artiq_compile: make print() write to core log, not an invalid op.

Fixes #710.

artiq/compiler/embedding.py

@@ -699,7 +699,7 @@ class TypedtreeHasher(algorithm.Visitor):
         return hash(tuple(freeze(getattr(node, field_name)) for field_name in fields))
 
 class Stitcher:
-    def __init__(self, core, dmgr, engine=None):
+    def __init__(self, core, dmgr, engine=None, print_as_rpc=True):
         self.core = core
         self.dmgr = dmgr
         if engine is None:
@@ -715,7 +715,8 @@ class Stitcher:
         # We don't want some things from the prelude as they are provided in
         # the host Python namespace and gain special meaning when quoted.
         self.prelude = prelude.globals()
-        self.prelude.pop("print")
+        if print_as_rpc:
+            self.prelude.pop("print")
         self.prelude.pop("array")
 
         self.functions = {}

artiq/coredevice/core.py

@@ -81,11 +81,13 @@ class Core:
         self.core = self
         self.comm.core = self
 
-    def compile(self, function, args, kwargs, set_result=None, attribute_writeback=True):
+    def compile(self, function, args, kwargs, set_result=None,
+                attribute_writeback=True, print_as_rpc=True):
         try:
             engine = _DiagnosticEngine(all_errors_are_fatal=True)
 
-            stitcher = Stitcher(engine=engine, core=self, dmgr=self.dmgr)
+            stitcher = Stitcher(engine=engine, core=self, dmgr=self.dmgr,
+                                print_as_rpc=print_as_rpc)
             stitcher.stitch_call(function, args, kwargs, set_result)
             stitcher.finalize()
 

artiq/frontend/artiq_compile.py

@@ -55,7 +55,7 @@ def main():
 
         object_map, kernel_library, _, _ = \
             core.compile(exp.run, [exp_inst], {},
-                         attribute_writeback=False)
+                         attribute_writeback=False, print_as_rpc=False)
     except CompileError as error:
         return
     finally: