Sk4f: add floor()

src/opts/SkNx_sse.h

Index: src/opts/SkNx_sse.h
diff --git a/src/opts/SkNx_sse.h b/src/opts/SkNx_sse.h
index 889253b9d8cfff1fbb56266863386190db3fec14..def7ba17191d9225f195bace7422a6d584c561a5 100644
--- a/src/opts/SkNx_sse.h
+++ b/src/opts/SkNx_sse.h
@@ -13,6 +13,17 @@
 
 #define SKNX_IS_FAST
 
+// SSE 4.1 has _mm_floor_ps to floor 4 floats.  We emulate it:
+//   - round by adding (1<<23) with our sign, then subtracting it;
+//   - if that rounded value is bigger than our input, subtract 1.
+static inline __m128 sse2_mm_floor_ps(__m128 v) {
+    __m128 sign = _mm_and_ps(v, _mm_set1_ps(-0.0f));
+    __m128 bias = _mm_or_ps(sign, _mm_set1_ps(1<<23));
+    __m128 rounded = _mm_sub_ps(_mm_add_ps(v, bias), bias);
+    __m128 too_big = _mm_cmpgt_ps(rounded, v);
+    return _mm_sub_ps(rounded, _mm_and_ps(too_big, _mm_set1_ps(1.0f)));
+}
+
 template <>
 class SkNx<2, float> {
 public:
@@ -92,6 +103,7 @@ public:
     static SkNx Max(const SkNx& l, const SkNx& r) { return _mm_max_ps(l.fVec, r.fVec); }
 
     SkNx abs() const { return _mm_andnot_ps(_mm_set1_ps(-0.0f), fVec); }
+    SkNx floor() const { return sse2_mm_floor_ps(fVec); }
 
     SkNx  sqrt () const { return _mm_sqrt_ps (fVec);  }
     SkNx rsqrt0() const { return _mm_rsqrt_ps(fVec); }