POV-Ray : Newsgroups : povray.beta-test : Technical verification build "v3.8.0-beta.668" - Unix package! : Re: Technical verification build "v3.8.0-beta.668" - Unix package! Server Time
2 Aug 2021 03:00:55 EDT (-0400)
  Re: Technical verification build "v3.8.0-beta.668" - Unix package!  
From: William F Pokorny
Date: 1 Jul 2021 07:42:01
Message: <60ddaa09@news.povray.org>
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.

Bill P.


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