Am 25.02.2016 um 11:15 schrieb Iago Toral Quiroga:
> From the OpenGL 4.2 spec:
>
> "When a constructor is used to convert any integer or floating-point type to a
> bool, 0 and 0.0 are converted to false, and non-zero values are converted to
> true."
>
> Thus, even the smallest non-zero floating value should be translated to true.
> This behavior has been verified also with proprietary NVIDIA drivers.
>
> Currently, we implement this conversion as a cmp.nz operation with floats,
> subject to floating-point precision limitations, and as a result, relatively
> small non-zero floating point numbers return false instead of true.
>
> This patch fixes the problem by getting rid of the sign bit (to cover the case
> of -0.0) and testing the result against 0u using an integer comparison
> instead.
> ---
> src/mesa/drivers/dri/i965/brw_fs_nir.cpp | 15 ++++++++++++---
> 1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
> b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
> index db20c71..7d62d7e 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
> @@ -913,9 +913,18 @@ fs_visitor::nir_emit_alu(const fs_builder &bld,
> nir_alu_instr *instr)
> bld.MOV(result, negate(op[0]));
> break;
>
> - case nir_op_f2b:
> - bld.CMP(result, op[0], brw_imm_f(0.0f), BRW_CONDITIONAL_NZ);
> - break;
> + case nir_op_f2b: {
> + /* Because comparing to 0.0f is subject to precision limitations, do
> the
> + * comparison using integers (we need to get rid of the sign bit for
> that)
> + */
> + if (devinfo->gen >= 8)
> + op[0] = resolve_source_modifiers(op[0]);
> + op[0] = retype(op[0], BRW_REGISTER_TYPE_UD);
> + bld.AND(op[0], op[0], brw_imm_ud(0x7FFFFFFFu));
> + bld.CMP(result, op[0], brw_imm_ud(0u), BRW_CONDITIONAL_NZ);
> + break;
> + }
> +
> case nir_op_i2b:
> bld.CMP(result, op[0], brw_imm_d(0), BRW_CONDITIONAL_NZ);
> break;
>
Does that fix anything? I don't really see a problem with the existing
logic. Yes any "non-zero value" should be converted to true. But surely
that definition cannot include denorms, which you are always allowed to
flush to zero.
(Albeit I can't tell what the result would be with NaNs with the float
compare, nor what the result actually should be in this case since glsl
doesn't require NaNs neither.)
Roland
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