Re: [Caml-list] The verdict on "%identity"

["Vincent HUGOT" <vincent.hugot@gmail.com>]

Is there some place where this fabulous -dmcc switch is documented?
ocamlopt's man and --help pages, as well as the manual, are utterly  
uninformative (either don't mention it or say simply "undocumented").

V.



On Tue, 20 Nov 2012 17:19:34 +0100, Gabriel Scherer  
<gabriel.scherer@gmail.com> wrote:

> This is good advice in general, and using ocamlobjinfo to get inlining
> information from the .cmx is indeed a very good idea.
>
> Regarding -dcmm vs. -S, I generally use -dcmm myself (much more
> readable), but it is not the right tool in this case. The cmm produced
> for my test.ml code does make a difference between the three styles:
>      (let testA/1046 (let x/1075 "camlTest__3" x/1075)
>        (store (+a "camlTest" 20) testA/1046))
>      (let testB/1047 (let prim/1076 "camlTest__2" prim/1076)
>        (store (+a "camlTest" 24) testB/1047))
>      (let testD/1048 "camlTest__1"
>         (store (+a "camlTest" 28) testD/1048))
>
> In fact, the removal of the trivial (let x = foo in x) does not happen
> during the inlining passes in closure.ml, but much later at the
> register allocation phase, where there is indeed a strong preference
> for eg. testA/1046 and x/1705 to be given the same register, and the
> useless move is erased. I make no claim of how robust this behavior
> will be in a different case (eg. with higher register pressure), but
> I'm not sure I really care.
> I'd rather have people study the behavior on the compiler the real
> performance-critical applications and suggest potential style changes
> in the program (or optimization changes in the compiler) in cases
> where this really make a performance difference. Writing code in a
> certain way because "the generated code is nicer" is usually not worth
> the trouble.