2 files changed, 124 insertions, 0 deletions

diff --git a/src/ast/bits.h b/src/ast/bits.h
new file mode 100644
index 000000000..d88cb5edb
--- /dev/null
+++ b/src/ast/bits.h
@@ -0,0 +1,47 @@
+/*
+ * Copyright 2017 WebAssembly Community Group participants
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef wasm_ast_bits_h
+#define wasm_ast_bits_h
+
+#include "support/bits.h"
+
+namespace wasm {
+
+struct Bits {
+  // get a mask to keep only the low # of bits
+  static int32_t lowBitMask(int32_t bits) {
+    uint32_t ret = -1;
+    if (bits >= 32) return ret;
+    return ret >> (32 - bits);
+  }
+
+  // checks if the input is a mask of lower bits, i.e., all 1s up to some high bit, and all zeros
+  // from there. returns the number of masked bits, or 0 if this is not such a mask
+  static uint32_t getMaskedBits(uint32_t mask) {
+    if (mask == uint32_t(-1)) return 32; // all the bits
+    if (mask == 0) return 0; // trivially not a mask
+    // otherwise, see if adding one turns this into a 1-bit thing, 00011111 + 1 => 00100000
+    if (PopCount(mask + 1) != 1) return 0;
+    // this is indeed a mask
+    return 32 - CountLeadingZeroes(mask);
+  }
+};
+
+} // namespace wasm
+
+#endif // wasm_ast_bits_h
+
diff --git a/src/ast/properties.h b/src/ast/properties.h
index 9834c73d0..9121deba5 100644
--- a/src/ast/properties.h
+++ b/src/ast/properties.h
@@ -18,6 +18,7 @@
 #define wasm_ast_properties_h
 
 #include "wasm.h"
+#include "ast/bits.h"
 
 namespace wasm {
 
@@ -52,6 +53,82 @@ struct Properties {
       default: return false;
     }
   }
+
+  // Check if an expression is a sign-extend, and if so, returns the value
+  // that is extended, otherwise nullptr
+  static Expression* getSignExtValue(Expression* curr) {
+    if (auto* outer = curr->dynCast<Binary>()) {
+      if (outer->op == ShrSInt32) {
+        if (auto* outerConst = outer->right->dynCast<Const>()) {
+          if (auto* inner = outer->left->dynCast<Binary>()) {
+            if (inner->op == ShlInt32) {
+              if (auto* innerConst = inner->right->dynCast<Const>()) {
+                if (outerConst->value == innerConst->value) {
+                  return inner->left;
+                }
+              }
+            }
+          }
+        }
+      }
+    }
+    return nullptr;
+  }
+
+  // gets the size of the sign-extended value
+  static Index getSignExtBits(Expression* curr) {
+    return 32 - curr->cast<Binary>()->right->cast<Const>()->value.geti32();
+  }
+
+  // Check if an expression is almost a sign-extend: perhaps the inner shift
+  // is too large. We can split the shifts in that case, which is sometimes
+  // useful (e.g. if we can remove the signext)
+  static Expression* getAlmostSignExt(Expression* curr) {
+    if (auto* outer = curr->dynCast<Binary>()) {
+      if (outer->op == ShrSInt32) {
+        if (auto* outerConst = outer->right->dynCast<Const>()) {
+          if (auto* inner = outer->left->dynCast<Binary>()) {
+            if (inner->op == ShlInt32) {
+              if (auto* innerConst = inner->right->dynCast<Const>()) {
+                if (outerConst->value.leU(innerConst->value).geti32()) {
+                  return inner->left;
+                }
+              }
+            }
+          }
+        }
+      }
+    }
+    return nullptr;
+  }
+
+  // gets the size of the almost sign-extended value, as well as the
+  // extra shifts, if any
+  static Index getAlmostSignExtBits(Expression* curr, Index& extraShifts) {
+    extraShifts = curr->cast<Binary>()->left->cast<Binary>()->right->cast<Const>()->value.geti32() -
+                  curr->cast<Binary>()->right->cast<Const>()->value.geti32();
+    return getSignExtBits(curr);
+  }
+
+  // Check if an expression is a zero-extend, and if so, returns the value
+  // that is extended, otherwise nullptr
+  static Expression* getZeroExtValue(Expression* curr) {
+    if (auto* binary = curr->dynCast<Binary>()) {
+      if (binary->op == AndInt32) {
+        if (auto* c = binary->right->dynCast<Const>()) {
+          if (Bits::getMaskedBits(c->value.geti32())) {
+            return binary->right;
+          }
+        }
+      }
+    }
+    return nullptr;
+  }
+
+  // gets the size of the sign-extended value
+  static Index getZeroExtBits(Expression* curr) {
+    return Bits::getMaskedBits(curr->cast<Binary>()->right->cast<Const>()->value.geti32());
+  }
 };
 
 } // wasm