What's the preferred way of doing bitwise rotate of an integral value in D?

Are there intrinsics for bitwise rotation available in LDC?

On Monday, 7 January 2019 at 14:39:07 UTC, Per Nordlöw wrote:
> What's the preferred way of doing bitwise rotate of an integral value in D?
>
> Are there intrinsics for bitwise rotation available in LDC?

I just found this

    ulong rotateLeft(ulong x, ubyte bits)
    {
        return (x << bits) | (x >> (64 - bits));
    }

Questions:
- Is this a good implementation for the ulong case?
- Should we use >> or >>> ?
- What's the preferred type for `bits`, ubyte, or uint or making it a template parameter?

On Monday, 7 January 2019 at 14:43:29 UTC, Per Nordlöw wrote:
> On Monday, 7 January 2019 at 14:39:07 UTC, Per Nordlöw wrote:
>> What's the preferred way of doing bitwise rotate of an integral value in D?
>>
>> Are there intrinsics for bitwise rotation available in LDC?
>
> I just found this
>
>     ulong rotateLeft(ulong x, ubyte bits)
>     {
>         return (x << bits) | (x >> (64 - bits));
>     }
>
> Questions:
> - Is this a good implementation for the ulong case?
> - Should we use >> or >>> ?
> - What's the preferred type for `bits`, ubyte, or uint or making it a template parameter?

There are
https://dlang.org/phobos/core_bitop.html#.rol
and
https://dlang.org/phobos/core_bitop.html#.ror

But it seems the implementation is like yours.

On Monday, 7 January 2019 at 14:53:26 UTC, Alex wrote:
> There are
> https://dlang.org/phobos/core_bitop.html#.rol
> and
> https://dlang.org/phobos/core_bitop.html#.ror
>
> But it seems the implementation is like yours.

Ahh, missed that. Thanks.

On 1/7/19 10:10 AM, Per Nordlöw wrote:
> On Monday, 7 January 2019 at 14:53:26 UTC, Alex wrote:
>> There are
>> https://dlang.org/phobos/core_bitop.html#.rol
>> and
>> https://dlang.org/phobos/core_bitop.html#.ror
>>
>> But it seems the implementation is like yours.
> 
> Ahh, missed that. Thanks.

Use the core.bitop ones. Chances are they are intrinsics on certain platforms.

-Steve

On Monday, 7 January 2019 at 14:39:07 UTC, Per Nordlöw wrote:
> What's the preferred way of doing bitwise rotate of an integral value in D?
>
> Are there intrinsics for bitwise rotation available in LDC?

Turns out you don't need any:

https://d.godbolt.org/z/C_Sk_-

Generates ROL instruction.

On Mon, Jan 07, 2019 at 11:13:37PM +0000, Guillaume Piolat via Digitalmars-d-learn wrote:
> On Monday, 7 January 2019 at 14:39:07 UTC, Per Nordlöw wrote:
> > What's the preferred way of doing bitwise rotate of an integral value in D?
> > 
> > Are there intrinsics for bitwise rotation available in LDC?
> 
> Turns out you don't need any:
> 
> https://d.godbolt.org/z/C_Sk_-
> 
> Generates ROL instruction.

There's a certain pattern that dmd looks for, that it transforms into a ROL instruction. Similarly for ROR.  Deviate too far from this pattern, though, and it might not recognize it as it should.  To be sure, always check the disassembly.


T

-- 
I don't trust computers, I've spent too long programming to think that they can get anything right. -- James Miller

On Monday, 7 January 2019 at 23:20:57 UTC, H. S. Teoh wrote:
> On Mon, Jan 07, 2019 at 11:13:37PM +0000, Guillaume Piolat via Digitalmars-d-learn wrote:
>> On Monday, 7 January 2019 at 14:39:07 UTC, Per Nordlöw wrote:
>> > What's the preferred way of doing bitwise rotate of an integral value in D?
>> > 
>> > Are there intrinsics for bitwise rotation available in LDC?
>> 
>> Turns out you don't need any:
>> 
>> https://d.godbolt.org/z/C_Sk_-
>> 
>> Generates ROL instruction.
>
> There's a certain pattern that dmd looks for, that it transforms into a ROL instruction. Similarly for ROR.  Deviate too far from this pattern, though, and it might not recognize it as it should.  To be sure, always check the disassembly.
>
Are you sure it's dmd looking for the pattern. Playing with the godbolt link shows that dmd doesn't generate the rol code (gdc 4.8.2 neither).

On Monday, 7 January 2019 at 14:39:07 UTC, Per Nordlöw wrote:
> What's the preferred way of doing bitwise rotate of an integral value in D?
>
> Are there intrinsics for bitwise rotation available in LDC?

LDC does not expose this intrinsic currently, but you can use LLVM's fshl:
https://llvm.org/docs/LangRef.html#llvm-fshl-intrinsic
It's a fairly new intrinsic, so won't work with older LLVM versions.

https://d.godbolt.org/z/SBnJFJ

As noted by others, the optimizer is strong enough to recognize what you are doing (without intrinsic) and use ror/rol if it deems it advantageous to do so.

-Johan

On Tue, Jan 08, 2019 at 09:15:09AM +0000, Patrick Schluter via Digitalmars-d-learn wrote:
> On Monday, 7 January 2019 at 23:20:57 UTC, H. S. Teoh wrote:
[...]
> > There's a certain pattern that dmd looks for, that it transforms into a ROL instruction. Similarly for ROR.  Deviate too far from this pattern, though, and it might not recognize it as it should. To be sure, always check the disassembly.
> > 
> Are you sure it's dmd looking for the pattern. Playing with the godbolt link shows that dmd doesn't generate the rol code (gdc 4.8.2 neither).

I vaguely remember a bug about this. There is definitely explicit checking for this in dmd; I don't remember if it was a bug in the pattern matching code itself, or some other problem, that made it fail. You may need to specify -O for the code to actually be active. Walter could point you to the actual function that does this optimization.


T

-- 
Never wrestle a pig. You both get covered in mud, and the pig likes it.