[Mesa-dev] [PATCH 4/4] RFC: nir: add lowering for idiv/udiv/umod
Rob Clark
robdclark at gmail.com
Tue Mar 31 18:44:27 PDT 2015
On Tue, Mar 31, 2015 at 9:03 PM, Roland Scheidegger <sroland at vmware.com> wrote:
> Am 01.04.2015 um 00:57 schrieb Rob Clark:
>> From: Rob Clark <robclark at freedesktop.org>
>>
>> Based on the algo from NV50LegalizeSSA::handleDIV() and handleMOD().
>> See also trans_idiv() in freedreno/ir3/ir3_compiler.c (which was an
>> adaptation of the nv50 code from Ilia).
>>
>> Just sending as an rfc right now, since I'm not quite at the point to be
>> able to test it on actual hw.
>>
>> Signed-off-by: Rob Clark <robclark at freedesktop.org>
>> ---
>> src/glsl/Makefile.sources | 1 +
>> src/glsl/nir/nir.h | 1 +
>> src/glsl/nir/nir_lower_idiv.c | 212 ++++++++++++++++++++++++++++++++++++++++++
>> 3 files changed, 214 insertions(+)
>> create mode 100644 src/glsl/nir/nir_lower_idiv.c
>>
>> diff --git a/src/glsl/Makefile.sources b/src/glsl/Makefile.sources
>> index 18fff38..e426970 100644
>> --- a/src/glsl/Makefile.sources
>> +++ b/src/glsl/Makefile.sources
>> @@ -32,6 +32,7 @@ NIR_FILES = \
>> nir/nir_lower_atomics.c \
>> nir/nir_lower_global_vars_to_local.c \
>> nir/nir_lower_locals_to_regs.c \
>> + nir/nir_lower_idiv.c \
>> nir/nir_lower_io.c \
>> nir/nir_lower_phis_to_scalar.c \
>> nir/nir_lower_samplers.cpp \
>> diff --git a/src/glsl/nir/nir.h b/src/glsl/nir/nir.h
>> index cd03d6b..e002d6f 100644
>> --- a/src/glsl/nir/nir.h
>> +++ b/src/glsl/nir/nir.h
>> @@ -1605,6 +1605,7 @@ void nir_lower_samplers(nir_shader *shader,
>>
>> void nir_lower_system_values(nir_shader *shader);
>> void nir_lower_tex_projector(nir_shader *shader);
>> +void nir_lower_idiv(nir_shader *shader);
>>
>> void nir_lower_atomics(nir_shader *shader);
>> void nir_lower_to_source_mods(nir_shader *shader);
>> diff --git a/src/glsl/nir/nir_lower_idiv.c b/src/glsl/nir/nir_lower_idiv.c
>> new file mode 100644
>> index 0000000..e95c57e
>> --- /dev/null
>> +++ b/src/glsl/nir/nir_lower_idiv.c
>> @@ -0,0 +1,212 @@
>> +/*
>> + * Copyright © 2015 Red Hat
>> + *
>> + * Permission is hereby granted, free of charge, to any person obtaining a
>> + * copy of this software and associated documentation files (the "Software"),
>> + * to deal in the Software without restriction, including without limitation
>> + * the rights to use, copy, modify, merge, publish, distribute, sublicense,
>> + * and/or sell copies of the Software, and to permit persons to whom the
>> + * Software is furnished to do so, subject to the following conditions:
>> + *
>> + * The above copyright notice and this permission notice (including the next
>> + * paragraph) shall be included in all copies or substantial portions of the
>> + * Software.
>> + *
>> + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
>> + * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
>> + * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
>> + * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
>> + * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
>> + * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
>> + * IN THE SOFTWARE.
>> + *
>> + * Authors:
>> + * Rob Clark <robclark at freedesktop.org>
>> + */
>> +
>> +#include "nir.h"
>> +#include "nir_builder.h"
>> +
>> +/* Lowers idiv/udiv/umod
>> + * Based on NV50LegalizeSSA::handleDIV()
>> + *
>> + * Note that this is probably not enough precision for compute shaders.
>> + * Perhaps we want a second higher precision (looping) version of this?
>> + * Or perhaps we assume if you can do compute shaders you can also
>> + * branch out to a pre-optimized shader library routine..
>
> So if this is not enough precision, maybe should state how large the
> error can be?
>
tbh, if I knew what the error for this approach was, I would have
included it. I'm not the original author, but this is based on
nouveau codegen code (as mentioned in the comment). I guess it is
better than converting to float and dividing and converting back, but
worse than an iterative (ie. looping, ie. divergent flow control)
approach. It is apparently enough to keep piglit happy.
The original algo in nv50 lowering code is from
322bc7ed68ed92233c97168c036d0aa50c11a20e (ie. 'nv50/ir: import nv50
target') which doesn't really give more clue about the origin..
if anyone knows, I'm all ears and will add relevant links/info to comment..
