Am 02.07.2014 18:44, schrieb Le_Forgeron:

> Rendering benchmark gives the attached pictures (the icpc seems ok,
> within usual time, but gcc is bogus and too fast, far too fast)

I finally found out what's going on here.

The implementation of adaptive area lights uses an N*M cache for 
"lightlet" data it has already computed. Previously, yet-uncomputed data 
was flagged by setting the respective colour's red channel to -1. I had 
changed this to use a NaN ("Not-a-Number") value instead, to avoid 
problems when a user deliberately sets a light source's colour to 
negative red.

Now of course this NaN value has to be tested for; according to IEEE 
standard, a NaN value has the property that it is non-equal to anything, 
even itself. So I wrote the test: "if (red != red)...".

Well, it turns out that the g++ compiler optimizes this comparison away 
when in "-ffast_math" mode.

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