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, 10 insertions, 32 deletions

diff --git a/tests/MathTest.cpp b/tests/MathTest.cpp
index ec1f936b9d..23785f742d 100644
--- a/tests/MathTest.cpp
+++ b/tests/MathTest.cpp
@@ -209,34 +209,29 @@ static float nextFloat(SkRandom& rand) {
 /*  returns true if a == b as resulting from (int)x. Since it is undefined
  what to do if the float exceeds 2^32-1, we check for that explicitly.
  */
-static bool equal_float_native_skia(float x, uint32_t ni, uint32_t si) {
-    if (!(x == x)) {    // NAN
-        return ((int32_t)si) == SK_MaxS32 || ((int32_t)si) == SK_MinS32;
+static bool equal_float_native_skia(float x, int32_t ni, int32_t si) {
+    // When the float is out of integer range (NaN, above, below),
+    // the C cast is undefined, but Skia's methods should have clamped.
+    if (!(x == x)) {    // NaN
+        return si == SK_MaxS32 || si == SK_MinS32;
     }
-    // for out of range, C is undefined, but skia always should return NaN32
     if (x > SK_MaxS32) {
-        return ((int32_t)si) == SK_MaxS32;
+        return si == SK_MaxS32;
     }
-    if (x < -SK_MaxS32) {
-        return ((int32_t)si) == SK_MinS32;
+    if (x < SK_MinS32) {
+        return si == SK_MinS32;
     }
     return si == ni;
 }
 
 static void assert_float_equal(skiatest::Reporter* reporter, const char op[],
-                               float x, uint32_t ni, uint32_t si) {
+                               float x, int32_t ni, int32_t si) {
     if (!equal_float_native_skia(x, ni, si)) {
         ERRORF(reporter, "%s float %g bits %x native %x skia %x\n",
                op, x, SkFloat2Bits(x), ni, si);
     }
 }
 
-static void test_float_cast(skiatest::Reporter* reporter, float x) {
-    int ix = (int)x;
-    int iix = SkFloatToIntCast(x);
-    assert_float_equal(reporter, "cast", x, ix, iix);
-}
-
 static void test_float_floor(skiatest::Reporter* reporter, float x) {
     int ix = (int)floor(x);
     int iix = SkFloatToIntFloor(x);
@@ -257,23 +252,17 @@ static void test_float_ceil(skiatest::Reporter* reporter, float x) {
 }
 
 static void test_float_conversions(skiatest::Reporter* reporter, float x) {
-    test_float_cast(reporter, x);
     test_float_floor(reporter, x);
     test_float_round(reporter, x);
     test_float_ceil(reporter, x);
 }
 
-static void test_int2float(skiatest::Reporter* reporter, int ival) {
-    float x0 = (float)ival;
-    float x1 = SkIntToFloatCast(ival);
-    REPORTER_ASSERT(reporter, x0 == x1);
-}
-
 static void unittest_fastfloat(skiatest::Reporter* reporter) {
     SkRandom rand;
     size_t i;
 
     static const float gFloats[] = {
+        0.f/0.f, -0.f/0.f, 1.f/0.f, -1.f/0.f,
         0.f, 1.f, 0.5f, 0.499999f, 0.5000001f, 1.f/3,
         0.000000001f, 1000000000.f,     // doesn't overflow
         0.0000000001f, 10000000000.f    // does overflow
@@ -289,17 +278,6 @@ static void unittest_fastfloat(skiatest::Reporter* reporter) {
             float x = nextFloat(rand);
             test_float_conversions(reporter, x);
         }
-
-        test_int2float(reporter, 0);
-        test_int2float(reporter, 1);
-        test_int2float(reporter, -1);
-        for (i = 0; i < 100000; i++) {
-            // for now only test ints that are 24bits or less, since we don't
-            // round (down) large ints the same as IEEE...
-            int ival = rand.nextU() & 0xFFFFFF;
-            test_int2float(reporter, ival);
-            test_int2float(reporter, -ival);
-        }
     }
 }