Remove redundant assignments with nested ifs (closes hylang#842)

hy/compiler.py

@@ -375,6 +375,7 @@ def __init__(self, module_name):
         self.anon_var_count = 0
         self.imports = defaultdict(set)
         self.module_name = module_name
+        self.temp_if = None
         if not module_name.startswith("hy.core"):
             # everything in core needs to be explicit.
             load_stdlib()
@@ -998,22 +999,30 @@ def _compile_catch_expression(self, expr, var):
 
     @builds("if")
     @checkargs(min=2, max=3)
-    def compile_if(self, expression):
+    def compile_if(self, expression, temp=None):
         expression.pop(0)
         cond = self.compile(expression.pop(0))
 
         body = self.compile(expression.pop(0))
         orel = Result()
+        nested = root = False
         if expression:
-            orel = self.compile(expression.pop(0))
+            orel_expr = expression.pop(0)
+            if isinstance(orel_expr, HyExpression) and isinstance(orel_expr[0],
+               HySymbol) and orel_expr[0] == 'if':
+                # Nested ifs: don't waste temporaries
+                root = self.temp_if is None
+                nested = True
+                self.temp_if = self.temp_if or self.get_anon_var()
+            orel = self.compile(orel_expr)
 
         # We want to hoist the statements from the condition
         ret = cond
 
         if body.stmts or orel.stmts:
             # We have statements in our bodies
             # Get a temporary variable for the result storage
-            var = self.get_anon_var()
+            var = self.temp_if or self.get_anon_var()
             name = ast.Name(id=ast_str(var), arg=ast_str(var),
                             ctx=ast.Store(),
                             lineno=expression.start_line,
@@ -1026,10 +1035,12 @@ def compile_if(self, expression):
                                col_offset=expression.start_column)
 
             # and of the else clause
-            orel += ast.Assign(targets=[name],
-                               value=orel.force_expr,
-                               lineno=expression.start_line,
-                               col_offset=expression.start_column)
+            if not nested or not orel.stmts or (not root and
+               var != self.temp_if):
+                orel += ast.Assign(targets=[name],
+                                   value=orel.force_expr,
+                                   lineno=expression.start_line,
+                                   col_offset=expression.start_column)
 
             # Then build the if
             ret += ast.If(test=ret.force_expr,
@@ -1052,6 +1063,10 @@ def compile_if(self, expression):
                              orelse=orel.force_expr,
                              lineno=expression.start_line,
                              col_offset=expression.start_column)
+
+        if root:
+            self.temp_if = None
+
         return ret
 
     @builds("break")