when nimvm stmt proof of concept

compiler/ccgexprs.nim

@@ -2069,6 +2069,9 @@ proc expr(p: BProc, n: PNode, d: var TLoc) =
   of nkStmtList:
     for i in countup(0, sonsLen(n) - 1): genStmts(p, n.sons[i])
   of nkIfExpr, nkIfStmt: genIf(p, n, d)
+  of nkWhen:
+    # This should be a "when nimvm" node.
+    expr(p, n.sons[1].sons[0], d)
   of nkObjDownConv: downConv(p, n, d)
   of nkObjUpConv: upConv(p, n, d)
   of nkChckRangeF: genRangeChck(p, n, d, "chckRangeF")

compiler/jsgen.nim

@@ -1687,6 +1687,9 @@ proc gen(p: PProc, n: PNode, r: var TCompRes) =
       gen(p, lastSon(n), r)
   of nkBlockStmt, nkBlockExpr: genBlock(p, n, r)
   of nkIfStmt, nkIfExpr: genIf(p, n, r)
+  of nkWhen:
+    # This is "when nimvm" node
+    gen(p, n.sons[1].sons[0], r)
   of nkWhileStmt: genWhileStmt(p, n)
   of nkVarSection, nkLetSection: genVarStmt(p, n)
   of nkConstSection: discard

compiler/semexprs.nim

@@ -1764,22 +1764,39 @@ proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
     if semCheck: result = semStmt(c, e) # do not open a new scope!
     else: result = e
 
+  # Check if the node is "when nimvm"
+  # when nimvm:
+  #   ...
+  # else:
+  #   ...
+  let whenNimvm = n.sons.len == 2 and n.sons[0].kind == nkElifBranch and
+      n.sons[1].kind == nkElse and n.sons[0].sons[0].kind == nkIdent and
+      lookUp(c, n.sons[0].sons[0]).magic == mNimvm
+
   for i in countup(0, sonsLen(n) - 1):
     var it = n.sons[i]
     case it.kind
     of nkElifBranch, nkElifExpr:
       checkSonsLen(it, 2)
-      var e = semConstExpr(c, it.sons[0])
-      if e.kind != nkIntLit:
-        # can happen for cascading errors, assume false
-        # InternalError(n.info, "semWhen")
-        discard
-      elif e.intVal != 0 and result == nil:
-        setResult(it.sons[1])
+      if whenNimvm:
+        if semCheck:
+          it.sons[1] = semStmt(c, it.sons[1])
+        result = n # when nimvm is not elimited until codegen
+      else:
+        var e = semConstExpr(c, it.sons[0])
+        if e.kind != nkIntLit:
+          # can happen for cascading errors, assume false
+          # InternalError(n.info, "semWhen")
+          discard
+        elif e.intVal != 0 and result == nil:
+          setResult(it.sons[1])
     of nkElse, nkElseExpr:
       checkSonsLen(it, 1)
-      if result == nil:
-        setResult(it.sons[0])
+      if result == nil or whenNimvm:
+        if semCheck:
+          it.sons[0] = semStmt(c, it.sons[0])
+        if result == nil:
+          result = it.sons[0]
     else: illFormedAst(n)
   if result == nil:
     result = newNodeI(nkEmpty, n.info)
@@ -2162,7 +2179,11 @@ proc semExpr(c: PContext, n: PNode, flags: TExprFlags = {}): PNode =
       result = semWhen(c, n, true)
     else:
       result = semWhen(c, n, false)
-      result = semExpr(c, result, flags)
+      if result == n:
+        # This is a "when nimvm" stmt.
+        result = semWhen(c, n, true)
+      else:
+        result = semExpr(c, result, flags)
   of nkBracketExpr:
     checkMinSonsLen(n, 1)
     var s = qualifiedLookUp(c, n.sons[0], {checkUndeclared})

compiler/semparallel.nim

@@ -363,7 +363,7 @@ proc analyse(c: var AnalysisCtx; n: PNode) =
             else: internalError(it.info, "slot already has a lower bound")
         if not isSpawned: analyse(c, value)
   of nkCaseStmt: analyseCase(c, n)
-  of nkIfStmt, nkIfExpr: analyseIf(c, n)
+  of nkWhen, nkIfStmt, nkIfExpr: analyseIf(c, n)
   of nkWhileStmt:
     analyse(c, n.sons[0])
     # 'while true' loop?

compiler/sempass2.nim

@@ -735,7 +735,7 @@ proc track(tracked: PEffects, n: PNode) =
       # since 'var (a, b): T = ()' is not even allowed, there is always type
       # inference for (a, b) and thus no nil checking is necessary.
   of nkCaseStmt: trackCase(tracked, n)
-  of nkIfStmt, nkIfExpr: trackIf(tracked, n)
+  of nkWhen, nkIfStmt, nkIfExpr: trackIf(tracked, n)
   of nkBlockStmt, nkBlockExpr: trackBlock(tracked, n.sons[1])
   of nkWhileStmt:
     track(tracked, n.sons[0])

compiler/vmgen.nim

@@ -1664,7 +1664,10 @@ proc gen(c: PCtx; n: PNode; dest: var TDest; flags: TGenFlags = {}) =
   of nkBracketExpr: genArrAccess(c, n, dest, flags)
   of nkDerefExpr, nkHiddenDeref: genAddrDeref(c, n, dest, opcLdDeref, flags)
   of nkAddr, nkHiddenAddr: genAddrDeref(c, n, dest, opcAddrNode, flags)
-  of nkWhenStmt, nkIfStmt, nkIfExpr: genIf(c, n, dest)
+  of nkIfStmt, nkIfExpr: genIf(c, n, dest)
+  of nkWhenStmt:
+      # This is "when nimvm" node. Chose the first branch.
+      gen(c, n.sons[0].sons[1], dest)
   of nkCaseStmt: genCase(c, n, dest)
   of nkWhileStmt:
     unused(n, dest)