1 files changed, 47 insertions, 0 deletions
diff --git a/src/ast/bits.h b/src/ast/bits.h
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--- /dev/null
+++ b/src/ast/bits.h
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+/*
+ * 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
+