[LLVMdev] rol/ror llvm instruction set

[Kasra]

--- On Tue, 2/3/09, Bill Wendling <isanbard at gmail.com> wrote:

> From: Bill Wendling <isanbard at gmail.com>
> Subject: Re: [LLVMdev] rol/ror llvm instruction set
> To: kasra_n500 at yahoo.com, "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Date: Tuesday, February 3, 2009, 4:17 PM
> On Tue, Feb 3, 2009 at 3:54 PM, Kasra
> <kasra_n500 at yahoo.com> wrote:
> >
> > I guess the backends could know about the
> instructions. But I am not convinced why it is beneficial
> not to have ROR and ROL instructions within llvm.
> >
> I guess I could ask you the opposite question: What is the
> benefit of
> having these? They would have to be mappable to the source
> language in
> some way. I'm not sure about Ada, but I don't know
> of a "rotate"
> operator for any of the C variants, or any other high-level
> language.
> (This could be my lack of knowledge about other languages.)
> In C, you
> specify a rotate by doing shifts and bit-wise operations.
> 
> This isn't to say that LLVM IR is C-specific. Just that
> if you did
> have an LLVM rotate instruction, it would have to be
> generated by the
> front-end -- currently by recognizing the same things that
> the DAG
> combiner recognizes. And then it may need to be
> "lowered" for various
> platforms that don't support it, which is greater than
> the number of
> platforms that don't have shifts.
> 
> If a language came along that had rotate as a primitive and
> that
> generated LLVM IR, then you could probably convince people
> that having
> the rotates as LLVM IR instructions would be a benefit.
> We're not
> above changing the language to support good things. :-)
> 
> -bw

You could not be more right. However, rotations was not widely implemented on machines when C and C++ was evolved. Python and other high level languages are just too high level (hence, inefficient) to have such semantics.

The argument sounds fine, however, remember that the number of platforms that don't implement an instruction could not be a point of simile. 

Say we have 100 Linux distribution that do not have a certain feature, we can't conclude that that feature is not a common feature. We should consider the mainstream Linux distributions (and even if we may the giant of current desktops Windows).

Since x86 is about (if I am not wrong) 95% of the desktop PC's we could say rotation is implemented on most of the machines where LLVM will be running.

I bet out of the people who are reading/following this thread majority are running under x86 any way :D

So I guess what I am saying is rotations is very wide spread except that there are many machines out there that do not implement it however, the number of machine that do is far grater.


-- Kasra