Re: [PATCH vkd3d 5/7] vkd3d-shader/hlsl: Write SM4 casts to float.

[Matteo Bruni]

On Thu, Sep 9, 2021 at 6:02 AM Zebediah Figura  wrote:
> Signed-off-by: Zebediah Figura
>
> libs/vkd3d-shader/hlsl_sm4.c | 34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
>
> diff --git a/libs/vkd3d-shader/hlsl_sm4.c b/libs/vkd3d-shader/hlsl_sm4.c
> index 160da47fd..cac71ef7e 100644
> --- a/libs/vkd3d-shader/hlsl_sm4.c
> +++ b/libs/vkd3d-shader/hlsl_sm4.c
> @@ -981,6 +981,40 @@ static void write_sm4_expr(struct hlsl_ctx *ctx,
>          {
>              switch (expr->op)
>              {
> -
>            case HLSL_OP1_CAST:
> -
>            {
> -
>                const struct hlsl_type *src_type = arg1->data_type;
> -
> -
>                /* Narrowing casts need to be lowered. */
> -
>                if (src_type->dimx != expr->node.data_type->dimx)
> -
>                    hlsl_fixme(ctx, expr->node.loc, "Narrowing cast.\n");
> -
> -
>                switch (src_type->base_type)
> -
>                {
> -
>                    case HLSL_TYPE_HALF:
> -
>                    case HLSL_TYPE_FLOAT:
> -
>                        write_sm4_unary_op(buffer, VKD3D_SM4_OP_MOV, &expr->node, arg1, 0);
> -
>                        break;
> -
> -
>                    case HLSL_TYPE_BOOL:
> -
>                    case HLSL_TYPE_INT:
> -
>                        write_sm4_unary_op(buffer, VKD3D_SM4_OP_ITOF, &expr->node, arg1, 0);
> -
>                        break;

As far as I'm aware, booleans in SM4+ are represented in quite a
peculiar way: false is represented as 0, true is canonically
0xffffffff. I think the idea is to be able to use the result of a
conditional expression directly as a mask and such, but don't quote me
on that. For sure it's different from SM1-3, which use the traditional
0 and 1 values for false and true.

That's probably the reason why native uses MOVC to convert from bool
(to float or anything else) and, most importantly, NE for the opposite
direction (with 0 as the other comparand).