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Re: question regarding div / std::div implementation
- From: Daniel Gutson <daniel dot gutson at tallertechnologies dot com>
- To: Adhemerval Zanella <adhemerval dot zanella at linaro dot org>
- Cc: GNU C Library <libc-alpha at sourceware dot org>
- Date: Wed, 20 Apr 2016 17:36:15 -0300
- Subject: Re: question regarding div / std::div implementation
- Authentication-results: sourceware.org; auth=none
- References: <CAF5HaEXKZ7j-gbZPiWPhDpx7=R0zm1xYvXNYCNUMG4WeZS532Q at mail dot gmail dot com> <5717DF65 dot 5060606 at linaro dot org> <CAF5HaEWdpAGiXtCO36u3F0QGAXfVHL+qkY+RLsszpv7paPVdMg at mail dot gmail dot com> <5717E68D dot 2020905 at linaro dot org>
On Wed, Apr 20, 2016 at 5:29 PM, Adhemerval Zanella
> On 20-04-2016 17:07, Daniel Gutson wrote:
>> On Wed, Apr 20, 2016 at 4:58 PM, Adhemerval Zanella
>> <firstname.lastname@example.org> wrote:
>>> On 20-04-2016 16:44, Daniel Gutson wrote:
>>>> is there any reason that std::div / cstdlib div is not implemented
>>>> in such a way that it is expanded to
>>>> the assembly instruction -when available- that calculates both the
>>>> remainder and the quotient,
>>>> e.g. x86' div ?
>>>> For example, why not an inline function with inline assembly? Or,
>>>> should this require a gcc built-in?
>>> I believe because nobody really implemented this optimization and
>>> my felling is if this is being a hotspot in your application you
>>> will probably get more gains trying to rewrite it than using the
>>> libc call.
>> then it won't be portable, or optimally-portable, meaning that the optimization
>> would show up whenever my target supports it. Suppose I need to provide
>> my application for several architectures, I would expect that I should
>> be able to
>> write my application using standard functions, and that it will get
>> optimized for each platform.
>> I'm reporting it in bugzilla and asking to assign it to one of my team members.
> I do not really get what exactly you are referring as non-portable,
> since glibc div code is implemented as stdlib/div.c and these will
> generate idivl instruction on x86_64 for all supported chips. And
I don't see it generating the idivl instruction, but
callq 400430 <div@plt>
so I think it should be implemented as an inline function maybe with
(or rely on the pattern recognition as you suggest below).
> afaik these are true for all supported architectures (I am not
> aware of any architecture that added a more optimized
> division/modulus operation with a *different* opcode).
Could you please post an example and the gcc command line call where
you do get the idiv?
> I mean to use the integer operation directly instead of using the
> libcall. The code is quite simple:
> div (int numer, int denom)
> div_t result;
> result.quot = numer / denom;
> result.rem = numer % denom;
> return result;
> You can try to add an inline version on headers, as such the one
> for string.h, but I would strongly recommend you to either work on
> your application if these are the hotspot (either by calling the
> operations directly instead) or on compiler side to make it
> handling it as builtin (and thus avoid the libcall).
Why should this be a builtin? I can implement it on gcc, but I still
don't see why should I pass the burden to the compiler
whereas it is a matter of library implementation.
Daniel F. Gutson
San Lorenzo 47, 3rd Floor, Office 5
Phone: +54 351 4217888 / +54 351 4218211