refactor approx_{log2,pow2,powf}

- Move to SkJumper_vectors.h
    - Fold the -127 and +2.774485010.
    - approx_powf(F,F) instead of approx_powf(F,float) for consistency.
    - A little layout reformatting.

Change-Id: If9cb3d62a097cb6ecf89f157a1dde672c1516371
Reviewed-on: https://skia-review.googlesource.com/13865
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>

src/jumper/SkJumper_stages.cpp

@@ -671,30 +671,6 @@ STAGE(table_g) { g = table(g, ctx); }
 STAGE(table_b) { b = table(b, ctx); }
 STAGE(table_a) { a = table(a, ctx); }
 
-// See http://www.machinedlearnings.com/2011/06/fast-approximate-logarithm-exponential.html.
-SI F approx_log2(F x) {
-    // e is a fair approximation of log2(x) in its own right...
-    F e = cast(bit_cast<U32>(x)) * C(1.0f / (1<<23)) - 127.0_f;
-
-    // ... but using the mantissa to refine its error is _much_ better.
-    F m = bit_cast<F>((bit_cast<U32>(x) & 0x007fffff_i) | 0x3f000000_i);
-    return e
-         + 2.774485010_f
-         - 1.498030302_f * m
-         - 1.725879990_f / (0.3520887068_f + m);
-}
-SI F approx_pow2(F x) {
-    F f = fract(x);
-    return bit_cast<F>(round(C(1.0f * (1<<23)),
-                x + 121.2740575_f
-                - 1.490129070_f * f
-                + 27.72802330_f / (4.84252568_f - f)));
-}
-
-SI F approx_powf(F x, float g) {
-    return approx_pow2(approx_log2(x) * g);
-}
-
 SI F parametric(F v, const SkJumper_ParametricTransferFunction* ctx) {
     F r = if_then_else(v <= ctx->D, mad(ctx->C, v, ctx->F)
                                   , approx_powf(mad(ctx->A, v, ctx->B), ctx->G) + ctx->E);

src/jumper/SkJumper_vectors.h

@@ -628,4 +628,28 @@ SI U16 bswap(U16 x) {
 
 SI F fract(F v) { return v - floor_(v); }
 
+// See http://www.machinedlearnings.com/2011/06/fast-approximate-logarithm-exponential.html.
+SI F approx_log2(F x) {
+    // e - 127 is a fair approximation of log2(x) in its own right...
+    F e = cast(bit_cast<U32>(x)) * C(1.0f / (1<<23));
+
+    // ... but using the mantissa to refine its error is _much_ better.
+    F m = bit_cast<F>((bit_cast<U32>(x) & 0x007fffff_i) | 0x3f000000_i);
+    return e
+         - 124.225514990_f
+         -   1.498030302_f * m
+         -   1.725879990_f / (0.3520887068_f + m);
+}
+SI F approx_pow2(F x) {
+    F f = fract(x);
+    return bit_cast<F>(round(C(1.0f * (1<<23)),
+                             x + 121.274057500_f
+                               -   1.490129070_f * f
+                               +  27.728023300_f / (4.84252568_f - f)));
+}
+
+SI F approx_powf(F x, F y) {
+    return approx_pow2(approx_log2(x) * y);
+}
+
 #endif//SkJumper_vectors_DEFINED