[Mesa-dev] [PATCH 5/5] ac: use correct LLVM opcodes for ordered comparisons

[Marek Olšák]

On Fri, Feb 23, 2018 at 6:39 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
> On Fri, Feb 23, 2018 at 8:30 AM, Bas Nieuwenhuizen <basni at chromium.org> wrote:
>>
>>
>> On Thu, Feb 15, 2018 at 8:54 AM, Connor Abbott <cwabbott0 at gmail.com> wrote:
>>>
>>> On Wed, Feb 14, 2018 at 11:53 PM, Timothy Arceri <tarceri at itsqueeze.com>
>>> wrote:
>>> >
>>> >
>>> > On 15/02/18 04:39, Marek Olšák wrote:
>>> >>
>>> >> Reviewed-by: Marek Olšák <marek.olsak at amd.com>
>>> >>
>>> >> Marek
>>> >>
>>> >> On Wed, Feb 14, 2018 at 7:29 AM, Timothy Arceri <tarceri at itsqueeze.com>
>>> >> wrote:
>>> >>>
>>> >>> Fixes glsl-1.30/execution/isinf-and-isnan* piglit tests for
>>> >>> radeonsi and should fix SPIRV errors when LLVM optimises away
>>> >>> the workarounds in vtn_handle_alu() for handling ordered
>>> >>> comparisons.
>>> >>>
>>> >>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104905
>>> >>> ---
>>> >>>   src/amd/common/ac_nir_to_llvm.c | 8 ++++----
>>> >>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>> >>>
>>> >>> diff --git a/src/amd/common/ac_nir_to_llvm.c
>>> >>> b/src/amd/common/ac_nir_to_llvm.c
>>> >>> index a0c5680205..e81f86bb08 100644
>>> >>> --- a/src/amd/common/ac_nir_to_llvm.c
>>> >>> +++ b/src/amd/common/ac_nir_to_llvm.c
>>> >>> @@ -1792,16 +1792,16 @@ static void visit_alu(struct ac_nir_context
>>> >>> *ctx,
>>> >>> const nir_alu_instr *instr)
>>> >>>                  result = emit_int_cmp(&ctx->ac, LLVMIntUGE, src[0],
>>> >>> src[1]);
>>> >>>                  break;
>>> >>>          case nir_op_feq:
>>> >>> -               result = emit_float_cmp(&ctx->ac, LLVMRealUEQ, src[0],
>>> >>> src[1]);
>>> >>> +               result = emit_float_cmp(&ctx->ac, LLVMRealOEQ, src[0],
>>> >>> src[1]);
>>> >>>                  break;
>>> >>>          case nir_op_fne:
>>> >>> -               result = emit_float_cmp(&ctx->ac, LLVMRealUNE, src[0],
>>> >>> src[1]);
>>> >>> +               result = emit_float_cmp(&ctx->ac, LLVMRealONE, src[0],
>>> >>> src[1]);
>>> >
>>> >
>>> > It seems we need to leave this one as is to avoid regressions. This is
>>> > also
>>> > what radeonsi does.
>>>
>>> So, the thing you have to understand is that in LLVM unordered
>>> comparisons are precisely the inverse of the ordered comparisons. That
>>> is, (a <=(ordered) b) == !(a >(unordered b), (a ==(ordered) b) == !(a
>>> !=(unordered) b), and  so on. C defines that all comparsions are
>>> ordered except !=, so that (a == b) == !(a != b) always holds true.
>>> Most hardware follows this convention -- offhand, x86 SSE is the only
>>> ISA I know of with separate ordered and unordered comparisons, and
>>> LLVM appears to have copied the distinction from them, but no one else
>>> has both. I'm not even sure if it's in the IEEE spec. GLSL follows the
>>> C convention, so glsl_to_nir just uses nir_op_fne to mean unordered
>>> not-equal. spirv_to_nir generates some extra instructions, which then
>>> get stripped away later... sigh.
>>>
>>> I think the right way to untangle this mess is to define that the NIR
>>> opcodes should always match the C convention. The separate ordered and
>>> unordered opcodes are unnecesary, since one is just the logical
>>> negation of the other, and LLVM was a little overzealous -- I'm sure
>>> they would get rid of the distinction if they had the chance -- and
>>> then they were blindly copied to SPIR-V. spirv_to_nir should just
>>> negate the result if necessary rather than emitting the extra code to
>>> handle NaN, and ac should use ordered except for not-equals.
>>
>>
>> I think we should also use ordered for not-equal. Otherwise we have no way
>> to contruct an unordered equal  or ordered not-equal using the not-trick. I
>> think that would be more important than trying to keep it in sync with C?
>
> I was thinking about that too... but all the backends (except for
> radv), frontends, opt_algebraic patterns, etc. currently assume fne
> means unordered not-equals. We'd have to rewrite a lot of stuff to
> flip the meaning. But if you're willing to do all the mechanical
> rewriting, sure :).

not-equal is unordered in DirectX, GLSL, and C. Let's keep it that
way. Ordered not-equal can't be generated as far as I know.

Marek