add benchmarks for bilerp study

Change-Id: I6e69a902cc660154b56a82cae727751ec6f5e806
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/448273
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>

experimental/lowp-basic/CMakeLists.txt

@@ -6,3 +6,5 @@ set(CMAKE_CXX_STANDARD 17)
 add_executable(lowp lowp_experiments.cpp)
 
 add_executable(lerp lerp-study.cpp)
+
+add_executable(bilerp bilerp-study.cpp)

experimental/lowp-basic/bilerp-study.cpp

@@ -0,0 +1,98 @@
+/*
+ * Copyright 2021 Google LLC
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include <algorithm>
+#include <cmath>
+#include <cstdio>
+#include <cstdint>
+
+#include "QMath.h"
+
+struct Stats {
+    int64_t diff_8_bits = 0;
+    int64_t max_diff = 0;
+    int64_t min_diff = 0;
+    int64_t total = 0;
+
+    void log(int16_t golden, int16_t candidate) {
+        int64_t diff = candidate - golden;
+        max_diff = std::max(max_diff, diff);
+        min_diff = std::min(min_diff, diff);
+        diff_8_bits += candidate != golden;
+        total++;
+    }
+
+    void print() const {
+        printf("8-bit diff: %lld - %g%%\n", diff_8_bits, 100.0 * diff_8_bits / total);
+        printf("differences min: %lld max: %lld\n", min_diff, max_diff);
+        printf("total: %lld\n", total);
+    }
+};
+
+// This has all kinds of rounding issues.
+// TODO(herb): figure out rounding problems with this code.
+static float golden_bilerp(float tx, float ty, int16_t p00, int16_t p10, int16_t p01, int16_t p11) {
+    return (1.0f-tx) * (1.0f-ty) * p00
+         + (1.0f-tx) * ty * p01
+         + (1.0f-ty) * tx * p10
+         + tx * ty * p11;
+}
+
+static double golden_bilerp2(
+        float tx, float ty, int16_t p00, int16_t p10, int16_t p01, int16_t p11) {
+    // Double is needed to avoid rounding of lower bits.
+    double dtx(tx), dty(ty);
+
+    double top = (1.0 - dtx) * p00 + dtx * p10;
+    double bottom = (1.0 - dtx) * p01 + dtx * p11;
+
+    return (1.0 - dty) * top + dty * bottom;
+}
+
+static int16_t full_res_bilerp(
+        float tx, float ty, int16_t p00, int16_t p10, int16_t p01, int16_t p11) {
+    int32_t ftx(floor(tx * 65536.0f + 0.5f));
+    int64_t top = ftx * (p10 - p00) + 65536 * p00;
+    int64_t bottom = ftx * (p11 - p01) + 65536 * p01;
+
+    int64_t fty(floor(ty * 65536.0f + 0.5f));
+    int64_t temp = fty * (bottom - top) + top * 65536LL;
+    int64_t rounded = temp + (1LL << 31);
+    return rounded >> 32;
+}
+
+template <typename Bilerp>
+static Stats check_bilerp(Bilerp bilerp) {
+    Stats stats;
+    const int step = 1;
+    for (float tx : {0.0f, 0.25f, 0.5f, 0.75f, 1.0f - 1.0f/65536.0f})
+    for (float ty : {0.0f, 0.25f, 0.5f, 0.75f, 1.0f - 1.0f/65536.0f})
+    for (int p00 = 0; p00 < 256; p00 += step)
+    for (int p01 = 0; p01 < 256; p01 += step)
+    for (int p10 = 0; p10 < 256; p10 += step)
+    for (int p11 = 0; p11 < 256; p11 += step) {
+        // Having this be double causes the proper rounding.
+        double l = golden_bilerp2(tx, ty, p00, p10, p01, p11);
+        int16_t golden = floor(l + 0.5);
+        l = golden_bilerp(tx, ty, p00, p10, p01, p11);
+        //int16_t golden2 = floor(l + 0.5f);
+        int16_t candidate = bilerp(tx, ty, p00, p10, p01, p11);
+        stats.log(golden, candidate);
+    }
+    return stats;
+}
+
+int main() {
+    Stats stats;
+
+    printf("\nUsing full_res_bilerp...\n");
+    stats = check_bilerp(full_res_bilerp);
+    stats.print();
+
+    printf("Done.\n");
+    return 0;
+}