[Mesa-dev] [PATCH 2/2] radeonsi: Implement ddx/ddy on VI using ds_bpermute

[Marek Olšák]

On Sat, Apr 16, 2016 at 8:17 PM, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
> On 16.04.2016 05:20, Marek Olšák wrote:
>>
>> On Sat, Apr 16, 2016 at 8:04 AM, Michel Dänzer <michel at daenzer.net> wrote:
>>>
>>> On 16.04.2016 14:51, Michel Dänzer wrote:
>>>>
>>>> On 16.04.2016 11:39, Tom Stellard wrote:
>>>>>
>>>>> The ds_bpermute instruction allows threads to transfer data directly
>>>>> to or from the vgprs of other threads.  These instructions use the lds
>>>>> hardware to transfer data, but do not read or write lds memory.
>>>>>
>>>>> DDX BEFORE:                        |  DDX AFTER:
>>>>>                                     |
>>>>> v_mbcnt_lo_u32_b32_e64 v2, -1, 0   |  v_mbcnt_lo_u32_b32_e64 v2, -1, 0
>>>>> v_mbcnt_hi_u32_b32_e64 v2, -1, v2  |  v_mbcnt_hi_u32_b32_e64 v2, -1, v2
>>>>> v_lshlrev_b32_e32 v4, 2, v2        |  v_and_b32_e32 v2, 0x3ffffffc, v2
>>>>> v_and_b32_e32 v2, -4, v2           |  v_lshlrev_b32_e32 v2, 2, v2
>>>>> v_lshlrev_b32_e32 v3, 2, v2        |  ds_bpermute_b32 v3, v2, v0
>>>>> s_mov_b32 m0, -1                   |  ds_bpermute_b32 v0, v2, v0
>>>>> offset:4
>>>>> ds_write_b32 v4, v0                |  s_waitcnt lgkmcnt(0)
>>>>> s_waitcnt lgkmcnt(0)               |
>>>>> v_or_b32_e32 v0, 1, v2             |
>>>>> v_lshlrev_b32_e32 v0, 2, v0        |
>>>>> ds_read_b32 v1, v3                 |
>>>>> ds_read_b32 v0, v0                 |
>>>>> s_waitcnt lgkmcnt(0)               |
>>>>>                                     |
>>>>> LDS: 1 blocks                      |  LDS: 0 blocks
>>>>
>>>>
>>>> Nice.
>>>>
>>>>
>>>> Were these intrinsics already available in LLVM 3.6? If not, the old
>>>> code needs to be kept for backwards compatibility.
>>>
>>>
>>> I can see now that you're taking care of this for the bpermute
>>> intrinsic, but AFAICT the mbcnt intrinsics were only added in LLVM 3.8.
>>
>>
>> How do you feel about increasing the requirement to LLVM 3.8 for Mesa git?
>
>
> Please no.
>
> In addition to Gentoo and Arch mentioned by Ilia, Ubuntu also still ships
> 3.7. This will change soon enough, but even then, we should give people a
> few months to update.
>
> Let's not scare people away too much.

OK. Sounds good.

Marek