jumper, to_2dot2 and from_2dot2

Nothing too tricky here.

Change-Id: I2a10548efc75a6fd875fcb242790880d9b9a28fd
Reviewed-on: https://skia-review.googlesource.com/11388
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Matt Sarett <msarett@google.com>

src/jumper/SkJumper.cpp

@@ -78,6 +78,8 @@ static K kConstants = {
     M(unpremul)           \
     M(from_srgb)          \
     M(to_srgb)            \
+    M(from_2dot2)         \
+    M(to_2dot2)           \
     M(scale_1_float)      \
     M(scale_u8)           \
     M(lerp_1_float)       \

src/jumper/SkJumper_stages.cpp

@@ -484,6 +484,35 @@ STAGE(to_srgb) {
     b = fn(b);
 }
 
+STAGE(from_2dot2) {
+    auto fn = [](F x) {
+        // x^(141/64) = x^(2.20312) is a great approximation of the true value, x^(2.2).
+        // (note: x^(35/16) = x^(2.1875) is an okay one as well and would be quicker)
+        F x16 = rsqrt(rsqrt(rsqrt(rsqrt(x)))),    // x^(1/16) = x^(4/64);
+          x64 = rsqrt(rsqrt(x16));                // x^(1/64)
+
+        // 141/64 = 128/64 + 12/64 + 1/64
+        return max((x*x) * (x16*x16*x16) * x64, 0);
+    };
+    r = fn(r);
+    g = fn(g);
+    b = fn(b);
+}
+STAGE(to_2dot2) {
+    auto fn = [](F x) {
+        // x^(29/64) is a very good approximation of the true value, x^(1/2.2).
+        F x2  = rsqrt(x),                         // x^(-1/2)
+          x32 = rsqrt(rsqrt(rsqrt(rsqrt(x2)))),   // x^(-1/32)
+          x64 = rsqrt(x32);                       // x^(+1/64)
+
+        // 29/64 = 32/64 - 2/64 - 1/64
+        return max(rcp(x2) * x32 * rcp(x64), 0);
+    };
+    r = fn(r);
+    g = fn(g);
+    b = fn(b);
+}
+
 STAGE(scale_1_float) {
     auto c = *(const float*)ctx;