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Fix fast_exp2 with MSVC and sse4.2 #3804

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Apr 15, 2023
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Fix fast_exp2 with MSVC and sse4.2 #3804

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14 changes: 13 additions & 1 deletion src/include/OpenImageIO/fmath.h
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Original file line number Diff line number Diff line change
Expand Up @@ -2068,7 +2068,19 @@ OIIO_FORCEINLINE OIIO_HOSTDEVICE T fast_exp2 (const T& xval) {
r = madd(x, r, kD);
r = madd(x, r, kE);
r = madd(x, r, one);
return bitcast_to_float (bitcast_to_int(r) + (m << 23));

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// Original code was:
// return bitcast_to_float (bitcast_to_int(r) + (m << 23));
// This was producing the wrong results when called via `sRGB_to_linear()`
// and `linear_to_sRGB()` on some Windows MSVC builds.
// We found that it was fixed by using a temporary intN, as below.
// Presumed to be an optimizer bug in MSVC versions 16.11.x
// and earlier.
// See PR #3804

intN i = bitcast_to_int(r);
T f = bitcast_to_float(i + (m << 23));
return f;
#else
T r;
for (int i = 0; i < r.elements; ++i)
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