[PATCH vkd3d 2/3] vkd3d-shader/hlsl: Take writemask into account when emitting a SM4 constant.

[Zebediah Figura (she/her)]

On 11/17/21 03:52, Matteo Bruni wrote:
> On Tue, Nov 16, 2021 at 10:23 AM Giovanni Mascellani
> <gmascellani at codeweavers.com> wrote:
>>
>> Hi,
>>
>> On 15/11/21 18:48, Zebediah Figura (she/her) wrote:
>>> This is probably quibbling, but how about this?
>>>
>>> j = 0;
>>> for (i = 0; i < 4; ++i)
>>> {
>>>       if (instr.dsts[0].writemask & (1u << i))
>>>           instr.srcs[0].reg.immconst_uint[i] = constant->value[j++].u;
>>> }
>>>
>>> Feels a bit simpler to me.
>>
>> I agree it's simpler, but as written it's also wrong. :-)
>>
>> The problem is that when dimx == 1, we set the register to have
>> dimension VKD3D_SM4_DIMENSION_SCALAR, which means that the constant must
>> be written in the first register, no matter the writemask.
> 
> That's probably right but it raises all kinds of red flags for me. Why
> is dimx == 1 such a special case (aside from the SCALAR / VEC4
> dimension)? We're basically saying that we ignore the writemask for
> scalar constants. Why?
> 
> The code being convoluted, or mostly duplicated, is a consequence of
> the above. It feels like we should fix that instead.
> 

dimx == 1 is a special case because it's a special case in DXBC. We're 
not ignoring the writemask; it's just that for scalars the writemask is 
only reflected in the destination register, whereas for vectors it's 
reflected in the source registers as well.

As Giovanni puts it, write_sm4_instruction() is mostly just a 
serialization helper. hlsl_ir_node structures categorically contain 
"unmapped" data—so the "writemask" field of hlsl_reg is a misnomer; on 
the other hand, not all source registers are "mapped" (e.g. ld/sample 
coords) so we can't categorically do mapping in write_sm4_instruction(). 
Hence we do that translation in the individual callers, like this one.

We should be doing a similar thing for normal arithmetic register 
swizzles, but we currently aren't.