Re: [Mesa-dev] [PATCH] nir: mark dsqrt/drsqrt/drcp implementation as exact

[Connor Abbott]

On Thu, Apr 28, 2016 at 11:26 AM, Ilia Mirkin  wrote:
> On Thu, Apr 28, 2016 at 11:21 AM, Connor Abbott  wrote:
>> On Thu, Apr 28, 2016 at 11:20 AM, Ilia Mirkin  wrote:
>>> What if the existing operations were already exact?
>>
>> They aren't -- the builder makes them non-exact by default, and we
>> initialize the builder ourselves in this pass.
>
> Hmm... OK. I'm just thinking of the case
>
> precise c  = op(a, b)
>
> And op gets lowered. The resulting operations should also be exact. If
> that's what's happening, then all's well.

The resulting operations should be exact regardless of whether the
original one was exact -- they're relying on the extra precision that
fma() gives us in order to get the right precision in the result, and
like in GLSL, there's no guarantee that a non-exact fma won't be split
into multiply + add. Once we lower the operation, the compiler can't
really apply any optimizations to the operation as a whole (except
maybe constant folding) so it basically becomes exact anyways.

>
>   -ilia
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Re: [Mesa-dev] [PATCH] nir: mark dsqrt/drsqrt/drcp implementation as exact

[Ilia Mirkin]

On Thu, Apr 28, 2016 at 11:21 AM, Connor Abbott  wrote:
> On Thu, Apr 28, 2016 at 11:20 AM, Ilia Mirkin  wrote:
>> What if the existing operations were already exact?
>
> They aren't -- the builder makes them non-exact by default, and we
> initialize the builder ourselves in this pass.

Hmm... OK. I'm just thinking of the case

precise c  = op(a, b)

And op gets lowered. The resulting operations should also be exact. If
that's what's happening, then all's well.

  -ilia
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Re: [Mesa-dev] [PATCH] nir: mark dsqrt/drsqrt/drcp implementation as exact

[Connor Abbott]

On Thu, Apr 28, 2016 at 11:20 AM, Ilia Mirkin  wrote:
> What if the existing operations were already exact?

They aren't -- the builder makes them non-exact by default, and we
initialize the builder ourselves in this pass.

>
> On Thu, Apr 28, 2016 at 11:17 AM, Connor Abbott  wrote:
>> The floating-point operations used to implement these have been
>> carefully chosen to minimize rounding error while still getting decent
>> performance. We don't want any optimizations to mess with them. While
>> this shouldn't affect anything now, it seems like a good idea.
>>
>> Signed-off-by: Connor Abbott 
>> ---
>>  src/compiler/nir/nir_lower_double_ops.c | 4 
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/src/compiler/nir/nir_lower_double_ops.c 
>> b/src/compiler/nir/nir_lower_double_ops.c
>> index 7505fa3..42b94bb 100644
>> --- a/src/compiler/nir/nir_lower_double_ops.c
>> +++ b/src/compiler/nir/nir_lower_double_ops.c
>> @@ -142,8 +142,10 @@ lower_rcp(nir_builder *b, nir_ssa_def *src)
>>  * See https://en.wikipedia.org/wiki/Division_algorithm for more details.
>>  */
>>
>> +   b->exact = true;
>> ra = nir_ffma(b, ra, nir_ffma(b, ra, src, nir_imm_double(b, -1)), ra);
>> ra = nir_ffma(b, ra, nir_ffma(b, ra, src, nir_imm_double(b, -1)), ra);
>> +   b->exact = false;
>>
>> return fix_inv_result(b, ra, src, new_exp);
>>  }
>> @@ -267,6 +269,7 @@ lower_sqrt_rsq(nir_builder *b, nir_ssa_def *src, bool 
>> sqrt)
>>  * (https://en.wikipedia.org/wiki/Methods_of_computing_square_roots).
>>  */
>>
>> +b->exact = true;
>>  nir_ssa_def *one_half = nir_imm_double(b, 0.5);
>>  nir_ssa_def *h_0 = nir_fmul(b, one_half, ra);
>>  nir_ssa_def *g_0 = nir_fmul(b, src, ra);
>> @@ -283,6 +286,7 @@ lower_sqrt_rsq(nir_builder *b, nir_ssa_def *src, bool 
>> sqrt)
>> one_half);
>> res = nir_ffma(b, y_1, r_1, y_1);
>>  }
>> +b->exact = false;
>>
>>  if (sqrt) {
>> /* Here, the special cases we need to handle are
>> --
>> 2.5.0
>>
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Re: [Mesa-dev] [PATCH] nir: mark dsqrt/drsqrt/drcp implementation as exact

[Ilia Mirkin]

What if the existing operations were already exact?

