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diff --git a/runtime/c/i64_smulh.c b/runtime/c/i64_smulh.c
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+/*****************************************************************
+ *
+ *               The Compcert verified compiler
+ *
+ *           Xavier Leroy, INRIA Paris-Rocquencourt
+ *
+ * Copyright (c) 2013 Institut National de Recherche en Informatique et
+ *  en Automatique.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name of the <organization> nor the
+ *       names of its contributors may be used to endorse or promote products
+ *       derived from this software without specific prior written permission.
+ * 
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL <COPYRIGHT
+ * HOLDER> BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+ * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+ * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ **********************************************************************/
+
+/* Helper functions for 64-bit integer arithmetic. Reference C implementation */
+
+#include "i64.h"
+
+typedef signed long long s64;
+typedef unsigned long long u64;
+
+/* Signed multiply high */
+
+/* Hacker's Delight section 8.3:
+ * - compute high 64 bits of the unsigned product X * Y
+ * - subtract X if Y < 0
+ * - subtract Y if X < 0 
+ */
+
+s64 __i64_smulh(s64 x, s64 y)
+{
+  s64 t = (s64) __i64_umulh(x, y);
+  if (y < 0) t = t - x;
+  if (x < 0) t = t - y;
+  return t;
+}