crypto: Add generic 64-bit carry-less multiply routine

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

crypto/clmul.c

@@ -92,3 +92,20 @@ uint64_t clmul_32(uint32_t n, uint32_t m32)
     }
     return r;
 }
+
+Int128 clmul_64_gen(uint64_t n, uint64_t m)
+{
+    uint64_t rl = 0, rh = 0;
+
+    /* Bit 0 can only influence the low 64-bit result.  */
+    if (n & 1) {
+        rl = m;
+    }
+
+    for (int i = 1; i < 64; ++i) {
+        uint64_t mask = -((n >> i) & 1);
+        rl ^= (m << i) & mask;
+        rh ^= (m >> (64 - i)) & mask;
+    }
+    return int128_make128(rl, rh);
+}