remove useless loop in a function body

[Roman Marynchak]

I cannot imagine the simpler example than this one:

(defun completely-useless ()
    (loop for i from 1 to 10000000000 do (= 1 1)))

However, it does iterates in that loop... Is there some misunderstanding from my side, or SBCL just gives up any loop optimizations? I will not even try more complex examples from the famous book "Optimizing Compilers for Modern Architectures: A Dependence-based Approach", since they require more advanced loop analysis logic in the compiler than SBCL seems to have these days.

Regards,
Roman

[Nathan Froyd (froydnj)]

SBCL does not do any classic loop optimizations (e.g. unrolling), let alone any of the higher-level transforms described in Allen and Kennedy.

[Nikodemus Siivola (nikodemus)]

SBCL does rudimentary loop analysis to aid register allocation and aligning loop heads -- but that's it.

In this case it's particularly sad as the (= 1 1) is optimized away -- so the loop is quite breathtaking:

; B0: L0: 4883C108 ADD RCX, 8
; B4: L1: 483B0D25000000 CMP RCX, [RIP+37]
; BB: 7EF3 JLE L0