llvm dev - Oct 2017 - Why x86_64 divq is not used for 128-bit by 64-bit division?

Hi there,

Let's have this C code:

unsigned long div(unsigned __int128 n, unsigned long d)
{
return n / d;
}

I would assume that the divq is the perfect match here. But the compiler
generates the
code that calls the __udivti3 procedure which performs 128-bit by 128-bit
division.

Why is divq not used here?

- Paweł
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divq only produces a 64-bit result. There's way for the compiler to know
you didn't divide a greater than 64-bit number by 1 or some other value
that required a large result.

~Craig

On Thu, Oct 19, 2017 at 1:47 PM, Paweł Bylica via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi there,
>
> Let's have this C code:
>
> unsigned long div(unsigned __int128 n, unsigned long d)
> {
> return n / d;
> }
>
> I would assume that the divq is the perfect match here. But the compiler
> generates the
> code that calls the __udivti3 procedure which performs 128-bit by 128-bit
> division.
>
> Why is divq not used here?
>
> - Paweł
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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In my example, I'm truncating the result.

But anyway, are you saying that when the result of divq does not fit in
64-bit the result stored in the register is undefined?

On Thu, Oct 19, 2017 at 10:54 PM, Craig Topper <craig.topper at gmail.com>
wrote:
> divq only produces a 64-bit result. There's way for the compiler to
know
> you didn't divide a greater than 64-bit number by 1 or some other value
> that required a large result.
>
> ~Craig
>
> On Thu, Oct 19, 2017 at 1:47 PM, Paweł Bylica via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi there,
>>
>> Let's have this C code:
>>
>> unsigned long div(unsigned __int128 n, unsigned long d)
>> {
>> return n / d;
>> }
>>
>> I would assume that the divq is the perfect match here. But the
compiler
>> generates the
>> code that calls the __udivti3 procedure which performs 128-bit by
>> 128-bit division.
>>
>> Why is divq not used here?
>>
>> - Paweł
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
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