Fold subtraction of sums or differences from constants (#3295)

`C1 - (x + C2)`  ->  `(C1 - C2) - x`
`C1 - (x - C2)`  ->  `(C1 + C2) - x`
`C1 - (C2 - x)`  ->  `x + (C1 - C2)`

1 files changed, 33 insertions, 0 deletions

diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp
index 57ef42913..20cdeee19 100644
--- a/src/passes/OptimizeInstructions.cpp
+++ b/src/passes/OptimizeInstructions.cpp
@@ -1734,6 +1734,39 @@ private:
       return curr->left;
     }
     {
+      // C1 - (x + C2)  ==>  (C1 - C2) - x
+      Const *c1, *c2;
+      Expression* x;
+      if (matches(curr,
+                  binary(Sub, ival(&c1), binary(Add, any(&x), ival(&c2))))) {
+        left->value = c1->value.sub(c2->value);
+        curr->right = x;
+        return curr;
+      }
+      // C1 - (x - C2)  ==>  (C1 + C2) - x
+      if (matches(curr,
+                  binary(Sub, ival(&c1), binary(Sub, any(&x), ival(&c2))))) {
+        left->value = c1->value.add(c2->value);
+        curr->right = x;
+        return curr;
+      }
+      // C1 - (C2 - x)  ==>   x + (C1 - C2)
+      if (matches(curr,
+                  binary(Sub, ival(&c1), binary(Sub, ival(&c2), any(&x))))) {
+        left->value = c1->value.sub(c2->value);
+        if (left->value.isNegative()) {
+          // -C1 - (C2 - x)  ==>  x - (C1 - C2)
+          left->value = left->value.neg();
+          curr->op = Abstract::getBinary(type, Sub);
+        } else {
+          curr->op = Abstract::getBinary(type, Add);
+        }
+        curr->right = x;
+        std::swap(curr->left, curr->right);
+        return curr;
+      }
+    }
+    {
       // fval(C) / -x   ==>  -C / x
       Expression* right;
       if (matches(curr, binary(DivS, fval(), unary(Neg, any(&right))))) {