BR,
-R
> Roland
>
>> + *
>> + * vec4 af, bf, a, b, q, r;
>> + *
>> + * if (op == idiv) {
>> + * af = i2f numerator
>> + * bf = i2f denominator
>> + * af = fabs af
>> + * bf = fabs bf
>> + * a = iabs numerator
>> + * b = iabs denominator
>> + * } else {
>> + * af = u2f numerator
>> + * bf = u2f denominator
>> + * a = numerator
>> + * b = denominator
>> + * }
>> + *
>> + * ; get first result
>> + * bf = frcp bf
>> + * bf = iadd bf, -2 ; yes, subtract 2 as integer from float
>> + * q = fmul af, bf
>> + *
>> + * if (op == idiv) {
>> + * q = f2i q
>> + * } else {
>> + * q = f2u q
>> + * }
>> + *
>> + * ; get error of first result:
>> + * r = imul q, b
>> + * r = isub a, r
>> + * r = u2f r
>> + *
>> + * r = fmul r, bf
>> + * r = f2u r
>> + * q = iadd q, r
>> + *
>> + * ; correction: if modulus >= divisor, add 1
>> + * t = imul q, b
>> + * r = isub a, t
>> + *
>> + * r = fge r, b
>> + * r = b2i r
>> + *
>> + * q = isub q, r
>> + * if (op == idiv) {
>> + * r = ixor a, b
>> + * r = ushr r, 31
>> + * r = i2b r
>> + * b = ineg q
>> + * q = bcsel r, b, q
>> + * }
>> + *
>> + * if (op == umod) {
>> + * ; division result in q
>> + * r = imul q, b
>> + * q = isub a, r
>> + * }
>> + *
>> + * dst = q
>> + */
>> +
>> +static void
>> +convert_instr(nir_builder *bld, nir_alu_instr *alu)
>> +{
>> + nir_ssa_def *numer, *denom, *af, *bf, *a, *b, *q, *r;
>> + nir_op op = alu->op;
>> +
>> + if ((op != nir_op_idiv) &&
>> + (op != nir_op_udiv) &&
>> + (op != nir_op_umod))
>> + return;
>> +
>> + nir_builder_insert_before_instr(bld, &alu->instr);
>> +
>> + numer = nir_ssa_for_src(bld, alu->src[0].src,
>> + nir_ssa_alu_instr_src_components(alu, 0));
>> + denom = nir_ssa_for_src(bld, alu->src[1].src,
>> + nir_ssa_alu_instr_src_components(alu, 1));
>> +
>> + if (op == nir_op_idiv) {
>> + af = nir_i2f(bld, numer);
>> + bf = nir_i2f(bld, denom);
>> + af = nir_fabs(bld, af);
>> + bf = nir_fabs(bld, bf);
>> + a = nir_iabs(bld, numer);
>> + b = nir_iabs(bld, denom);
>> + } else {
>> + af = nir_u2f(bld, numer);
>> + bf = nir_u2f(bld, denom);
>> + a = numer;
>> + b = denom;
>> + }
>> +
>> + /* get first result: */
>> + bf = nir_frcp(bld, bf);
>> + bf = nir_iadd(bld, bf, nir_imm_int(bld, -2)); /* yes, sub 2 as int from float */
>> + q = nir_fmul(bld, af, bf);
>> +
>> + if (op == nir_op_idiv) {
>> + q = nir_f2i(bld, q);
>> + } else {
>> + q = nir_f2u(bld, q);
>> + }
>> +
>> + /* get error of first result: */
>> + r = nir_imul(bld, q, b);
>> + r = nir_isub(bld, a, r);
>> + r = nir_u2f(bld, r);
>> +
>> + r = nir_fmul(bld, r, bf);
>> + r = nir_f2u(bld, r);
>> +
>> + /* correction: if modulus >= divisor, add 1 */
>> + r = nir_imul(bld, q, b);
>> + r = nir_isub(bld, a, r);
>> +
>> + r = nir_fge(bld, r, b);
>> + r = nir_b2i(bld, r);
>> +
>> + q = nir_isub(bld, q, r);
>> + if (op == nir_op_idiv) { /* ie. udiv or umod */
>> + r = nir_ixor(bld, a, b);
>> + r = nir_ushr(bld, r, nir_imm_int(bld, 31));
>> + r = nir_i2b(bld, r);
>> + b = nir_ineg(bld, q);
>> + q = nir_bcsel(bld, r, b, q);
>> + }
>> +
>> + if (op == nir_op_umod) {
>> + /* division result in q */
>> + r = nir_imul(bld, q, b);
>> + q = nir_isub(bld, a, r);
>> + }
>> +
>> + /* not quite sure if frob'ing the existing instr is what we
>> + * are *supposed* to do.. but wasn't sure a better way:
>> + */
>> + alu->op = nir_op_imov;
>> + nir_instr_rewrite_src(&alu->instr, &alu->src[0].src, nir_src_for_ssa(q));
>> +}
>> +
>> +static bool
>> +convert_block(nir_block *block, void *state)
>> +{
>> + nir_builder *b = state;
>> +
>> + nir_foreach_instr_safe(block, instr) {
>> + if (instr->type == nir_instr_type_alu)
>> + convert_instr(b, nir_instr_as_alu(instr));
>> + }
>> +
>> + return true;
>> +}
>> +
>> +static void
>> +convert_impl(nir_function_impl *impl)
>> +{
>> + nir_builder b;
>> + nir_builder_init(&b, impl);
>> +
>> + nir_foreach_block(impl, convert_block, &b);
>> + nir_metadata_preserve(impl, nir_metadata_block_index |
>> + nir_metadata_dominance);
>> +}
>> +
>> +void
>> +nir_lower_idiv(nir_shader *shader)
>> +{
>> + nir_foreach_overload(shader, overload) {
>> + if (overload->impl)
>> + convert_impl(overload->impl);
>> + }
>> +
>> + exec_list_make_empty(&shader->system_values);
>> +}
>>
>
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