introduce Sheng-McClellan hybrid

src/util/bitutils.h

@@ -471,4 +471,55 @@ u32 rank_in_mask64(u64a mask, u32 bit) {
     return popcount64(mask);
 }
 
+#if defined(__BMI2__) || (defined(_WIN32) && defined(__AVX2__))
+#define HAVE_PEXT
+#endif
+
+static really_inline
+u32 pext32(u32 x, u32 mask) {
+#if defined(HAVE_PEXT)
+    // Intel BMI2 can do this operation in one instruction.
+    return _pext_u32(x, mask);
+#else
+
+    u32 result = 0, num = 1;
+    while (mask != 0) {
+        u32 bit = findAndClearLSB_32(&mask);
+        if (x & (1U << bit)) {
+            assert(num != 0); // more than 32 bits!
+            result |= num;
+        }
+        num <<= 1;
+    }
+    return result;
+#endif
+}
+
+static really_inline
+u64a pext64(u64a x, u64a mask) {
+#if defined(HAVE_PEXT) && defined(ARCH_64_BIT)
+    // Intel BMI2 can do this operation in one instruction.
+    return _pext_u64(x, mask);
+#else
+
+    u32 result = 0, num = 1;
+    while (mask != 0) {
+        u32 bit = findAndClearLSB_64(&mask);
+        if (x & (1ULL << bit)) {
+            assert(num != 0); // more than 32 bits!
+            result |= num;
+        }
+        num <<= 1;
+    }
+    return result;
+#endif
+}
+
+#if defined(HAVE_PEXT) && defined(ARCH_64_BIT)
+static really_inline
+u64a pdep64(u64a x, u64a mask) {
+    return _pdep_u64(x, mask);
+}
+#endif
+
 #endif // BITUTILS_H