[Mesa-dev] [PATCH 5/5] ac: use correct LLVM opcodes for ordered comparisons
Bas Nieuwenhuizen
bas at basnieuwenhuizen.nl
Fri Feb 23 19:18:46 UTC 2018
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 :).
Well, nir_opt_algebraic completely disregards whether it is ordered
and will happily flip it the wrong way
(https://cgit.freedesktop.org/mesa/mesa/tree/src/compiler/nir/nir_opt_algebraic.py#n137)
and it seems like spirv does try to be extra careful by doing the NaN
checks manually for all the comparisons (but in the process still
relies on feq being ordered). So I'd argue that we need some
non-backend work either way.
I agree that switching over all backends is annoying though,
especially because not all of them have a great instruction selector
that we can trust to optimize out any of the not's if we don't lower
them in nir_opt_algebraic to instructions with explicit orderedness.
So radv switching over to what intel uses seems OK for now.
btw should we document this somewhere? Either in the instruction name,
or at least in the definition in nir_opcodes.py?
>
>>>
>>>
>>> >
>>> >
>>> >>> break;
>>> >>> case nir_op_flt:
>>> >>> - result = emit_float_cmp(&ctx->ac, LLVMRealULT, src[0],
>>> >>> src[1]);
>>> >>> + result = emit_float_cmp(&ctx->ac, LLVMRealOLT, src[0],
>>> >>> src[1]);
>>> >>> break;
>>> >>> case nir_op_fge:
>>> >>> - result = emit_float_cmp(&ctx->ac, LLVMRealUGE, src[0],
>>> >>> src[1]);
>>> >>> + result = emit_float_cmp(&ctx->ac, LLVMRealOGE, src[0],
>>> >>> src[1]);
>>> >>> break;
>>> >>> case nir_op_ufeq:
>>> >>> result = emit_float_cmp(&ctx->ac, LLVMRealUEQ,
>>> >>> src[0],
>>> >>> src[1]);
>>> >>> --
>>> >>> 2.14.3
>>> >>>
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>>
>>
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