[Haskell-cafe] Glome raytracer bug: bad output with -O2 -fasm

[Luke Palmer]

On Fri, Mar 28, 2008 at 6:28 AM, Jim Snow <jsnow at cs.pdx.edu> wrote:
> I was trying to get Blinn highlights working with my raytracer, and kept
>  getting ugly artifacts.  After trying a bunch of things, I finally
>  compiled without -O2, and the artifacts went away.
>
>  Here's what I mean:
>  http://syn.cs.pdx.edu/~jsnow/glome/Glome.hs-noartifact.png
>  http://syn.cs.pdx.edu/~jsnow/glome/Glome.hs-artifact.png
>
>  Here's the offending code, run "./make" and "./run" and you should see
>  the artifacts if your setup is the same as mine. (Requires OpenGL.)
>  http://syn.cs.pdx.edu/~jsnow/glome/glome.hs-0.2-bug.tar.gz
>
>  The artifacts also go away if I use -fvia-C.  It doesn't seem to matter
>  whether I use Floats or Doubles in the rendering code.  The artifacts
>  also show up with -O1.  Have I stumbled across a known compiler bug?  Or
>  perhaps an OpenGL bug?  (The bug could, of course, be in my code, but
>  then one might expect to get the same erroneous output every time
>  regardless of compiler flags.)
>
>  To reiterate, I'm using ghc 8.6.2.

You probably mean 6.8.2.

Works for me in all cases.

% uname -a
Linux madhatter 2.6.22-gentoo-r8 #6 PREEMPT Sat Oct 20 04:19:22 GMT
2007 i686 AMD Turion(tm) 64 Mobile Technology ML-40 AuthenticAMD
GNU/Linux
% ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.8.2

Looks to me like Glome is depending on some very fine details of
floating point arithmetic.

Luke