[cfe-dev] Floating-point performance question

[Richard Hadsell]

On 09/09/2013 11:49 AM, Richard Hadsell wrote:
> This may not be necessary.  I found another clue here, in section 1.1: https://sourceware.org/newlib/libm.html
>
> When I tested the initial value of libm's |_LIB_VERSION, I found that my G++ version starts up with _IEEE_, the fastest mode (no exception handling, no warnings, ignoring errno.  The Clang++ version starts up with _POSIX_, which sets errno correctly.
>
> I will test our code's behavior, when I set ||_LIB_VERSION to ||_IEEE_ explicitly, and I'll let you know if this explains the difference.
> | 
Unfortunately, that did not help.  The version change did not improve the behavior of exp2 -- it is still slower than pow.  I will have to stick with my workaround, giving pow a const double with the value 2.0 instead of a literal 2.0.

You may continue to consider how Clang++ could be controlled to avoid converting to the slower function.

-- 
Dick Hadsell			203-992-6320  Fax: 203-992-6001
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