Shut up as much compiler warnings as possible through firstprivate

Cython/Compiler/Nodes.py

@@ -6146,8 +6146,20 @@ class ParallelRangeNode(ParallelStatNode):
         # Note: nsteps is private in an outer scope if present
         code.putln("%(nsteps)s = (%(stop)s - %(start)s) / %(step)s;" % fmt_dict)
 
+        # The target iteration variable might not be initialized, do it only if
+        # we are executing at least 1 iteration, otherwise we should leave the
+        # target unaffected. The target iteration variable is firstprivate to
+        # shut up compiler warnings caused by lastprivate, as the compiler
+        # erroneously believes that nsteps may be <= 0, leaving the private
+        # target index uninitialized
+        code.putln("if (%(nsteps)s > 0)" % fmt_dict)
+        code.begin_block()
+        code.putln("%(target)s = 0;" % fmt_dict)
+
         self.generate_loop(code, fmt_dict)
 
+        code.end_block()
+
         # And finally, release our privates and write back any closure
         # variables
         for temp in start_stop_step:
@@ -6173,6 +6185,8 @@ class ParallelRangeNode(ParallelStatNode):
             if op and op in "+*-&^|" and entry != self.target.entry:
                 code.put(" reduction(%s:%s)" % (op, entry.cname))
             else:
+                if entry == self.target.entry:
+                    code.put(" firstprivate(%s)" % entry.cname)
                 code.put(" lastprivate(%s)" % entry.cname)
 
         if self.schedule:

tests/run/sequential_parallel.pyx

@@ -2,7 +2,7 @@
 
 cimport cython.parallel
 from cython.parallel import prange, threadid
-from libc.stdlib cimport malloc, free
+from libc.stdlib cimport malloc, free, abort
 from libc.stdio cimport puts
 
 import sys
@@ -67,11 +67,7 @@ def test_propagation():
 
 def test_unsigned_operands():
     """
-    This test is disabled, as this currently does not work (neither does it
-    for 'for i from x < i < y:'. I'm not sure we should strife to support
-    this, at least the C compiler gives a warning.
-
-    test_unsigned_operands()
+    >>> test_unsigned_operands()
     10
     """
     cdef int i
@@ -83,6 +79,8 @@ def test_unsigned_operands():
 
     for i in prange(start, stop, step, nogil=True):
         steps_taken += 1
+        if steps_taken > 10:
+            abort()
 
     return steps_taken