fix nim-lang#13513 +% now gives correct values in semfold + VM

compiler/semfold.nim

@@ -172,6 +172,14 @@ proc fitLiteral(c: ConfigRef, n: PNode): PNode {.deprecated: "no substitute".} =
   if typ.kind in tyUInt..tyUInt32:
     result.intVal = result.intVal and castToInt64(lastOrd(c, typ))
 
+proc castToSizeSigned*(a: BiggestInt, numBits: int): BiggestInt =
+  # consider moving somewhere more general
+  if numBits == 8: result = cast[int8](a)
+  elif numBits == 16: result = cast[int16](a)
+  elif numBits == 32: result = cast[int32](a)
+  elif numBits == 64: result = cast[int64](a)
+  else: doAssert false, $numBits
+
 proc evalOp(m: TMagic, n, a, b, c: PNode; g: ModuleGraph): PNode =
   # b and c may be nil
   result = nil
@@ -297,7 +305,7 @@ proc evalOp(m: TMagic, n, a, b, c: PNode; g: ModuleGraph): PNode =
   of mBitorI, mOr: result = newIntNodeT(bitor(getInt(a), getInt(b)), n, g)
   of mBitxorI, mXor: result = newIntNodeT(bitxor(getInt(a), getInt(b)), n, g)
   of mAddU:
-    let val = maskBytes(getInt(a) + getInt(b), int(n.typ.size))
+    let val = castToSizeSigned(cast[BiggestInt](getInt(a) + getInt(b)), int(n.typ.size)*8)
     result = newIntNodeT(val, n, g)
   of mSubU:
     let val = maskBytes(getInt(a) - getInt(b), int(n.typ.size))

compiler/vm.nim

@@ -18,7 +18,7 @@ import
   vmmarshal, gorgeimpl, lineinfos, tables, btrees, macrocacheimpl,
   modulegraphs, sighashes, int128
 
-from semfold import leValueConv, ordinalValToString
+from semfold import leValueConv, ordinalValToString, castToSizeSigned
 from evaltempl import evalTemplate
 
 const
@@ -1493,15 +1493,8 @@ proc rawExecute(c: PCtx, start: int, tos: PStackFrame): TFullReg =
         stackTrace(c, tos, pc, "unhandled exception: value out of range")
     of opcNarrowU:
       decodeB(rkInt)
-      var ret = regs[ra].intVal
-      ret = ret and ((1'i64 shl rb)-1)
-      echo0b (rb, ret)
-      if rb == 8: ret = cast[int8](ret)
-      elif rb == 16: ret = cast[int16](ret)
-      elif rb == 32: ret = cast[int32](ret)
-      elif rb == 64: ret = cast[int64](ret)
-      else: doAssert false, $rb
-      regs[ra].intVal = ret
+      var ret = regs[ra].intVal and ((1'i64 shl rb)-1)
+      regs[ra].intVal = castToSizeSigned(ret, rb)
     of opcSignExtend:
       # like opcNarrowS, but no out of range possible
       decodeB(rkInt)

tests/vm/tarithmetics.nim

@@ -0,0 +1,20 @@
+template testAll(isSemFold: bool) =
+  block:
+    template test(T) =
+      proc fun(): T {.gensym.} =
+        when isSemFold:
+          high(T) +% 1
+        else:
+          let a = high(T)
+          a +% 1
+      const a1 = fun()
+      let a2 = fun()
+      doAssert a1 <= high(T)
+      doAssert $a1 == $a2
+      doAssert a1 == a2
+    test(int8)
+    test(int16)
+    test(int32)
+
+testAll(true) # will call semfold.nim `of mAddU:`
+testAll(false) # will call vm.nim `of opcNarrowU:`