[Mesa-dev] [PATCH] i965/fs: Fix extract_i8/u8 to a 64-bit destination

[Matt Turner]

On Mon, Nov 13, 2017 at 10:06 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
> On Mon, Nov 13, 2017 at 3:23 PM, Matt Turner <mattst88 at gmail.com> wrote:
>>
>> The MOV instruction can extract bytes to words/double words, and
>> words/double words to quadwords, but not byte to quadwords.
>>
>> For unsigned byte to quadword, we can read them as words and AND off the
>> high byte and extract to quadword in one instruction. For signed bytes,
>> we need to first sign extend to word and the sign extend that word to a
>> quadword.
>>
>> Fixes the following test on CHV, BXT, and GLK:
>>    KHR-GL46.shader_ballot_tests.ShaderBallotBitmasks
>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103628
>> ---
>>  src/intel/compiler/brw_fs_nir.cpp | 20 ++++++++++++++++++--
>>  1 file changed, 18 insertions(+), 2 deletions(-)
>>
>> diff --git a/src/intel/compiler/brw_fs_nir.cpp
>> b/src/intel/compiler/brw_fs_nir.cpp
>> index 38d0d357e8..e266837ece 100644
>> --- a/src/intel/compiler/brw_fs_nir.cpp
>> +++ b/src/intel/compiler/brw_fs_nir.cpp
>> @@ -1395,10 +1395,26 @@ fs_visitor::nir_emit_alu(const fs_builder &bld,
>> nir_alu_instr *instr)
>>
>>     case nir_op_extract_u8:
>>     case nir_op_extract_i8: {
>> -      const brw_reg_type type = brw_int_type(1, instr->op ==
>> nir_op_extract_i8);
>>        nir_const_value *byte = nir_src_as_const_value(instr->src[1].src);
>>        assert(byte != NULL);
>> -      bld.MOV(result, subscript(op[0], type, byte->u32[0]));
>> +
>> +      /* MOV does not support a 64-bit destination and a byte source */
>> +      if (nir_dest_bit_size(instr->dest.dest) == 64) {
>
>
> Do we also want to predicate this on "devinfo->is_cherrytrail ||
> gen_device_info_is_gen9lp(devinfo)" as well as bit size?  Big-core can
> handle it just fine.

Actually, the documentation says it isn't allowed:

BDW+
There is no direct conversion from B/UB to Q/UQ or Q/UQ to B/UB. Use
two instructions and a word or DWord intermediate integer type.