Better error message when Theano isn't able to use cuDNN

theano/gof/cmodule.py

@@ -1788,7 +1788,8 @@ class GCC_compiler(object):
         return cxxflags
 
     @staticmethod
-    def try_compile_tmp(src_code, tmp_prefix='', flags=(), try_run=False):
+    def try_compile_tmp(src_code, tmp_prefix='', flags=(),
+                        try_run=False, output=False):
         """Try to compile (and run) a test program.
 
         This is useful in various occasions, to check if libraries
@@ -1799,6 +1800,7 @@ class GCC_compiler(object):
 
         If try_run is False, returns the compilation status.
         If try_run is True, returns a (compile_status, run_status) pair.
+        If output is there, we append the stdout and stderr to the output.
         """
         if not theano.config.cxx:
             return False
@@ -1818,14 +1820,14 @@ class GCC_compiler(object):
                 os.write(fd, src_code)
                 os.close(fd)
                 fd = None
-                p_ret = call_subprocess_Popen(
+                out, err, p_ret = output_subprocess_Popen(
                     ['g++', path, '-o', exe_path] + flags)
                 if p_ret != 0:
                     compilation_ok = False
                 elif try_run:
                     # Try to execute the program
                     try:
-                        p_ret = call_subprocess_Popen([exe_path])
+                        out, err, p_ret = output_subprocess_Popen([exe_path])
                         run_ok = (p_ret == 0)
                     finally:
                         os.remove(exe_path)
@@ -1839,13 +1841,17 @@ class GCC_compiler(object):
         except OSError, e:
             compilation_ok = False
 
-        if not try_run:
+        if not try_run and not output:
             return compilation_ok
-        else:
+        elif not try_run and output:
+            return (compilation_ok, out, err)
+        elif not output:
             return (compilation_ok, run_ok)
+        else:
+            return (compilation_ok, run_ok, out, err)
 
     @staticmethod
-    def try_flags(flag_list, try_run=False, preambule=""):
+    def try_flags(flag_list, preambule="", try_run=False, output=False):
         '''
         Try to compile a dummy file with these flags.
 
@@ -1863,7 +1869,8 @@ class GCC_compiler(object):
         }
         """ % locals())
         return GCC_compiler.try_compile_tmp(code, tmp_prefix='try_flags_',
-                                            flags=flag_list, try_run=try_run)
+                                            flags=flag_list, try_run=try_run,
+                                            output=output)
 
     @staticmethod
     def compile_str(module_name, src_code, location=None,

theano/sandbox/cuda/dnn.py

@@ -25,9 +25,17 @@ def dnn_available():
             dnn_available.msg = "Device not supported by cuDNN"
             dnn_available.avail = False
         else:
-            dnn_available.msg = "Can not find the cuDNN library"
-            dnn_available.avail = all(gof.cmodule.GCC_compiler.try_flags(
-                ["-l", "cudnn"], try_run=True, preambule="#include <cudnn.h>"))
+            comp, run, out, err = gof.cmodule.GCC_compiler.try_flags(
+                ["-l", "cudnn"], preambule="#include <cudnn.h>",
+                try_run=True, output=True)
+
+            dnn_available.avail = comp and run
+            if dnn_available.avail:
+                dnn_available.msg = "cuDNN should work"
+            else:
+                dnn_available.msg = (
+                    "Theano is not able to use cuDNN. We got this error: \n" +
+                    err)
     return dnn_available.avail
 
 
@@ -1040,6 +1048,7 @@ if cuda_available:
             """ Raise a RuntimeError if cudnn can't be used"""
             if not dnn_available():
                 raise RuntimeError(
-                    "cuDNN optimization was enabled, but cuDNN is not available. " +
+                    "cuDNN optimization was enabled, but Theano was not able"
+                    " to use it. We got this error: \n" +
                     dnn_available.msg)
     gpu_seqopt.register("NoCuDNNRaise", NoCuDNNRaise(), 0, 'cudnn')