Add clz functions (Count number of Leading Zero bits), 32-and 64-bit variants

webrtc/common_audio/signal_processing/spl_inl.c

+/*
+ *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include <stdint.h>
+
+#include "webrtc/common_audio/signal_processing/include/spl_inl.h"
+
+// Table used by WebRtcSpl_CountLeadingZeros32_NotBuiltin. For each uint32_t n
+// that's a sequence of 0 bits followed by a sequence of 1 bits, the entry at
+// index (n * 0x8c0b2891) >> 26 in this table gives the number of zero bits in
+// n.
+const int8_t kWebRtcSpl_CountLeadingZeros32_Table[64] = {
+    32, 8,  17, -1, -1, 14, -1, -1, -1, 20, -1, -1, -1, 28, -1, 18,
+    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 0,  26, 25, 24,
+    4,  11, 23, 31, 3,  7,  10, 16, 22, 30, -1, -1, 2,  6,  13, 9,
+    -1, 15, -1, 21, -1, 29, 19, -1, -1, -1, -1, -1, 1,  27, 5,  12,

[tlegrand-webrtc, 2016/05/26 12:01:46]

I would prefer keeping it as in Patch Set 1, since this add memory usage for a
function that should mostly be used in tests, right?

[kwiberg-webrtc, 2016/05/26 12:17:59]

Yes, this global table adds 64 bytes to the program (about the same as a few
lines of code). But at each call site, the inlined code is ~50 instead of ~100
bytes (unless we use GCC or clang, in which case the inlined code is ~10 bytes
and doesn't use this table).

No, WebRtcSpl_CountLeadingZeros32_NotBuiltin isn't just used in tests (then what
would be the point?)---it's used wherever we don't use gcc or clang, such as on
Windows.

I could preprocessor-magick this table away if we're on GCC or clang, but it's
going to be ugly since we still want the tests. Frankly, I'd rather lose the 64
bytes...

+};