jumper, rgb<->hsl

I've rearranged while porting, I hope making the logic clearer.
Exactly one gm is affected, highcontrastfilter.

The most interesting line is this, from hsl_to_rgb:

  F t2 = if_then_else(t < 0.0_f, t + 1.0_f,

I had to write 0.0_f (instead of the usual 0) to force Clang to compare
against a zero register instead of a 16-byte zero constant in memory.
Register pressure is high in hsl_to_rgb, so something must have kicked
in to prefer memory over zeroing a register.

No big deal.  It makes the code read more symmetrically anyway.

Change-Id: I1a5ced72216234587760c6f803fb69315d18fae0
Reviewed-on: https://skia-review.googlesource.com/13242
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>

src/jumper/SkJumper.cpp

@@ -80,6 +80,8 @@ static K kConstants = {
     M(to_srgb)            \
     M(from_2dot2)         \
     M(to_2dot2)           \
+    M(rgb_to_hsl)         \
+    M(hsl_to_rgb)         \
     M(scale_1_float)      \
     M(scale_u8)           \
     M(lerp_1_float)       \

src/jumper/SkJumper_stages.cpp

@@ -512,6 +512,50 @@ STAGE(to_2dot2) {
     b = fn(b);
 }
 
+STAGE(rgb_to_hsl) {
+    F mx = max(max(r,g), b),
+      mn = min(min(r,g), b),
+      d = mx - mn,
+      d_rcp = 1.0_f / d;
+
+    F h = C(1/6.0f) *
+          if_then_else(mx == mn, 0,
+          if_then_else(mx ==  r, (g-b)*d_rcp + if_then_else(g < b, 6.0_f, 0),
+          if_then_else(mx ==  g, (b-r)*d_rcp + 2.0_f,
+                                 (r-g)*d_rcp + 4.0_f)));
+
+    F l = (mx + mn) * 0.5_f;
+    F s = if_then_else(mx == mn, 0,
+                       d / if_then_else(l > 0.5_f, 2.0_f-mx-mn, mx+mn));
+
+    r = h;
+    g = s;
+    b = l;
+}
+STAGE(hsl_to_rgb) {
+    F h = r,
+      s = g,
+      l = b;
+
+    F q = if_then_else(l < 0.5_f, l*(1.0_f + s), l + s - l*s),
+      p = 2.0_f*l - q;
+
+    auto hue_to_rgb = [&](F t) {
+        F t2 = if_then_else(t < 0.0_f, t + 1.0_f,
+               if_then_else(t > 1.0_f, t - 1.0_f,
+                                       t));
+
+        return if_then_else(t2 < C(1/6.0f),  p + (q-p)*6.0_f*t,
+               if_then_else(t2 < C(3/6.0f),  q,
+               if_then_else(t2 < C(4/6.0f),  p + (q-p)*6.0_f*(C(4/6.0f) - t2),
+                                             p)));
+    };
+
+    r = if_then_else(s == 0, l, hue_to_rgb(h + C(1/3.0f)));
+    g = if_then_else(s == 0, l, hue_to_rgb(h            ));
+    b = if_then_else(s == 0, l, hue_to_rgb(h - C(1/3.0f)));
+}
+
 STAGE(scale_1_float) {
     auto c = *(const float*)ctx;