On Wed 12 Feb 2020 12:50, Ludovic Courtès <l...@gnu.org> writes: > Hello! > > Consider this loop: > > (let loop ((n (expt 2 18)) > (i 1)) > (unless (zero? n) > (loop (- n 1) > (logior 0 (ash i 1))))) > > Guile 2.2 strips away the computation of ‘i’ (it cannot throw, has no > side effects, and the result is unused): > [...] > L1: > 27 (handle-interrupts) at (unknown > file):21:11 > 28 (sub/immediate 4 4 1) > 29 (br-if-u64-=-scm 3 4 #t -2) ;; -> L1 at (unknown > file):18:11
Like specifically, it removes the logior call. > However, 3.0.0 keeps the computation of ‘i’: > [...] > L3: > 53 (instrument-loop 139) > 55 (handle-interrupts) > 56 (call-scm<-scm-uimm 5 5 1 3) at (unknown > file):388:11 > 58 (call-scm<-scm-uimm 3 3 1 34) at (unknown > file):389:21 > 60 (call-scm<-scm-scm 3 4 3 10) at (unknown > file):389:11 > 62 (=? 5 4) at (unknown > file):385:11 > 63 (jne -10) ;; -> L3 Hoo, we need to fix the disassembler to output something more sensible than this :P IP 56 appears to be the 1-, 58 is the lsh/immediate, and 60 is the logior. > I’m not sure where the optimization should be taking place. Perhaps > it’s just a matter of amount-of-work threshold somewhere? It's not an amount-of-work, that's only in peval which does nothing to either of these (though it certainly could). I took a look. I just pushed something to make (logior 0 INT) reduce to INT, but it doesn't remove the loop variable. Then I thought it was surely dead code elimination that just wasn't doing its thing. The value is unused, so it must be that it thought that the `ash' was effectful. That `ash' gets compiled to `lsh/immediate', which does indeed raise an exception if the operand isn't an int; but here we prove that it is. The problem was a missing "type checker" implementation for lsh/immediate, a problem introduced in the refactored compilation of `ash'. So, fixed in git now: L3: 45 (instrument-loop 135) 47 (handle-interrupts) 48 (call-scm<-scm-uimm 5 5 1 3) at (unknown file):4:12 50 (=? 5 4) at (unknown file):3:12 51 (jne -6) ;; -> L3 Cheers, Andy
