At a closer inspection, it seems that the culprit is that I use |, not +. + is optimized, while | isn't. That should be easy to fix, shouldn't it?

(mapping size 100000:)

gauge { m|=([-1:17]); };

Result: 0.160000

gauge { m=([-2:17])|m; };

Result: 0.170000

gauge { m+=([-3:17]); };

Result: 0.000000 <- lo!

gauge { m=([-4:17])+m; };

Result: 0.030000