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

[Nicolai Hähnle]

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.

Nicolai