ARM Skia NEON patches - 18 - Preparation work for BitmapProcState

src/core/SkBitmapProcState_utils.h

+#ifndef SkBitmapProcState_utils_DEFINED
+#define SkBitmapProcState_utils_DEFINED
+
+// Helper to ensure that when we shift down, we do it w/o sign-extension
+// so the caller doesn't have to manually mask off the top 16 bits
+//
+static unsigned SK_USHIFT16(unsigned x) {
+    return x >> 16;
+}
+
+/*
+ *  The decal_ functions require that
+ *  1. dx > 0
+ *  2. [fx, fx+dx, fx+2dx, fx+3dx, ... fx+(count-1)dx] are all <= maxX
+ *
+ *  In addition, we use SkFractionalInt to keep more fractional precision than
+ *  just SkFixed, so we will abort the decal_ call if dx is very small, since
+ *  the decal_ function just operates on SkFixed. If that were changed, we could
+ *  skip the very_small test here.
+ */
+static inline bool can_truncate_to_fixed_for_decal(SkFractionalInt frX,
+                                                   SkFractionalInt frDx,
+                                                   int count, unsigned max) {
+    SkFixed dx = SkFractionalIntToFixed(frDx);
+
+    // if decal_ kept SkFractionalInt precision, this would just be dx <= 0
+    // I just made up the 1/256. Just don't want to perceive accumulated error
+    // if we truncate frDx and lose its low bits.
+    if (dx <= SK_Fixed1 / 256) {
+        return false;
+    }
+
+    // We cast to unsigned so we don't have to check for negative values, which
+    // will now appear as very large positive values, and thus fail our test!
+    SkFixed fx = SkFractionalIntToFixed(frX);
+    return (unsigned)SkFixedFloorToInt(fx) <= max &&
+           (unsigned)SkFixedFloorToInt(fx + dx * (count - 1)) < max;
+}
+
+#endif /* #ifndef SkBitmapProcState_utils_DEFINED */