On 06/30/2016 03:20 PM, Matt Turner wrote:
> On Wed, Jun 29, 2016 at 2:04 PM, Ian Romanick <i...@freedesktop.org> wrote:
>> From: Ian Romanick <ian.d.roman...@intel.com>
>>
>> Signed-off-by: Ian Romanick <ian.d.roman...@intel.com>
>> ---
>> src/mesa/drivers/dri/i965/brw_fs_nir.cpp | 50 ++++++++++++++++++++++------
>> src/mesa/drivers/dri/i965/brw_vec4_nir.cpp | 52
>> +++++++++++++++++++++++-------
>> 2 files changed, 81 insertions(+), 21 deletions(-)
>>
>> diff --git a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
>> b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
>> index f15bf3e..f8db28a 100644
>> --- a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
>> @@ -623,8 +623,32 @@ fs_visitor::nir_emit_find_msb_using_lzd(const
>> fs_builder &bld,
>> bool is_signed)
>> {
>> fs_inst *inst;
>> + fs_reg temp = src;
>>
>> - bld.LZD(retype(result, BRW_REGISTER_TYPE_UD), src);
>> + if (is_signed) {
>> + /* LZD of an absolute value source almost always does the right
>> + * thing. There are two problem values:
>> + *
>> + * * 0x80000000. Since abs(0x80000000) == 0x80000000, LZD returns
>> + * 0. However, findMSB(int(0x80000000)) == 30.
>> + *
>> + * * 0xffffffff. Since abs(0xffffffff) == 1, LZD returns
>> + * 31. Section 8.8 (Integer Functions) of the GLSL 4.50 spec says:
>> + *
>> + * For a value of zero or negative one, -1 will be returned.
>> + *
>> + * For all negative number cases, including 0x80000000 and
>> + * 0xffffffff, the correct value is obtained from LZD if instead of
>
> Interesting, both the G45 and IVB docs (I didn't check others, I
> suspect they say the same) say
>
> "If the source is signed, the abs source modifier must be used to
> convert any negative source value to a positive value."
>
> I suppose that means you're supposed to retype the src to UD below?
Yes, probably. The above instructions make sure that it is positive,
but we could still have a value of 0x80000000. I can add the retype()
below.
>> + * negating the (already negative) value the logical-not is used. A
>> + * conditonal logical-not can be achieved in two instructions.
>> + */
>> + temp = vgrf(glsl_type::int_type);
>> +
>> + bld.ASR(temp, src, brw_imm_d(31));
>> + bld.XOR(temp, temp, src);
>> + }
>> +
>> + bld.LZD(retype(result, BRW_REGISTER_TYPE_UD), temp);
>
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