[Mesa-dev] [RFC PATCH] nir: Transform 4*x into x << 2 during late optimizations.
Ian Romanick
idr at freedesktop.org
Fri May 8 11:11:30 PDT 2015
On 05/08/2015 03:36 AM, Kenneth Graunke wrote:
> According to Glenn, shifts on R600 have 5x the throughput as multiplies.
>
> Intel GPUs have strange integer multiplication restrictions - on most
> hardware, MUL actually only does a 32-bit x 16-bit multiply. This
> means the arguments aren't commutative, which can limit our constant
> propagation options. SHL has no such restrictions.
>
> Shifting is probably reasonable on most people's hardware, so let's just
> do that.
>
> i965 shader-db results (using NIR for VS):
> total instructions in shared programs: 7432587 -> 7388982 (-0.59%)
> instructions in affected programs: 1360411 -> 1316806 (-3.21%)
> helped: 5772
> HURT: 0
>
> Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> Cc: mattst88 at gmail.com
> Cc: jason at jlekstrand.net
> ---
> src/glsl/nir/nir_opt_algebraic.py | 5 +++++
> 1 file changed, 5 insertions(+)
>
> So...I found a bizarre issue with this patch.
>
> (('imul', 4, a), ('ishl', a, 2)),
>
> totally optimizes things. However...
>
> (('imul', a, 4), ('ishl', a, 2)),
>
> doesn't seem to do anything, even though imul is commutative, and nir_search
> should totally handle that...
>
> ▄▄ ▄▄ ▄▄ ▄▄▄▄▄▄▄▄ ▄▄▄▄▄ ▄▄
> ██ ██ ████ ▀▀▀██▀▀▀ █▀▀▀▀██ ██
> ▀█▄ ██ ▄█▀ ████ ██ ▄█▀ ██
> ██ ██ ██ ██ ██ ██ ▄██▀ ██
> ███▀▀███ ██████ ██ ██ ▀▀
> ███ ███ ▄██ ██▄ ██ ▄▄ ▄▄
> ▀▀▀ ▀▀▀ ▀▀ ▀▀ ▀▀ ▀▀ ▀▀
>
> If you know why, let me know, otherwise I may have to look into it when more
> awake.
I've noticed a couple other weird things that I have been unable to
understand. Shaders like the one below end with fmul/ffma instaed of
flrp, for example. I understand why that happens from GLSL IR
opt_algebraic, but it seems like nir_opt_algebraic should handle it.
[require]
GLSL >= 1.30
[vertex shader]
in vec4 v;
in vec2 tc_in;
out vec2 tc;
void main() {
gl_Position = v;
tc = tc_in;
}
[fragment shader]
in vec2 tc;
out vec4 color;
uniform sampler2D s;
uniform float a;
uniform vec3 base_color;
void main() {
vec3 tex_color = texture(s, tc).xyz;
color.xyz = (base_color * a) + (tex_color * (1.0 - a));
color.a = 1.0;
}
> diff --git a/src/glsl/nir/nir_opt_algebraic.py b/src/glsl/nir/nir_opt_algebraic.py
> index 400d60e..350471f 100644
> --- a/src/glsl/nir/nir_opt_algebraic.py
> +++ b/src/glsl/nir/nir_opt_algebraic.py
> @@ -247,6 +247,11 @@ late_optimizations = [
> (('fge', ('fadd', a, b), 0.0), ('fge', a, ('fneg', b))),
> (('feq', ('fadd', a, b), 0.0), ('feq', a, ('fneg', b))),
> (('fne', ('fadd', a, b), 0.0), ('fne', a, ('fneg', b))),
> +
> + # Multiplication by 4 comes up fairly often in indirect offset calculations.
> + # Some GPUs have weird integer multiplication limitations, but shifts should work
> + # equally well everywhere.
> + (('imul', 4, a), ('ishl', a, 2)),
This should be conditionalized on whether the platform has native integers.
> ]
>
> print nir_algebraic.AlgebraicPass("nir_opt_algebraic", optimizations).render()
>
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