Improved Yosys binary discovery and error reporting.

Signed-off-by: Maciej Kurc <mkurc@antmicro.com>

v2x/__main__.py

diff --git a/v2x/__main__.py b/v2x/__main__.py
index 8ab0174..38ba16d 100644
--- a/v2x/__main__.py
+++ b/v2x/__main__.py
@@ -5,8 +5,16 @@
 import argparse
 import sys
 
+from .yosys.run import get_yosys
+
 
 def main(args):
+
+    # Check if Yosys can be found. Print an error message if not.
+    if get_yosys() is None:
+        print("ERROR: Cannot find the Yosys binary or its not executable.")
+        return -1
+
     if args.mode == "pb_type":
         with open(args.outfile, "w") as fp:
             fp.write(vlog_to_pbtype.vlog_to_pbtype(

v2x/yosys/run.py

diff --git a/v2x/yosys/run.py b/v2x/yosys/run.py
index 3703eeb..91e361d 100755
--- a/v2x/yosys/run.py
+++ b/v2x/yosys/run.py
@@ -19,9 +19,37 @@
 
 
 def get_yosys():
-    """Return how to execute Yosys: the value of $YOSYS if set, otherwise just
-    `yosys`."""
-    return os.getenv("YOSYS", "yosys")
+    """
+    Searches for the Yosys binary. If the env. var. "YOSYS" is set, then it
+    checks if it points to a valid executable binary. Otherwise it searches
+    in PATH for binaries named "yosys" and returns the first one found.
+    """
+
+    def is_exe(fpath):
+        """
+        Returns True if a file exists and is executable.
+        """
+        return os.path.isfile(fpath) and os.access(fpath, os.X_OK)
+
+    # The environmental variable "YOSYS" is set. It should point to the Yosys
+    # executable.
+    if "YOSYS" in os.environ:
+        fpath = os.environ["YOSYS"]
+        if not is_exe(fpath):
+            return None
+
+        return fpath
+
+    # Look for the 'yosys' binary in the current PATH but only if the PATH
+    # variable is available.
+    elif "PATH" in os.environ:
+        for path in os.environ["PATH"].split(os.pathsep):
+            fpath = os.path.join(path, "yosys")
+            if is_exe(fpath):
+                return fpath
+
+    # Couldn't find Yosys.
+    return None
 
 
 def get_yosys_common_args():