optimize a loop which uses some linear expression as a condition

[Roman Marynchak]

SBCL fails to optimize this code at compile time and becomes very busy at runtime, printing nothing after long calculations:

(defun g()
 (loop for i from 2 to 100000 do
  (loop for j from i to 100000
    when (= j (- i 1)) do (print "Hi"))))

When loop variables have a monotonic defined range, and an action condition may be expressed as 'k1x1 + b1 > k2x2 + b2', where x1 and x2 are loop indexes and k1,k2,b1,b2 are some constants, it is easy to say whether a condition will become valid for some pair (x1, x2) or not (recall your geometry course here for a clear analogy). I believe that SBCL as a modern optimizing compiler should analyse cases like this, they are quite simple.

Regards,
Roman