Clean up SkFloatBits

- remove dead code
- rewrite float -> int converters

The strategy for the new converters is:
   - convert input to double
   - floor/ceil/round in double space
   - pin that double to [SK_MinS32, SK_MaxS32]
   - truncate that double to int32_t

This simpler strategy does not work:
   - floor/ceil/round in float space
   - pin that float to [SK_MinS32, SK_MaxS32]
   - truncate that float to int32_t

SK_MinS32 and SK_MaxS32 are not representable as floats:
they round to the nearest float, ±2^31, which makes the
pin insufficient for floats near SK_MinS32 (-2^31+1) or
SK_MaxS32 (+2^31-1).

float only has 24 bits of precision, and we need 31.
double can represent all integers up to 50-something bits.

An alternative is to pin in float to ±2147483520, the last
exactly representable float before SK_MaxS32 (127 too small).

Our tests test that we round as floor(x+0.5), which can
return different numbers than round(x) for negative x.
So this CL explicitly uses floor(x+0.5).

I've updated the tests with ±inf and ±NaN, and tried to
make them a little clearer, especially using SK_MinS32
instead of -SK_MaxS32.

I have not timed anything here.  I have never seen any of these
methods in a profile.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2012333003

Review-Url: https://codereview.chromium.org/2012333003

1 files changed, 26 insertions, 0 deletions

diff --git a/bench/MathBench.cpp b/bench/MathBench.cpp
index 77ba2f04be..877d42663c 100644
--- a/bench/MathBench.cpp
+++ b/bench/MathBench.cpp
@@ -607,3 +607,29 @@ DEF_BENCH( return new CLZBench(true); )
 DEF_BENCH( return new NormalizeBench(); )
 
 DEF_BENCH( return new FixedMathBench(); )
+
+
+struct FloatToIntBench : public Benchmark {
+    enum { N = 1000000 };
+    float fFloats[N];
+    int   fInts  [N];
+
+    const char* onGetName() override { return "float_to_int"; }
+    bool isSuitableFor(Backend backend) override { return backend == kNonRendering_Backend; }
+
+    void onDelayedSetup() override  {
+        const auto f32 = 4294967296.0f;
+        for (int i = 0; i < N; ++i) {
+            fFloats[i] = -f32 + i*(2*f32/N);
+        }
+    }
+
+    void onDraw(int loops, SkCanvas*) override {
+        while (loops --> 0) {
+            for (int i = 0; i < N; i++) {
+                fInts[i] = SkFloatToIntFloor(fFloats[i]);
+            }
+        }
+    }
+};
+DEF_BENCH( return new FloatToIntBench; )