import splitFile
 
 when hasFFI:
@@ -761,6 +761,49 @@ proc genCard(c: PCtx; n: PNode; dest: var TDest) =
   c.gABC(n, opcCard, dest, tmp)
   c.freeTemp(tmp)
 
+proc genIntCast(c: PCtx; n: PNode; dest: var TDest) =
+  const allowedIntegers = {tyInt..tyInt64, tyUInt..tyUInt64, tyChar}
+  var signedIntegers = {tyInt8..tyInt32}
+  var unsignedIntegers = {tyUInt8..tyUInt32, tyChar}
+  let src = n.sons[1].typ.skipTypes(abstractRange)#.kind
+  let dst = n.sons[0].typ.skipTypes(abstractRange)#.kind
+  let src_size = src.getSize
+
+  if platform.intSize < 8:
+    signedIntegers.incl(tyInt)
+    unsignedIntegers.incl(tyUInt)
+  if src_size == dst.getSize and src.kind in allowedIntegers and
+                                 dst.kind in allowedIntegers:
+    let tmp = c.genx(n.sons[1])
+    var tmp2 = c.getTemp(n.sons[1].typ)
+    let tmp3 = c.getTemp(n.sons[1].typ)
+    if dest < 0: dest = c.getTemp(n[0].typ)
+    proc mkIntLit(ival: int): int =
+      result = genLiteral(c, newIntTypeNode(nkIntLit, ival, getSysType(tyInt)))
+    if src.kind in unsignedIntegers and dst.kind in signedIntegers:
+      # cast unsigned to signed integer of same size
+      # signedVal = (unsignedVal xor offset) -% offset
+      let offset = 1 shl (src_size * 8 - 1)
+      c.gABx(n, opcLdConst, tmp2, mkIntLit(offset))
+      c.gABC(n, opcBitxorInt, tmp3, tmp, tmp2)
+      c.gABC(n, opcSubInt, dest, tmp3, tmp2)
+    elif src.kind in signedIntegers and dst.kind in unsignedIntegers:
+      # cast signed to unsigned integer of same size
+      # unsignedVal = (offset +% signedVal +% 1) and offset
+      let offset = (1 shl (src_size * 8))  - 1
+      c.gABx(n, opcLdConst, tmp2, mkIntLit(offset))
+      c.gABx(n, opcLdConst, dest, mkIntLit(offset+1))
+      c.gABC(n, opcAddu, tmp3, tmp, dest)
+      c.gABC(n, opcNarrowU, tmp3, TRegister(src_size*8))
+      c.gABC(n, opcBitandInt, dest, tmp3, tmp2)
+    else:
+      c.gABC(n, opcAsgnInt, dest, tmp)
+    c.freeTemp(tmp)
+    c.freeTemp(tmp2)
+    c.freeTemp(tmp3)
+  else:
+    globalError(n.info, errGenerated, "VM is only allowed to 'cast' between integers of same size")
+
 proc genMagic(c: PCtx; n: PNode; dest: var TDest; m: TMagic) =
   case m
   of mAnd: c.genAndOr(n, opcFJmp, dest)
@@ -1844,7 +1887,7 @@ proc gen(c: PCtx; n: PNode; dest: var TDest; flags: TGenFlags = {}) =
     if allowCast in c.features:
       genConv(c, n, n.sons[1], dest, opcCast)
     else:
-      globalError(n.info, errGenerated, "VM is not allowed to 'cast'")
+      genIntCast(c, n, dest)
   of nkTypeOfExpr:
     genTypeLit(c, n.typ, dest)
   of nkComesFrom: