Cycles: Tweak to the expf() speed workaround
Add compile-time check for particular glibc version which fixed the issue. This makes it so own-compiled blender is the fastest in the world, and the only issue remains what should we do for release builds. After some discussion with Campbell we decided to keep it as is for now because slowdown is not that much noticeable. We'll disable this workaround for release builds when all the majority of the distros will switch to the new version of glibc.
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@ -25,10 +25,12 @@
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#include "util_half.h"
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#include "util_types.h"
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/* On 64bit linux single precision exponent is really slow comparing to the
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* double precision version, even with float<->double conversion involved.
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/* On x86_64, versions of glibc < 2.16 have an issue where expf is
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* much slower than the double version. This was fixed in glibc 2.16.
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*/
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#if !defined(__KERNEL_GPU__) && defined(__linux__) && defined(__x86_64__)
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#if !defined(__KERNEL_GPU__) && defined(__x86_64__) && defined(__x86_64__) && \
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defined(__GNU_LIBRARY__) && defined(__GLIBC__ ) && defined(__GLIBC_MINOR__) && \
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(__GLIBC__ <= 2 && __GLIBC_MINOR__ < 16)
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# define expf(x) ((float)exp((double)(x)))
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#endif
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