MIPS: MIPS32r1: Added optimizations for FastLog2

Functions VP8LFastLog2Slow and VP8LFastSLog2Slow

also: replaced some "% y" by "& (y-1)" in the C-version
(since y is a power-of-two)

Change-Id: I875170384e3c333812ca42d6ce7278aecabd60f0

src/dsp/lossless_mips32.c

@@ -0,0 +1,108 @@
+// Copyright 2014 Google Inc. All Rights Reserved.
+//
+// Use of this source code is governed by a BSD-style license
+// that can be found in the COPYING 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.
+// -----------------------------------------------------------------------------
+//
+// MIPS version of lossless functions
+//
+// Author(s):  Jovan Zelincevic (jovan.zelincevic@imgtec.com)
+
+#include "./dsp.h"
+#include "./lossless.h"
+
+#if defined(WEBP_USE_MIPS32)
+
+#include <math.h>
+#include <stdlib.h>
+#include <assert.h>
+
+#define APPROX_LOG_WITH_CORRECTION_MAX  65536
+#define APPROX_LOG_MAX                   4096
+#define LOG_2_RECIPROCAL 1.44269504088896338700465094007086
+
+static float FastSLog2SlowMIPS32(int v) {
+  assert(v >= LOG_LOOKUP_IDX_MAX);
+  if (v < APPROX_LOG_WITH_CORRECTION_MAX) {
+    int log_cnt, y, correction;
+    const int c24 = 24;
+    const float v_f = (float)v;
+    int temp;
+
+    // Xf = 256 = 2^8
+    // log_cnt is index of leading one in upper 24 bits
+    __asm__ volatile(
+      "clz      %[log_cnt], %[v]                      \n\t"
+      "addiu    %[y],       $zero,        1           \n\t"
+      "subu     %[log_cnt], %[c24],       %[log_cnt]  \n\t"
+      "sllv     %[y],       %[y],         %[log_cnt]  \n\t"
+      "srlv     %[temp],    %[v],         %[log_cnt]  \n\t"
+      : [log_cnt]"=&r"(log_cnt), [y]"=&r"(y),
+        [temp]"=r"(temp)
+      : [c24]"r"(c24), [v]"r"(v)
+    );
+
+    // vf = (2^log_cnt) * Xf; where y = 2^log_cnt and Xf < 256
+    // Xf = floor(Xf) * (1 + (v % y) / v)
+    // log2(Xf) = log2(floor(Xf)) + log2(1 + (v % y) / v)
+    // The correction factor: log(1 + d) ~ d; for very small d values, so
+    // log2(1 + (v % y) / v) ~ LOG_2_RECIPROCAL * (v % y)/v
+    // LOG_2_RECIPROCAL ~ 23/16
+
+    // (v % y) = (v % 2^log_cnt) = v & (2^log_cnt - 1)
+    correction = (23 * (v & (y - 1))) >> 4;
+    return v_f * (kLog2Table[temp] + log_cnt) + correction;
+  } else {
+    return (float)(LOG_2_RECIPROCAL * v * log((double)v));
+  }
+}
+
+static float FastLog2SlowMIPS32(int v) {
+  assert(v >= LOG_LOOKUP_IDX_MAX);
+  if (v < APPROX_LOG_WITH_CORRECTION_MAX) {
+    int log_cnt, y;
+    const int c24 = 24;
+    double log_2;
+    int temp;
+
+    __asm__ volatile(
+      "clz      %[log_cnt], %[v]                      \n\t"
+      "addiu    %[y],       $zero,        1           \n\t"
+      "subu     %[log_cnt], %[c24],       %[log_cnt]  \n\t"
+      "sllv     %[y],       %[y],         %[log_cnt]  \n\t"
+      "srlv     %[temp],    %[v],         %[log_cnt]  \n\t"
+      : [log_cnt]"=&r"(log_cnt), [y]"=&r"(y),
+        [temp]"=r"(temp)
+      : [c24]"r"(c24), [v]"r"(v)
+    );
+
+    log_2 = kLog2Table[temp] + log_cnt;
+    if (v >= APPROX_LOG_MAX) {
+      // Since the division is still expensive, add this correction factor only
+      // for large values of 'v'.
+
+      const int correction = (23 * (v & (y - 1))) >> 4;
+      log_2 += (double)correction / v;
+    }
+    return (float)log_2;
+  } else {
+    return (float)(LOG_2_RECIPROCAL * log((double)v));
+  }
+}
+
+#endif  // WEBP_USE_MIPS32
+
+//------------------------------------------------------------------------------
+// Entry point
+
+extern void VP8LDspInitMIPS32(void);
+
+void VP8LDspInitMIPS32(void) {
+#if defined(WEBP_USE_MIPS32)
+  VP8LFastSLog2Slow = FastSLog2SlowMIPS32;
+  VP8LFastLog2Slow = FastLog2SlowMIPS32;
+#endif  // WEBP_USE_MIPS32
+}