id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
2284,Stack-hack optimization causes much re-computation in GUI callbacks,sedillard,,"This is a duplicate of #1168, recorded for posterity here, at the request of Simon PJ,

http://www.haskell.org/pipermail/glasgow-haskell-users/2008-May/014739.html

An IO lambda is created within main's scope, and this is given to the GLUT GUI library (or it could be GTK or wxHaskell) as a callback to draw the contents of the window. The callback lambda captures a value from the outer scope, but the state-hack inlines the value's defining expression into the callback. Thus, the value is re-computed every time the callback is called.

-fno-state-hack fixes it.

The attached program is a 3D model viewer. I've attached two models, one is small, the other larger. The performance hit is quite noticeable on the large one. The models need to be unzipped before running.

{{{
gunzip torus.obj.gz
./ObjView torus.obj
}}}

use x y z to rotate and force a redraw. When compiled with -O0 or -fno-state-hack, you'll see ""BUILDING MESH"" output once, otherwise it will be output on every redraw.
 ",bug,closed,normal,_|_,Compiler,6.8.2,duplicate,,JulesBean,Unknown/Multiple,Unknown/Multiple,None/Unknown,Unknown,,,,