On Thu, Apr 28, 2016 at 11:17 AM, Connor Abbott  wrote:
> The floating-point operations used to implement these have been
> carefully chosen to minimize rounding error while still getting decent
> performance. We don't want any optimizations to mess with them. While
> this shouldn't affect anything now, it seems like a good idea.
>
> Signed-off-by: Connor Abbott 
> ---
>  src/compiler/nir/nir_lower_double_ops.c | 4 
>  1 file changed, 4 insertions(+)
>
> diff --git a/src/compiler/nir/nir_lower_double_ops.c 
> b/src/compiler/nir/nir_lower_double_ops.c
> index 7505fa3..42b94bb 100644
> --- a/src/compiler/nir/nir_lower_double_ops.c
> +++ b/src/compiler/nir/nir_lower_double_ops.c
> @@ -142,8 +142,10 @@ lower_rcp(nir_builder *b, nir_ssa_def *src)
>  * See https://en.wikipedia.org/wiki/Division_algorithm for more details.
>  */
>
> +   b->exact = true;
> ra = nir_ffma(b, ra, nir_ffma(b, ra, src, nir_imm_double(b, -1)), ra);
> ra = nir_ffma(b, ra, nir_ffma(b, ra, src, nir_imm_double(b, -1)), ra);
> +   b->exact = false;
>
> return fix_inv_result(b, ra, src, new_exp);
>  }
> @@ -267,6 +269,7 @@ lower_sqrt_rsq(nir_builder *b, nir_ssa_def *src, bool 
> sqrt)
>  * (https://en.wikipedia.org/wiki/Methods_of_computing_square_roots).
>  */
>
> +b->exact = true;
>  nir_ssa_def *one_half = nir_imm_double(b, 0.5);
>  nir_ssa_def *h_0 = nir_fmul(b, one_half, ra);
>  nir_ssa_def *g_0 = nir_fmul(b, src, ra);
> @@ -283,6 +286,7 @@ lower_sqrt_rsq(nir_builder *b, nir_ssa_def *src, bool 
> sqrt)
> one_half);
> res = nir_ffma(b, y_1, r_1, y_1);
>  }
> +b->exact = false;
>
>  if (sqrt) {
> /* Here, the special cases we need to handle are
> --
> 2.5.0
>
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[Mesa-dev] [PATCH] nir: mark dsqrt/drsqrt/drcp implementation as exact

[Connor Abbott]

The floating-point operations used to implement these have been
carefully chosen to minimize rounding error while still getting decent
performance. We don't want any optimizations to mess with them. While
this shouldn't affect anything now, it seems like a good idea.

Signed-off-by: Connor Abbott 
---
 src/compiler/nir/nir_lower_double_ops.c | 4 
 1 file changed, 4 insertions(+)

diff --git a/src/compiler/nir/nir_lower_double_ops.c 
b/src/compiler/nir/nir_lower_double_ops.c
index 7505fa3..42b94bb 100644
--- a/src/compiler/nir/nir_lower_double_ops.c
+++ b/src/compiler/nir/nir_lower_double_ops.c
@@ -142,8 +142,10 @@ lower_rcp(nir_builder *b, nir_ssa_def *src)
 * See https://en.wikipedia.org/wiki/Division_algorithm for more details.
 */
 
+   b->exact = true;
ra = nir_ffma(b, ra, nir_ffma(b, ra, src, nir_imm_double(b, -1)), ra);
ra = nir_ffma(b, ra, nir_ffma(b, ra, src, nir_imm_double(b, -1)), ra);
+   b->exact = false;
 
return fix_inv_result(b, ra, src, new_exp);
 }
@@ -267,6 +269,7 @@ lower_sqrt_rsq(nir_builder *b, nir_ssa_def *src, bool sqrt)
 * (https://en.wikipedia.org/wiki/Methods_of_computing_square_roots).
 */
 
+b->exact = true;
 nir_ssa_def *one_half = nir_imm_double(b, 0.5);
 nir_ssa_def *h_0 = nir_fmul(b, one_half, ra);
 nir_ssa_def *g_0 = nir_fmul(b, src, ra);
@@ -283,6 +286,7 @@ lower_sqrt_rsq(nir_builder *b, nir_ssa_def *src, bool sqrt)
one_half);
res = nir_ffma(b, y_1, r_1, y_1);
 }
+b->exact = false;
 
 if (sqrt) {
/* Here, the special cases we need to handle are
-- 
2.5.0

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