From cb9e7d15bf2a23d551702504ef4610883cf70c9b Mon Sep 17 00:00:00 2001
From: whitequark <whitequark@whitequark.org>
Date: Fri, 20 Nov 2015 22:15:03 +0800
Subject: [PATCH] compiler.iodelay: subtraction certainly shouldn't be
 commutative.

---
 artiq/compiler/iodelay.py | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/artiq/compiler/iodelay.py b/artiq/compiler/iodelay.py
index ec9a73729..47e04472a 100644
--- a/artiq/compiler/iodelay.py
+++ b/artiq/compiler/iodelay.py
@@ -158,18 +158,23 @@ class Add(BinOpFixpoint):
     _op       = lambda a, b: a + b
     _fixpoint = 0
 
-class Sub(BinOpFixpoint):
-    _priority = 2
-    _symbol   = "-"
-    _op       = lambda a, b: a - b
-    _fixpoint = 0
-
 class Mul(BinOpFixpoint):
     _priority = 1
     _symbol   = "*"
     _op       = lambda a, b: a * b
     _fixpoint = 1
 
+class Sub(BinOp):
+    _priority = 2
+    _symbol   = "-"
+    _op       = lambda a, b: a - b
+
+    def _fold_binop(self, lhs, rhs):
+        if isinstance(rhs, Const) and rhs.value == 0:
+            return lhs
+        else:
+            return super()._fold_binop(lhs, rhs)
+
 class Div(BinOp):
     def _fold_binop(self, lhs, rhs):
         if isinstance(rhs, Const) and rhs.value == 1: