[Mesa-dev] [PATCH 17/28] intel/nir: call nir_opt_constant_folding before nir_opt_algebraic is executed

[Samuel Iglesias Gonsálvez]

On 05/12/2018 19:22, Connor Abbott wrote:
> Why is this needed? In general, we shouldn't be relying on
> optimization ordering for correctness. It probably just means one of
> the optimizations is wrong, and you're working around that.

There is an issue when the shader tries to generate a NaN using 0/0,
opt_algebraic is replaces the 0 * NaN part for 0.

This is an example of denorm * NaN code, although it is happening for
other tests too. The optimization takes this code:

	vec1 32 ssa_0 = load_const (0x00000000 /* 0.000000 */)
	vec1 32 ssa_2 = load_const (0x00800000 /* 0.000000 */)
	vec1 32 ssa_3 = load_const (0x008003f0 /* 0.000000 */)
	vec1 32 ssa_4 = fneg ssa_2
	vec1 32 ssa_5 = fadd ssa_3, ssa_4 <<<< generate denorm
	vec1 32 ssa_7 = frcp ssa_0
	vec1 32 ssa_8 = fmul ssa_0, ssa_7 <<< generate NaN
	vec1 32 ssa_10 = fmul ssa_5, ssa_8 <<<< denorm * NaN

and converts it to the following:

	vec1 32 ssa_0 = load_const (0x00000000 /* 0.000000 */)
	vec1 32 ssa_2 = load_const (0x00800000 /* 0.000000 */)
	vec1 32 ssa_3 = load_const (0x008003f0 /* 0.000000 */)
	vec1 32 ssa_4 = fneg ssa_2
	vec1 32 ssa_5 = fadd ssa_3, ssa_4   <<<< build denorm
	vec1 32 ssa_7 = frcp ssa_0
	vec1 32 ssa_18 = load_const (0x00000000 /* 0.000000 */)
	vec1 32 ssa_19 = imov ssa_18       <<<< fmul (a, 0) = 0, so we miss NaN!
	vec1 32 ssa_10 = fmul ssa_5, ssa_19 <<<< denorm * 0

Is it possible to represent fmul (a, 0) = 0 if 'a' is not NaN? Or
fmul(a, NaN) = NaN? Do you have any other suggestion?

Sam

> On Wed, Dec 5, 2018 at 4:56 PM Samuel Iglesias Gonsálvez
> <siglesias at igalia.com> wrote:
>>
>> This would do constant folding and also flush to zero denorms operands before
>> the nir_opt_algebraic is executed.
>>
>> Signed-off-by: Samuel Iglesias Gonsálvez <siglesias at igalia.com>
>> ---
>>  src/intel/compiler/brw_nir.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/src/intel/compiler/brw_nir.c b/src/intel/compiler/brw_nir.c
>> index 0a5aa35c700..600f7a97df9 100644
>> --- a/src/intel/compiler/brw_nir.c
>> +++ b/src/intel/compiler/brw_nir.c
>> @@ -570,8 +570,8 @@ brw_nir_optimize(nir_shader *nir, const struct brw_compiler *compiler,
>>        OPT(nir_opt_cse);
>>        OPT(nir_opt_peephole_select, 0);
>>        OPT(nir_opt_intrinsics);
>> -      OPT(nir_opt_algebraic);
>>        OPT(nir_opt_constant_folding);
>> +      OPT(nir_opt_algebraic);
>>        OPT(nir_opt_dead_cf);
>>        if (OPT(nir_opt_trivial_continues)) {
>>           /* If nir_opt_trivial_continues makes progress, then we need to clean
>> --
>> 2.19.1
>>
>> _______________________________________________
>> mesa-dev mailing list
>> mesa-dev at lists.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
> 

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