conversions to unsigned numbers are not checked anymore (#12688) [bac…

…kport]

* conversions to unsigned numbers are not checked anymore; implements / fixes nim-lang/RFCs#175

* change the spec yet again to be less consistent but to make more sense; updated the changelog

(cherry picked from commit c98e0e2)

changelog.md

@@ -27,6 +27,9 @@
 
 ## Language changes
 
+- Conversions to unsigned integers are unchecked at runtime, imitating earlier Nim
+ versions. The documentation was improved to acknowledge this special case.
+ See https://github.com/nim-lang/RFCs/issues/175 for more details.
 
 
 ### Tool changes

compiler/ccgexprs.nim

@@ -1931,7 +1931,8 @@ proc genCast(p: BProc, e: PNode, d: var TLoc) =
 proc genRangeChck(p: BProc, n: PNode, d: var TLoc, magic: string) =
  var a: TLoc
  var dest = skipTypes(n.typ, abstractVar)
- if optRangeCheck notin p.options:
+ if optRangeCheck notin p.options or (dest.kind in {tyUInt..tyUInt64} and
+ checkUnsignedConversions notin p.config.legacyFeatures):
  initLocExpr(p, n.sons[0], a)
  putIntoDest(p, d, n, "(($1) ($2))" %
  [getTypeDesc(p.module, dest), rdCharLoc(a)], a.storage)

compiler/jsgen.nim

@@ -2155,7 +2155,10 @@ proc upConv(p: PProc, n: PNode, r: var TCompRes) =
 proc genRangeChck(p: PProc, n: PNode, r: var TCompRes, magic: string) =
  var a, b: TCompRes
  gen(p, n.sons[0], r)
- if optRangeCheck in p.options:
+ if optRangeCheck notin p.options or (skipTypes(n.typ, abstractVar).kind in {tyUInt..tyUInt64} and
+ checkUnsignedConversions notin p.config.legacyFeatures):
+ discard "XXX maybe emit masking instructions here"
+ else:
  gen(p, n.sons[1], a)
  gen(p, n.sons[2], b)
  useMagic(p, "chckRange")

compiler/options.nim

@@ -144,6 +144,10 @@ type
  ## Allows to modify a NimNode where the type has already been
  ## flagged with nfSem. If you actually do this, it will cause
  ## bugs.
+ checkUnsignedConversions
+ ## Historically and especially in version 1.0.0 of the language
+ ## conversions to unsigned numbers were checked. In 1.0.4 they
+ ## are not anymore.
 
  SymbolFilesOption* = enum
  disabledSf, writeOnlySf, readOnlySf, v2Sf

compiler/semfold.nim

@@ -433,25 +433,23 @@ proc foldConv(n, a: PNode; g: ModuleGraph; check = false): PNode =
  let dstTyp = skipTypes(n.typ, abstractRange - {tyTypeDesc})
  let srcTyp = skipTypes(a.typ, abstractRange - {tyTypeDesc})
 
-
  # if srcTyp.kind == tyUInt64 and "FFFFFF" in $n:
  # echo "n: ", n, " a: ", a
  # echo "from: ", srcTyp, " to: ", dstTyp, " check: ", check
  # echo getInt(a)
  # echo high(int64)
  # writeStackTrace()
-
- # XXX range checks?
  case dstTyp.kind
  of tyInt..tyInt64, tyUInt..tyUInt64:
  case srcTyp.kind
  of tyFloat..tyFloat64:
  result = newIntNodeT(BiggestInt(getFloat(a)), n, g)
  of tyChar, tyUInt..tyUInt64, tyInt..tyInt64:
  var val = a.getOrdValue
+
  if check: rangeCheck(n, val, g)
  result = newIntNodeT(val, n, g)
- if dstTyp.kind in {tyUInt .. tyUInt64}:
+ if dstTyp.kind in {tyUInt..tyUInt64}:
  result.kind = nkUIntLit
  else:
  result = a

doc/manual.rst

@@ -3185,6 +3185,7 @@ has lots of advantages:
 
 Type conversions
 ----------------
+
 Syntactically a `type conversion` is like a procedure call, but a
 type name replaces the procedure name. A type conversion is always
 safe in the sense that a failure to convert a type to another
@@ -3204,6 +3205,19 @@ A type conversion can also be used to disambiguate overloaded routines:
  let procVar = (proc(x: string))(p)
  procVar("a")
 
+Since operations on unsigned numbers wrap around and are unchecked so are
+type conversion to unsigned integers and between unsigned integers. The
+rationale for this is mostly better interoperability with the C Programming
+language when algorithms are ported from C to Nim.
+
+Exception: Values that are converted to an unsigned type at compile time
+are checked so that code like ``byte(-1)`` does not compile.
+
+**Note**: Historically the operations
+were unchecked and the conversions were sometimes checked but starting with
+the revision 1.0.4 of this document and the language implementation the
+conversions too are now *always unchecked*.
+
 
 Type casts
 ----------