Convert string litteral to bytes for python 3, add test

theano/gof/cmodule.py

@@ -25,7 +25,7 @@ import numpy.distutils  # TODO: TensorType should handle this
 
 import theano
 from theano.compat import any, PY3, next, decode, decode_iter
-from theano.compat.six import BytesIO, StringIO
+from theano.compat.six import b, BytesIO, StringIO
 from theano.gof.utils import flatten
 from theano.configparser import config
 from theano.gof.cc import hash_from_code
@@ -1843,12 +1843,12 @@ class GCC_compiler(object):
         if not theano.config.cxx:
             return False
 
-        code = """
+        code = b("""
         int main(int argc, char** argv)
         {
             return 0;
         }
-        """
+        """)
         return GCC_compiler.try_compile_tmp(code, tmp_prefix='try_flags_',
                 flags=flag_list, try_run=False)
 

theano/gof/tests/test_cmodule.py

@@ -7,6 +7,7 @@ deterministic based on the input type and the op.
 import numpy
 
 import theano
+from theano.gof.cmodule import GCC_compiler
 
 
 class MyOp(theano.compile.ops.DeepCopyOp):
@@ -57,3 +58,11 @@ def test_inter_process_cache():
         assert MyOp.nb_called == 0
     else:
         assert MyOp.nb_called == 1
+
+
+def test_flag_detection():
+    # Check that the code detecting blas flags does not raise any exception.
+    # It used to happen on python 3 because of improper string handling,
+    # but was not detected because that path is not usually taken,
+    # so we test it here directly.
+    GCC_compiler.try_flags(["-lblas"])