On Tue, Oct 9, 2018 at 11:32 AM Ian Romanick <i...@freedesktop.org> wrote:
> On 10/08/2018 02:14 PM, Jason Ekstrand wrote: > > On Mon, Oct 8, 2018 at 3:46 PM Ian Romanick <i...@freedesktop.org > > <mailto:i...@freedesktop.org>> wrote: > > > > On 10/05/2018 09:10 PM, Jason Ekstrand wrote: > > > --- > > > src/compiler/nir/nir_constant_expressions.py | 1 + > > > src/compiler/nir/nir_opcodes.py | 43 > > ++++++++++++++++++-- > > > 2 files changed, 40 insertions(+), 4 deletions(-) > > > > > > diff --git a/src/compiler/nir/nir_constant_expressions.py > > b/src/compiler/nir/nir_constant_expressions.py > > > index 118af9f7818..afc0739e8b2 100644 > > > --- a/src/compiler/nir/nir_constant_expressions.py > > > +++ b/src/compiler/nir/nir_constant_expressions.py > > > @@ -79,6 +79,7 @@ template = """\ > > > #include <math.h> > > > #include "util/rounding.h" /* for _mesa_roundeven */ > > > #include "util/half_float.h" > > > +#include "util/bigmath.h" > > > #include "nir_constant_expressions.h" > > > > > > /** > > > diff --git a/src/compiler/nir/nir_opcodes.py > > b/src/compiler/nir/nir_opcodes.py > > > index 4ef4ecc6f22..209f0c5509b 100644 > > > --- a/src/compiler/nir/nir_opcodes.py > > > +++ b/src/compiler/nir/nir_opcodes.py > > > @@ -443,12 +443,47 @@ binop("isub", tint, "", "src0 - src1") > > > binop("fmul", tfloat, commutative + associative, "src0 * src1") > > > # low 32-bits of signed/unsigned integer multiply > > > binop("imul", tint, commutative + associative, "src0 * src1") > > > + > > > # high 32-bits of signed integer multiply > > > -binop("imul_high", tint32, commutative, > > > - "(int32_t)(((int64_t) src0 * (int64_t) src1) >> 32)") > > > +binop("imul_high", tint, commutative, """ > > > > This will enable imul_high for all integer types (ditto for umul_high > > below). A later patch adds lowering for 64-bit integer type. Will > the > > backend do the right thing for [iu]mul_high of 16- or 8-bit types? > > > > > > That's a good question. Looks like lower_integer_multiplication in the > > back-end will do nothing whatsoever, and we'll emit an illegal opcode > > which will probably hang the GPU. For 8 and 16, it's easy enough to > > lower to a couple of conversions, a N*2-bit multiply, and a shift. It's > > also not obvious where the cut-off point for the optimization is. > > Certainly, it's better in 64-bits than doing the division algorithm in > > the shader and I think it's better for 32 but maybe not in 8 and 16? > > I'm not sure. I'm pretty sure my 32-bit benchmark gave positive results > > (about 40-50% faster) but it was very noisy. > > > > I don't think anything allows 8 and 16-bit arithmetic right now. Still, > > should probably fix it... > > Hm... if an extension adds GL or Vulkan support for 16-bit arithmetic, I > doubt it would add [iu]mul_high (e.g., GL_AMD_gpu_shader_int16). I'd > expect every GPU would support a 16x16=32 multiplier. Would it be > better to restrict this instruction to 32- and 64-bit and implement the > integer division optimization differently for 8- and 16-bit sources? > We don't currently have a mechanism in NIR to restrict an instruction to a particular set of bit-widths. Maybe we should consider adding that? As far as the optimization goes, I see a few different options; 1) Add a few lines in opt_algebraic to do the lowering. We could even make it unconditional for now. 2) Add a couple tiny helpers that switch on bit-width in opt_idiv_const and do the lowering on-the-fly. 3) Just disable the pass for 8 and 16-bit for now. It's questionable whether it's actually worth emitting 4-6 instructions for a simple division at those bit sizes anyway. I think my order of preference is 3 > 1 > 2. Especially when you consider that we have no good way to test the pass for anything other than 32 and 64-bit right now. --Jason
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