[Mesa-dev] [PATCH] st/glsl_to_tgsi: Translate float ir_unop_neg into float MOV with modifier.

[Francisco Jerez]

Ilia Mirkin <imirkin at alum.mit.edu> writes:

> Also, how is this happening in the first place? For example, we have:
>
>    case ir_unop_bitcast_f2i:
>    case ir_unop_bitcast_f2u:
>       /* Make sure we don't propagate the negate modifier to integer opcodes. */
>       if (op[0].negate || op[0].abs)
>          emit_asm(ir, TGSI_OPCODE_MOV, result_dst, op[0]);
>       else
>          result_src = op[0];
>
> Oh, but it's going directly into a ir_triop_csel, which is missing
> this logic. It should be added there instead, IMHO. OTOH, the same
> issue will hit in emit_block_mov() if you do. Would love to hear some
> other opinions... Marek, Brian, Roland?
>

I considered doing something like that but it will be somewhat more
involved than in the snippet above because you'll have to allocate
temporaries to hold the negated source results in case that any of the
csel sources has a modifier set -- Can look into it next week if you
think it's the right thing to do.

>   -ilia
>
>
> On Sat, Mar 4, 2017 at 2:24 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
>> Hmmm... I wonder if this should only be done for the native_integers
>> case. I'm concerned that this will cause perf regressions on weaker hw
>> like nv30 and r300, as the neg will no longer be inserted as a
>> modifier into the next instruction. Any opinion on this?
>>
>> On Sat, Mar 4, 2017 at 2:16 PM, Francisco Jerez <currojerez at riseup.net> wrote:
>>> Otherwise result_src may be provided to an integer instruction whose
>>> negate modifier has different semantics.  Example is UCMP as in the
>>> bug linked below, where an unrelated change in the GLSL built-in
>>> lowering code for atan2 (e9ffd12827ac11a2d2002a42fa8eb1df847153ba)
>>> caused the generation of floating-point ir_unop_neg instructions
>>> followed by ir_triop_csel, which is lowered into UCMP on back-ends
>>> with native integer support.
>>>
>>> Fixes a regression of piglit glsl-fs-tan-1 on softpipe introduced by
>>> the above-mentioned glsl front-end commit.  Even though the commit
>>> that triggered the regression doesn't seem to have made it to any
>>> stable branches yet, this seems worth back-porting since I don't see
>>> any reason why the bug couldn't have been reproduced before that
>>> point.
>>>
>>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=99817
>>> Tested-by: Vinson Lee <vlee at freedesktop.org>
>>> Cc: mesa-stable at lists.freedesktop.org
>>> ---
>>>  src/mesa/state_tracker/st_glsl_to_tgsi.cpp | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/src/mesa/state_tracker/st_glsl_to_tgsi.cpp b/src/mesa/state_tracker/st_glsl_to_tgsi.cpp
>>> index af41bdb..6bf3c89 100644
>>> --- a/src/mesa/state_tracker/st_glsl_to_tgsi.cpp
>>> +++ b/src/mesa/state_tracker/st_glsl_to_tgsi.cpp
>>> @@ -1633,7 +1633,7 @@ glsl_to_tgsi_visitor::visit_expression(ir_expression* ir, st_src_reg *op)
>>>           emit_asm(ir, TGSI_OPCODE_DNEG, result_dst, op[0]);
>>>        else {
>>>           op[0].negate = ~op[0].negate;
>>> -         result_src = op[0];
>>> +         emit_asm(ir, TGSI_OPCODE_MOV, result_dst, op[0]);
>>>        }
>>>        break;
>>>     case ir_unop_subroutine_to_int:
>>> --
>>> 2.10.2
>>>
>>> _______________________________________________
>>> mesa-dev mailing list
>>> mesa-dev at lists.freedesktop.org
>>> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
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