Add Noop, a better variant of Null for internal usages

Author: lwaern-intel

py/dml/codegen.py

@@ -210,7 +210,7 @@ def fail(self, site):
 class IgnoreFailure(Failure):
     '''Ignore exceptions'''
     def fail(self, site):
-        return mkNull(site)
+        return mkNoop(site)
 
 class ExitHandler(ABC):
     current = None
@@ -1916,9 +1916,10 @@ def codegen_statements(trees, *args):
     return stmts
 
 def codegen_statement(tree, *args):
-    rbrace_site = tree.args[1] if tree.kind == 'compound' else None
-    return mkCompound(tree.site, codegen_statements([tree], *args),
-                      rbrace_site)
+    stmts = codegen_statements([tree], *args)
+    if len(stmts) == 1 and not stmts[0].is_declaration:
+        return stmts[0]
+    return mkCompound(tree.site, stmts)
 
 @statement_dispatcher
 def stmt_compound(stmt, location, scope):
@@ -2190,7 +2191,7 @@ def stmt_saved(stmt, location, scope):
 
 @statement_dispatcher
 def stmt_null(stmt, location, scope):
-    return []
+    return [mkNull(stmt.site)]
 
 @statement_dispatcher
 def stmt_if(stmt, location, scope):
@@ -3207,7 +3208,7 @@ def stmt_while(stmt, location, scope):
     [cond, statement] = stmt.args
     cond = as_bool(codegen_expression(cond, location, scope))
     if stmt.site.dml_version() == (1, 2) and cond.constant and not cond.value:
-        return [mkNull(stmt.site)]
+        return [mkNoop(stmt.site)]
     else:
         with CLoopContext():
             res = mkWhile(stmt.site, cond,
@@ -3345,7 +3346,7 @@ def mkcall_method(site, func, indices):
 
 def common_inline(site, method, indices, inargs, outargs):
     if not verify_args(site, method.inp, method.outp, inargs, outargs):
-        return mkNull(site)
+        return mkNoop(site)
 
     if dml.globals.debuggable:
         if method.fully_typed and (
@@ -3999,7 +4000,7 @@ def codegen_call_traitmethod(site, expr, inargs, outargs):
     if not isinstance(expr, TraitMethodRef):
         raise ICE(site, "cannot call %r: not a trait method" % (expr,))
     if not verify_args(site, expr.inp, expr.outp, inargs, outargs):
-        return mkNull(site)
+        return mkNoop(site)
     def mkcall(args):
         rettype = c_rettype(expr.outp, expr.throws)
         # implicitly convert endian int arguments to integers
@@ -4011,7 +4012,7 @@ def mkcall(args):
 def codegen_call(site, meth_node, indices, inargs, outargs):
     '''Generate a call using a direct reference to the method node'''
     if not verify_args(site, meth_node.inp, meth_node.outp, inargs, outargs):
-        return mkNull(site)
+        return mkNoop(site)
     require_fully_typed(site, meth_node)
     func = method_instance(meth_node)
 

py/dml/ctree.py

@@ -40,7 +40,8 @@
     'param_bool_fixup',
 
     'mkCompound',
-    'mkNull', 'Null',
+    'mkNull',
+    'mkNoop',
     'mkLabel',
     'mkUnrolledLoop',
     'mkGoto',
@@ -432,6 +433,9 @@ def mkCompound(site, statements, rbrace_site=None):
         return Compound(site, collapsed, rbrace_site)
 
 class Null(Statement):
+    '''Should only be used to represent stand-alone ; present in DML code. For
+       all other purposes (artificially created empty statements) Noop should
+       be used instead.'''
     is_empty = True
     def toc_stmt(self):
         self.linemark()
@@ -441,6 +445,19 @@ def toc(self):
 
 mkNull = Null
 
+class Noop(Statement):
+    '''An empty statement represented by a pair of braces in generated C. This
+       avoids certain GCC and Coverity warnings that could manifest if ';' were
+       used instead.'''
+    is_empty = True
+    def toc_stmt(self):
+        self.linemark()
+        out('{}\n')
+    def toc(self):
+        pass
+
+mkNoop = Noop
+
 class Label(Statement):
     def __init__(self, site, label, unused=False):
         Statement.__init__(self, site)
@@ -664,7 +681,7 @@ def toc_stmt(self):
 
 def mkExpressionStatement(site, expr):
     if isinstance(expr, Constant):
-        return mkNull(site)
+        return mkNoop(site)
     return ExpressionStatement(site, expr)
 
 def toc_constsafe_pointer_assignment(site, source, target, typ):
@@ -840,7 +857,7 @@ def mkIf(site, cond, truebranch, falsebranch = None, else_site=None):
         elif falsebranch:
             return falsebranch
         else:
-            return mkNull(site)
+            return mkNoop(site)
     return If(site, cond, truebranch, falsebranch, else_site)
 
 class While(Statement):