Add test for QuickFDot6Div

This test will catch our (1 << 10) bug (which should be 1 << 9)

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4986

Change-Id: I25b607d1535a647284cee3b304a6f567f389e7f6
Reviewed-on: https://skia-review.googlesource.com/4986
Reviewed-by: Cary Clark <caryclark@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>

1 files changed, 37 insertions, 0 deletions

diff --git a/src/core/SkFDot6.h b/src/core/SkFDot6.h
index 726aa2e46d..b9a5c2a12d 100644
--- a/src/core/SkFDot6.h
+++ b/src/core/SkFDot6.h
@@ -75,4 +75,41 @@ inline SkFixed SkFDot6Div(SkFDot6 a, SkFDot6 b) {
     }
 }
 
+#include "SkFDot6Constants.h"
+
+class QuickFDot6Inverse {
+private:
+    static constexpr const SkFDot6* table = gFDot6INVERSE + kInverseTableSize;
+public:
+    inline static SkFixed Lookup(SkFDot6 x) {
+        SkASSERT(SkAbs32(x) < kInverseTableSize);
+        return table[x];
+    }
+};
+
+static inline SkFixed QuickSkFDot6Div(SkFDot6 a, SkFDot6 b) {
+    const int kMinBits = 3;  // abs(b) should be at least (1 << kMinBits) for quick division
+    const int kMaxBits = 31; // Number of bits available in signed int
+    // Given abs(b) <= (1 << kMinBits), the inverse of abs(b) is at most 1 << (22 - kMinBits) in
+    // SkFixed format. Hence abs(a) should be less than kMaxAbsA
+    const int kMaxAbsA = 1 << (kMaxBits - (22 - kMinBits));
+    SkFDot6 abs_a = SkAbs32(a);
+    SkFDot6 abs_b = SkAbs32(b);
+    if (abs_b >= (1 << kMinBits) && abs_b < kInverseTableSize && abs_a < kMaxAbsA) {
+        SkASSERT((int64_t)a * QuickFDot6Inverse::Lookup(b) <= SK_MaxS32
+                && (int64_t)a * QuickFDot6Inverse::Lookup(b) >= SK_MinS32);
+        SkFixed ourAnswer = (a * QuickFDot6Inverse::Lookup(b)) >> 6;
+        #ifdef SK_DEBUG
+        SkFixed directAnswer = SkFDot6Div(a, b);
+        SkASSERT(
+            (directAnswer == 0 && ourAnswer == 0) ||
+            SkFixedDiv(SkAbs32(directAnswer - ourAnswer), SkAbs32(directAnswer)) <= 1 << 10
+        );
+        #endif
+        return ourAnswer;
+    } else {
+        return SkFDot6Div(a, b);
+    }
+}
+
 #endif