Optimize negative one on LHS for some shift operations (#3292)

1 files changed, 13 insertions, 2 deletions

diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp
index d019a983f..57ef42913 100644
--- a/src/passes/OptimizeInstructions.cpp
+++ b/src/passes/OptimizeInstructions.cpp
@@ -764,7 +764,7 @@ struct OptimizeInstructions
           }
           // abs(0 - x)   ==>   abs(x),
           // only for fast math
-          if (getPassOptions().fastMath &&
+          if (fastMath &&
               binary->op == Abstract::getBinary(binary->type, Abstract::Sub)) {
             if (auto* c = binary->left->dynCast<Const>()) {
               if (c->value.isZero()) {
@@ -1716,12 +1716,23 @@ private:
     using namespace Match;
     using namespace Abstract;
 
+    auto type = curr->left->type;
     auto* left = curr->left->cast<Const>();
-    // 0 <<>> y   ==>   0
+    // 0 <<>> x   ==>   0
     if (Abstract::hasAnyShift(curr->op) && left->value.isZero() &&
         !effects(curr->right).hasSideEffects()) {
       return curr->left;
     }
+    // (signed)-1 >> x   ==>   -1
+    // rotl(-1, x)       ==>   -1
+    // rotr(-1, x)       ==>   -1
+    if ((curr->op == Abstract::getBinary(type, ShrS) ||
+         curr->op == Abstract::getBinary(type, RotL) ||
+         curr->op == Abstract::getBinary(type, RotR)) &&
+        left->value.getInteger() == -1LL &&
+        !effects(curr->right).hasSideEffects()) {
+      return curr->left;
+    }
     {
       // fval(C) / -x   ==>  -C / x
       Expression* right;