py: Fix bug in optimised for .. range.

Don't store final, failing value to the loop variable.  This fix also
makes for .. range a bit more efficient, as it uses less store/load
pairs for the loop variable.

py/compile.c

@@ -1534,35 +1534,41 @@ void compile_while_stmt(compiler_t *comp, mp_parse_node_struct_t *pns) {
 }
 
 // TODO preload end and step onto stack if they are not constants
-// TODO check if step is negative and do opposite test
+// Note that, as per semantics of for .. range, the final failing value should not be stored in the loop variable
+// And, if the loop never runs, the loop variable should never be assigned
 void compile_for_stmt_optimised_range(compiler_t *comp, mp_parse_node_t pn_var, mp_parse_node_t pn_start, mp_parse_node_t pn_end, mp_parse_node_t pn_step, mp_parse_node_t pn_body, mp_parse_node_t pn_else) {
     START_BREAK_CONTINUE_BLOCK
 
     int top_label = comp_next_label(comp);
     int entry_label = comp_next_label(comp);
 
-    // compile: var = start
+    // compile: start, duplicated on stack
     compile_node(comp, pn_start);
-    c_assign(comp, pn_var, ASSIGN_STORE);
+    EMIT(dup_top);
 
     EMIT_ARG(jump, entry_label);
     EMIT_ARG(label_assign, top_label);
 
+    // at this point we actually have 1 less element on the stack
+    EMIT_ARG(set_stack_size, EMIT(get_stack_size) - 1);
+
+    // store next value to var
+    c_assign(comp, pn_var, ASSIGN_STORE);
+
     // compile body
     compile_node(comp, pn_body);
 
     EMIT_ARG(label_assign, continue_label);
 
-    // compile: var += step
-    c_assign(comp, pn_var, ASSIGN_AUG_LOAD);
+    // compile: var + step, duplicated on stack
+    compile_node(comp, pn_var);
     compile_node(comp, pn_step);
     EMIT_ARG(binary_op, MP_BINARY_OP_INPLACE_ADD);
-    c_assign(comp, pn_var, ASSIGN_AUG_STORE);
+    EMIT(dup_top);
 
     EMIT_ARG(label_assign, entry_label);
 
     // compile: if var <cond> end: goto top
-    compile_node(comp, pn_var);
     compile_node(comp, pn_end);
     assert(MP_PARSE_NODE_IS_SMALL_INT(pn_step));
     if (MP_PARSE_NODE_LEAF_SMALL_INT(pn_step) >= 0) {
@@ -1572,6 +1578,9 @@ void compile_for_stmt_optimised_range(compiler_t *comp, mp_parse_node_t pn_var,
     }
     EMIT_ARG(pop_jump_if_true, top_label);
 
+    // discard final value of var that failed the loop condition
+    EMIT(pop_top);
+
     // break/continue apply to outer loop (if any) in the else block
     END_BREAK_CONTINUE_BLOCK
 

tests/basics/for2.py

+i = 'init'
+for i in range(0):
+    pass
+print(i) # should not have been modified
+
+for i in range(10):
+    pass
+print(i) # should be last successful value of loop

tests/misc/features.py

+# mad.py
+# Alf Clement 27-Mar-2014
+#
+zero=0
+three=3
+print("1")
+print("2")
+print(three)
+print("{}".format(4))
+five=25/5
+print(int(five))
+j=0
+for i in range(4):
+	j += i
+print(j)
+print(3+4)
+try:
+	a=4/zero
+except:
+	print(8)
+print("xxxxxxxxx".count("x"))
+def ten():
+	return 10
+print(ten())
+a=[]
+for i in range(13):
+	a.append(i)
+print(a[11])	
+print(a[-1])
+str="0123456789"
+print(str[1]+str[3])
+def p(s):
+	print(s)
+p("14")
+p(15)
+class A:
+	def __init__(self):
+		self.a=16
+	def print(self):
+		print(self.a)
+	def set(self,b):
+		self.a=b
+a=A()
+a.print()
+a.set(17)
+a.print()
+b=A()
+b.set(a.a + 1)
+b.print()
+for i in range(20):
+	pass
+print(i)
+if 20 > 30:
+	a="1"
+else:
+	a="2"
+if 0 < 4:
+	print(a+"0")
+else:
+	print(a+"1")
+a=[20,21,22,23,24]
+for i in a:
+	if i < 21:
+		continue
+	if i > 21:
+		break
+	print(i)
+b=[a,a,a]
+print(b[1][2])
+print(161//7)
+a=24
+while True:
+	try:
+		def gcheck():
+			global a
+			print(a)	
+		gcheck()
+		class c25():
+			x=25
+		x=c25()
+		print(x.x)
+		raise 
+	except:
+		print(26)
+	print(27+zero)
+	break
+print(28)
+k=29
+def f():
+	global k
+	k = yield k
+print(next(f()))
+while True:
+	k+= 1
+	if k < 30:
+		continue
+	break
+print(k)
+for i in [1,2,3]:
+	class A():
+		def __init__(self, c):
+			self.a = i+10*c
+	b = A(3)
+	print(b.a)
+print(34)
+p=0
+for i in range(35, -1, -1):
+	print(i)
+	p = p + 1
+	if p > 0:
+		break
+p=36
+while p == 36:
+	print(p)
+	p=37
+print(p)
+for i in [38]:
+	print(i)
+print(int(exec("def foo(): return 38") == None)+foo())
+d = {}
+exec("def bar(): return 40", d)
+print(d["bar"]())
+def fib2(n):
+	result = []
+	a, b = 0, 1
+	while a < n:
+		result.append(a)
+		a, b = b, a+b
+	return result
+print(fib2(100)[-2]-14)
+Answer={}
+Answer["ForAll"]=42
+print(Answer["ForAll"])
+i = 43
+def f(i=i):
+    print(i)
+i = 44
+f()
+print(i)
+while True:
+	try:
+		if None != True:
+			print(45)
+			break
+		else:
+			print(0)
+	except:
+		print(0)
+print(46)
+print(46+1)
+def u(p):
+	if p > 3:
+		return 3*p
+	else:
+		return u(2*p)-3*u(p)
+print(u(16))
+def u49():
+	return 49
+print(u49())