Add a bench to measure the best way to pack from int to uint16_t with SSE.

bench/pack_int_uint16_t_Bench.cpp

Index: bench/pack_int_uint16_t_Bench.cpp
diff --git a/bench/pack_int_uint16_t_Bench.cpp b/bench/pack_int_uint16_t_Bench.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..5e1527d7ca25c2cc6a761ddf7801be0293bfe134
--- /dev/null
+++ b/bench/pack_int_uint16_t_Bench.cpp
@@ -0,0 +1,93 @@
+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "Benchmark.h"
+#include "SkTypes.h"
+
+/**
+ * There's a good variety of ways to pack from int down to uint16_t with SSE,
+ * depending on the specific instructions available.
+ *
+ * SSE2 offers an int -> int16_t pack instruction.  We can use this in two ways:
+ *    - subtract off 32768, int -> int16_t, add 32768 back                                  (sse2_a)
+ *    - first artificially sign extend the (positive) value in our int, then int -> int16_t (sse2_b)
+ * SSSE3 adds a byte shuffle, so we just put the bytes where we want them.                  (ssse3)
+ * SSE41 added an int -> uint16_t pack instruction.                                         (sse41)
+ *
+ * Findings so far:
+ *   - sse41 < ssse3 <<< sse2_b < sse2_a;
+ *   - the ssse3 version is only slightly slower than the sse41 version, maybe not at all
+ *   - the sse2_a is only slightly slower than the sse2_b version
+ *   - the ssse3 and sse41 versions are about 3x faster than either sse2 version
+ *   - the sse41 version seems to cause some code generation trouble.
+ */
+
+#if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSE2
+
+#include <immintrin.h>
+
+template <__m128i (kernel)(__m128i)>
+class pack_int_uint16_t_Bench : public Benchmark {
+public:
+    pack_int_uint16_t_Bench(const char* impl) {
+        fName.append("pack_int_uint16_t_");
+        fName.append(impl);
+    }
+
+    bool isSuitableFor(Backend backend) override { return backend == kNonRendering_Backend; }
+    const char* onGetName() override { return fName.c_str(); }
+
+    void onDraw(int loops, SkCanvas*) override {
+        __m128i x = _mm_set1_epi32(0x42424242);
+        while (loops --> 0) {
+            x = kernel(x);
+        }
+
+        volatile int blackhole = 0;
+        blackhole ^= _mm_cvtsi128_si32(x);
+    }
+
+    SkString fName;
+};
+
+namespace {
+    __m128i sse2_a(__m128i x) {
+        x = _mm_sub_epi32(x, _mm_set1_epi32(0x8000));
+        return _mm_add_epi16(_mm_packs_epi32(x,x), _mm_set1_epi16((short)0x8000));
+    }
+}
+DEF_BENCH( return new pack_int_uint16_t_Bench<sse2_a>("sse2_a"); )
+
+namespace {
+    __m128i sse2_b(__m128i x) {
+        x = _mm_srai_epi32(_mm_slli_epi32(x, 16), 16);
+        return _mm_packs_epi32(x,x);
+    }
+}
+DEF_BENCH( return new pack_int_uint16_t_Bench<sse2_b>("sse2_b"); )
+
+#if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSSE3
+namespace {
+    __m128i ssse3(__m128i x) {
+        // TODO: Can we force the bench to load the mask inside the loop?  Would be more realistic.
+        const int _ = ~0;
+        return _mm_shuffle_epi8(x, _mm_setr_epi8(0,1, 4,5, 8,9, 12,13, _,_,_,_,_,_,_,_));
+    }
+}
+DEF_BENCH( return new pack_int_uint16_t_Bench<ssse3>("ssse3"); )
+#endif
+
+#if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSE41
+namespace {
+    __m128i sse41(__m128i x) {
+        return _mm_packus_epi32(x,x);
+    }
+}
+DEF_BENCH( return new pack_int_uint16_t_Bench<sse41>("sse41"); )
+#endif
+
+#endif  // SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSE2