On 6/29/21 4:15 AM, Thomas Debe wrote:
> I don't only compile but also test the runtime behavior of povray. In
> this context I noticed with clang builds of povray that the compiler
> flag -ffast-math has a degrading runtime behavior (more than four times
> longer runtime). This is not caused by the noise code and can't be
> explained at first, -Ofast has the same behavior.
What scene(s) were you running when you saw the performance degrade?
I've not seen this behavior - but I compile with clang more than I run
with clang compiled code.
FWIW. With respect to -Ofast and g++, I've seen inconsistent results.
Compiles are more often than not slower with the flag than without.
Never swings in performance as large as 4x though.
As for the 4x difference, a wild guess is clang is handling some
'-fast-math' allowed, undefined behavior, differently than gcc.
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