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bug#39573: [3.0.0] Compiler fails to optimize out side-effect-free expre
bug#39573: [3.0.0] Compiler fails to optimize out side-effect-free expression
Thu, 13 Feb 2020 12:35:59 +0100
Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux)
Andy Wingo <address@hidden> skribis:
>> However, 3.0.0 keeps the computation of ‘i’:
>> 53 (instrument-loop 139)
>> 55 (handle-interrupts)
>> 56 (call-scm<-scm-uimm 5 5 1 3) at (unknown
>> 58 (call-scm<-scm-uimm 3 3 1 34) at (unknown
>> 60 (call-scm<-scm-scm 3 4 3 10) at (unknown
>> 62 (=? 5 4) at (unknown
>> 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.
Oh, I hadn’t read that much into these lines. :-)
>> 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:
> 45 (instrument-loop 135)
> 47 (handle-interrupts)
> 48 (call-scm<-scm-uimm 5 5 1 3) at (unknown
> 50 (=? 5 4) at (unknown
> 51 (jne -6) ;; -> L3
Yay! It’s nice to see how 7dc90a17e03045c7cd8894b14b027b845b68aa4f is
short and clear.