[Mesa-dev] [PATCH 07/12] nir: Allow [iu]mul_high on non-32-bit types

[Jason Ekstrand]

On Mon, Oct 8, 2018 at 3:46 PM Ian Romanick <idr at 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...

--Jason


> > +if (bit_size == 64) {
> > +   /* We need to do a full 128-bit x 128-bit multiply in order for the
> sign
> > +    * extension to work properly.  The casts are kind-of annoying but
> needed
> > +    * to prevent compiler warnings.
> > +    */
> > +   uint32_t src0_u32[4] = {
> > +      src0,
> > +      (int64_t)src0 >> 32,
> > +      (int64_t)src0 >> 63,
> > +      (int64_t)src0 >> 63,
> > +   };
> > +   uint32_t src1_u32[4] = {
> > +      src1,
> > +      (int64_t)src1 >> 32,
> > +      (int64_t)src1 >> 63,
> > +      (int64_t)src1 >> 63,
> > +   };
> > +   uint32_t prod_u32[4];
> > +   ubm_mul_u32arr(prod_u32, src0_u32, src1_u32);
> > +   dst = (uint64_t)prod_u32[2] | ((uint64_t)prod_u32[3] << 32);
> > +} else {
> > +   dst = ((int64_t)src0 * (int64_t)src1) >> bit_size;
> > +}
> > +""")
> > +
> >  # high 32-bits of unsigned integer multiply
> > -binop("umul_high", tuint32, commutative,
> > -      "(uint32_t)(((uint64_t) src0 * (uint64_t) src1) >> 32)")
> > +binop("umul_high", tuint, commutative, """
> > +if (bit_size == 64) {
> > +   /* The casts are kind-of annoying but needed to prevent compiler
> warnings. */
> > +   uint32_t src0_u32[2] = { src0, (uint64_t)src0 >> 32 };
> > +   uint32_t src1_u32[2] = { src1, (uint64_t)src1 >> 32 };
> > +   uint32_t prod_u32[4];
> > +   ubm_mul_u32arr(prod_u32, src0_u32, src1_u32);
> > +   dst = (uint64_t)prod_u32[2] | ((uint64_t)prod_u32[3] << 32);
> > +} else {
> > +   dst = ((uint64_t)src0 * (uint64_t)src1) >> bit_size;
> > +}
> > +""")
> >
> >  binop("fdiv", tfloat, "", "src0 / src1")
> >  binop("idiv", tint, "", "src1 == 0 ? 0 : (src0 / src1)")
>
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