3 files changed, 71 insertions, 5 deletions

diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp
index 1e51adea9..85ab2a168 100644
--- a/src/passes/OptimizeInstructions.cpp
+++ b/src/passes/OptimizeInstructions.cpp
@@ -606,6 +606,18 @@ struct OptimizeInstructions
             }
           }
         }
+        if (binary->op == DivFloat32) {
+          float c = right->value.getf32();
+          if (Bits::isPowerOf2Float(c)) {
+            return optimizePowerOf2FDiv(binary, c);
+          }
+        }
+        if (binary->op == DivFloat64) {
+          double c = right->value.getf64();
+          if (Bits::isPowerOf2Float(c)) {
+            return optimizePowerOf2FDiv(binary, c);
+          }
+        }
       }
       // a bunch of operations on a constant left side can be simplified
       if (binary->left->is<Const>()) {
@@ -1260,6 +1272,15 @@ private:
   // but it's still worth doing since
   //  * Usually ands are more common than urems.
   //  * The constant is slightly smaller.
+  template<typename T> Expression* optimizePowerOf2URem(Binary* binary, T c) {
+    static_assert(std::is_same<T, uint32_t>::value ||
+                    std::is_same<T, uint64_t>::value,
+                  "type mismatch");
+    binary->op = std::is_same<T, uint32_t>::value ? AndInt32 : AndInt64;
+    binary->right->cast<Const>()->value = Literal(c - 1);
+    return binary;
+  }
+
   template<typename T> Expression* optimizePowerOf2UDiv(Binary* binary, T c) {
     static_assert(std::is_same<T, uint32_t>::value ||
                     std::is_same<T, uint64_t>::value,
@@ -1270,12 +1291,30 @@ private:
     return binary;
   }
 
-  template<typename T> Expression* optimizePowerOf2URem(Binary* binary, T c) {
-    static_assert(std::is_same<T, uint32_t>::value ||
-                    std::is_same<T, uint64_t>::value,
+  template<typename T> Expression* optimizePowerOf2FDiv(Binary* binary, T c) {
+    //
+    // x / C_pot    =>   x * (C_pot ^ -1)
+    //
+    // Explanation:
+    // Floating point numbers are represented as:
+    //    ((-1) ^ sign) * (2 ^ (exp - bias)) * (1 + significand)
+    //
+    // If we have power of two numbers, then the mantissa (significand)
+    // is all zeros. Let's focus on the exponent, ignoring the sign part:
+    //    (2 ^ (exp - bias))
+    //
+    // and for inverted power of two floating point:
+    //     1.0 / (2 ^ (exp - bias))   ->   2 ^ -(exp - bias)
+    //
+    // So inversion of C_pot is valid because it changes only the sign
+    // of the exponent part and doesn't touch the significand part,
+    // which remains the same (zeros).
+    static_assert(std::is_same<T, float>::value ||
+                    std::is_same<T, double>::value,
                   "type mismatch");
-    binary->op = std::is_same<T, uint32_t>::value ? AndInt32 : AndInt64;
-    binary->right->cast<Const>()->value = Literal(c - 1);
+    double invDivisor = 1.0 / (double)c;
+    binary->op = std::is_same<T, float>::value ? MulFloat32 : MulFloat64;
+    binary->right->cast<Const>()->value = Literal(static_cast<T>(invDivisor));
     return binary;
   }
 
diff --git a/src/support/bits.cpp b/src/support/bits.cpp
index 03fb66252..8a81a7e83 100644
--- a/src/support/bits.cpp
+++ b/src/support/bits.cpp
@@ -157,6 +157,30 @@ int ceilLog2(uint32_t v) { return 32 - countLeadingZeroes(v - 1); }
 
 int ceilLog2(uint64_t v) { return 64 - countLeadingZeroes(v - 1); }
 
+bool isPowerOf2Float(float v) {
+  // Power of two floating points should have zero as their significands,
+  // so here we just mask the exponent range of "v" and compare it with the
+  // unmasked input value. If they are equal, our value is a power of
+  // two. Also, we reject all values which are less than the minimal possible
+  // power of two or greater than the maximum possible power of two.
+  const uint32_t MIN_POT = 0x01U << 23;  // 0x1p-126
+  const uint32_t MAX_POT = 0xFDU << 23;  // 0x1p+126
+  const uint32_t EXP_MASK = 0xFFU << 23; // mask only exponent
+  const uint32_t SIGN_MASK = ~0U >> 1;   // mask everything except sign
+  auto u = bit_cast<uint32_t>(v) & SIGN_MASK;
+  return u >= MIN_POT && u <= MAX_POT && (u & EXP_MASK) == u;
+}
+
+bool isPowerOf2Float(double v) {
+  // See isPowerOf2Float(float)
+  const uint64_t MIN_POT = 0x001ULL << 52;  // 0x1p-1022
+  const uint64_t MAX_POT = 0x7FDULL << 52;  // 0x1p+1022
+  const uint64_t EXP_MASK = 0x7FFULL << 52; // mask only exponent
+  const uint64_t SIGN_MASK = ~0ULL >> 1;    // mask everything except sign
+  auto u = bit_cast<uint64_t>(v) & SIGN_MASK;
+  return u >= MIN_POT && u <= MAX_POT && (u & EXP_MASK) == u;
+}
+
 uint32_t log2(uint32_t v) {
   if (!isPowerOf2(v)) {
     WASM_UNREACHABLE("value should be a power of two");
diff --git a/src/support/bits.h b/src/support/bits.h
index 3231e1c96..333eb5cc2 100644
--- a/src/support/bits.h
+++ b/src/support/bits.h
@@ -77,6 +77,9 @@ template<typename T> bool isPowerOf2(T v) {
   return v != 0 && (v & (v - 1)) == 0;
 }
 
+bool isPowerOf2Float(float);
+bool isPowerOf2Float(double);
+
 template<typename T, typename U> inline static T rotateLeft(T val, U count) {
   auto value = typename std::make_unsigned<T>::type(val);
   U mask = sizeof(T) * CHAR_BIT - 1;