Add float-to-half (binary16) conversion functions.
Based on code by Fabian Giesen at
https://fgiesen.wordpress.com/2012/03/28/half-to-float-done-quic/.
These will be needed for creating binary16 textures from floating point data.
BUG=skia:3103
Review URL: https://codereview.chromium.org/760753003
diff --git a/src/core/SkHalf.cpp b/src/core/SkHalf.cpp
new file mode 100644
index 0000000..0db979b
--- /dev/null
+++ b/src/core/SkHalf.cpp
@@ -0,0 +1,97 @@
+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkHalf.h"
+#include "SkFloatBits.h"
+
+uint16_t halfMantissa(SkHalf h) {
+ return h & 0x03ff;
+}
+
+uint16_t halfExponent(SkHalf h) {
+ return (h >> 10) & 0x001f;
+}
+
+uint16_t halfSign(SkHalf h) {
+ return h >> 15;
+}
+
+union FloatUIntUnion {
+ uint32_t fUInt; // this must come first for the initializations below to work
+ float fFloat;
+};
+
+// based on Fabien Giesen's float_to_half_fast3()
+// see https://gist.github.com/rygorous/2156668
+SkHalf SkFloatToHalf(float f) {
+ static const uint32_t f32infty = { 255 << 23 };
+ static const uint32_t f16infty = { 31 << 23 };
+ static const FloatUIntUnion magic = { 15 << 23 };
+ static const uint32_t sign_mask = 0x80000000u;
+ static const uint32_t round_mask = ~0xfffu;
+ SkHalf o = 0;
+
+ FloatUIntUnion floatUnion;
+ floatUnion.fFloat = f;
+
+ uint32_t sign = floatUnion.fUInt & sign_mask;
+ floatUnion.fUInt ^= sign;
+
+ // NOTE all the integer compares in this function can be safely
+ // compiled into signed compares since all operands are below
+ // 0x80000000. Important if you want fast straight SSE2 code
+ // (since there's no unsigned PCMPGTD).
+
+ // Inf or NaN (all exponent bits set)
+ if (floatUnion.fUInt >= f32infty)
+ // NaN->qNaN and Inf->Inf
+ o = (floatUnion.fUInt > f32infty) ? 0x7e00 : 0x7c00;
+ // (De)normalized number or zero
+ else {
+ floatUnion.fUInt &= round_mask;
+ floatUnion.fFloat *= magic.fFloat;
+ floatUnion.fUInt -= round_mask;
+ // Clamp to signed infinity if overflowed
+ if (floatUnion.fUInt > f16infty) {
+ floatUnion.fUInt = f16infty;
+ }
+
+ o = floatUnion.fUInt >> 13; // Take the bits!
+ }
+
+ o |= sign >> 16;
+ return o;
+}
+
+// based on Fabien Giesen's half_to_float_fast2()
+// see https://fgiesen.wordpress.com/2012/03/28/half-to-float-done-quic/
+float SkHalfToFloat(SkHalf h) {
+ static const FloatUIntUnion magic = { 126 << 23 };
+ FloatUIntUnion o;
+
+ if (halfExponent(h) == 0)
+ {
+ // Zero / Denormal
+ o.fUInt = magic.fUInt + halfMantissa(h);
+ o.fFloat -= magic.fFloat;
+ }
+ else
+ {
+ // Set mantissa
+ o.fUInt = halfMantissa(h) << 13;
+ // Set exponent
+ if (halfExponent(h) == 0x1f)
+ // Inf/NaN
+ o.fUInt |= (255 << 23);
+ else
+ o.fUInt |= ((127 - 15 + halfExponent(h)) << 23);
+ }
+
+ // Set sign
+ o.fUInt |= (halfSign(h) << 31);
+ return o.fFloat;
+}