exact divide by 255 with NEON

Change-Id: Ib121eb0d5af1f22f48f517fe909112a77d92032e
Reviewed-on: https://skia-review.googlesource.com/113666
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>

2 files changed, 25 insertions, 0 deletions

diff --git a/src/opts/SkRasterPipeline_opts.h b/src/opts/SkRasterPipeline_opts.h
index 212a5d16aa..73624d1b02 100644
--- a/src/opts/SkRasterPipeline_opts.h
+++ b/src/opts/SkRasterPipeline_opts.h
@@ -2487,6 +2487,11 @@ static void start_pipeline(const size_t x0,     const size_t y0,
 SI U16 div255(U16 v) {
 #if 0
     return (v+127)/255;  // The ideal rounding divide by 255.
+#elif 1 && defined(__ARM_NEON)
+    // With NEON we can compute (v+127)/255 as (v + ((v+128)>>8) + 128)>>8
+    // just as fast as we can do the approximation below, so might as well be correct!
+    // First we compute v + ((v+128)>>8), then one more round of (...+128)>>8 to finish up.
+    return vrshrq_n_u16(vrsraq_n_u16(v, v, 8), 8);
 #else
     return (v+255)/256;  // A good approximation of (v+127)/255.
 #endif
diff --git a/tests/MathTest.cpp b/tests/MathTest.cpp
index 94b15207f7..565f76f819 100644
--- a/tests/MathTest.cpp
+++ b/tests/MathTest.cpp
@@ -750,3 +750,23 @@ DEF_TEST(DoubleSaturate32, reporter) {
         REPORTER_ASSERT(reporter, r.fExpectedInt == i);
     }
 }
+
+#if defined(__ARM_NEON)
+    #include <arm_neon.h>
+
+    DEF_TEST(NeonU16Div255, r) {
+
+        for (int v = 0; v <= 255*255; v++) {
+            int want = (v + 127)/255;
+
+            uint16x8_t V = vdupq_n_u16(v);
+            int got = vrshrq_n_u16(vrsraq_n_u16(V, V, 8), 8)[0];
+
+            if (got != want) {
+                SkDebugf("%d -> %d, want %d\n", v, got, want);
+            }
+            REPORTER_ASSERT(r, got == want);
+        }
+    }
+
+#endif