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

[eocallaghan at alterapraxis.com]

On 2016-04-16 20:20, 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?

+1 from me. Supporting more than two generations of LLVM is a bit much 
to carry imho.

> 
> Marek
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