[Mesa-dev] [PATCH 04/13] nir: add a pass to lower some double operations
Connor Abbott
cwabbott0 at gmail.com
Wed Apr 20 01:45:32 UTC 2016
On Tue, Apr 12, 2016 at 4:05 AM, Samuel Iglesias Gonsálvez
<siglesias at igalia.com> wrote:
> From: Connor Abbott <connor.w.abbott at intel.com>
>
> v2: Move to compiler/nir (Iago)
>
> Signed-off-by: Iago Toral Quiroga <itoral at igalia.com>
> ---
> src/compiler/Makefile.sources | 1 +
> src/compiler/nir/nir.h | 7 +
> src/compiler/nir/nir_lower_double_ops.c | 387 ++++++++++++++++++++++++++++++++
> 3 files changed, 395 insertions(+)
> create mode 100644 src/compiler/nir/nir_lower_double_ops.c
>
> diff --git a/src/compiler/Makefile.sources b/src/compiler/Makefile.sources
> index 6f09abf..db7ca3b 100644
> --- a/src/compiler/Makefile.sources
> +++ b/src/compiler/Makefile.sources
> @@ -187,6 +187,7 @@ NIR_FILES = \
> nir/nir_lower_alu_to_scalar.c \
> nir/nir_lower_atomics.c \
> nir/nir_lower_clip.c \
> + nir/nir_lower_double_ops.c \
> nir/nir_lower_double_packing.c \
> nir/nir_lower_global_vars_to_local.c \
> nir/nir_lower_gs_intrinsics.c \
> diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
> index ebac750..434d92b 100644
> --- a/src/compiler/nir/nir.h
> +++ b/src/compiler/nir/nir.h
> @@ -2282,6 +2282,13 @@ void nir_lower_to_source_mods(nir_shader *shader);
>
> bool nir_lower_gs_intrinsics(nir_shader *shader);
>
> +typedef enum {
> + nir_lower_drcp = (1 << 0),
> + nir_lower_dsqrt = (1 << 1),
> + nir_lower_drsq = (1 << 2),
> +} nir_lower_doubles_options;
> +
> +void nir_lower_doubles(nir_shader *shader, nir_lower_doubles_options options);
> void nir_lower_double_pack(nir_shader *shader);
>
> bool nir_normalize_cubemap_coords(nir_shader *shader);
> diff --git a/src/compiler/nir/nir_lower_double_ops.c b/src/compiler/nir/nir_lower_double_ops.c
> new file mode 100644
> index 0000000..4cd153c
> --- /dev/null
> +++ b/src/compiler/nir/nir_lower_double_ops.c
> @@ -0,0 +1,387 @@
> +/*
> + * Copyright © 2015 Intel Corporation
> + *
> + * 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.
> + *
> + */
> +
> +#include "nir.h"
> +#include "nir_builder.h"
> +#include "c99_math.h"
> +
> +/*
> + * Lowers some unsupported double operations, using only:
> + *
> + * - pack/unpackDouble2x32
> + * - conversion to/from single-precision
> + * - double add, mul, and fma
> + * - conditional select
> + * - 32-bit integer and floating point arithmetic
> + */
> +
> +/* Creates a double with the exponent bits set to a given integer value */
> +static nir_ssa_def *
> +set_exponent(nir_builder *b, nir_ssa_def *src, nir_ssa_def *exp)
> +{
> + /* Split into bits 0-31 and 32-63 */
> + nir_ssa_def *lo = nir_unpack_double_2x32_split_x(b, src);
> + nir_ssa_def *hi = nir_unpack_double_2x32_split_y(b, src);
> +
> + /* The exponent is bits 52-62, or 20-30 of the high word, so set those bits
> + * to 1023
> + */
> + nir_ssa_def *new_hi = nir_bfi(b, nir_imm_uint(b, 0x7ff00000),
> + exp, hi);
> + /* recombine */
> + return nir_pack_double_2x32_split(b, lo, new_hi);
> +}
> +
> +static nir_ssa_def *
> +get_exponent(nir_builder *b, nir_ssa_def *src)
> +{
> + /* get bits 32-63 */
> + nir_ssa_def *hi = nir_unpack_double_2x32_split_y(b, src);
> +
> + /* extract bits 20-30 of the high word */
> + return nir_ubitfield_extract(b, hi, nir_imm_int(b, 20), nir_imm_int(b, 11));
> +}
> +
> +/* Return infinity with the sign of the given source which is +/-0 */
> +
> +static nir_ssa_def *
> +get_signed_inf(nir_builder *b, nir_ssa_def *zero)
> +{
> + nir_ssa_def *zero_split = nir_unpack_double_2x32(b, zero);
> + nir_ssa_def *zero_hi = nir_swizzle(b, zero_split, (unsigned[]) {1}, 1, false);
This should be using nir_unpack_double_2x32_split_y and
nir_pack_double_2x32_split, or else it won't scalarize correctly. I'm
surprised a piglit test didn't catch this.
> +
> + /* The bit pattern for infinity is 0x7ff0000000000000, where the sign bit
> + * is the highest bit. Only the sign bit can be non-zero in the passed in
> + * source. So we essentially need to OR the infinity and the zero, except
> + * the low 32 bits are always 0 so we can construct the correct high 32
> + * bits and then pack it together with zero low 32 bits.
> + */
> + nir_ssa_def *inf_hi = nir_ior(b, nir_imm_uint(b, 0x7ff00000), zero_hi);
> + nir_ssa_def *inf_split = nir_vec2(b, nir_imm_int(b, 0), inf_hi);
> + return nir_pack_double_2x32(b, inf_split);
> +}
> +
> +/*
> + * Generates the correctly-signed infinity if the source was zero, and flushes
> + * the result to 0 if the source was infinity or the calculated exponent was
> + * too small to be representable.
> + */
> +
> +static nir_ssa_def *
> +fix_inv_result(nir_builder *b, nir_ssa_def *res, nir_ssa_def *src,
> + nir_ssa_def *exp)
> +{
> + /* If the exponent is too small or the original input was infinity/NaN,
> + * force the result to 0 (flush denorms) to avoid the work of handling
> + * denorms properly. Note that this doesn't preserve positive/negative
> + * zeros, but GLSL doesn't require it.
> + */
> + res = nir_bcsel(b, nir_ior(b, nir_ige(b, nir_imm_int(b, 0), exp),
> + nir_feq(b, nir_fabs(b, src),
> + nir_imm_double(b, INFINITY))),
> + nir_imm_double(b, 0.0f), res);
> +
> + /* If the original input was 0, generate the correctly-signed infinity */
> + res = nir_bcsel(b, nir_fne(b, src, nir_imm_double(b, 0.0f)),
> + res, get_signed_inf(b, src));
> +
> + return res;
> +
> +}
> +
> +static nir_ssa_def *
> +lower_rcp(nir_builder *b, nir_ssa_def *src)
> +{
> + /* normalize the input to avoid range issues */
> + nir_ssa_def *src_norm = set_exponent(b, src, nir_imm_int(b, 1023));
> +
> + /* cast to float, do an rcp, and then cast back to get an approximate
> + * result
> + */
> + nir_ssa_def *ra = nir_f2d(b, nir_frcp(b, nir_d2f(b, src_norm)));
> +
> + /* Fixup the exponent of the result - note that we check if this is too
> + * small below.
> + */
> + nir_ssa_def *new_exp = nir_isub(b, get_exponent(b, ra),
> + nir_isub(b, get_exponent(b, src),
> + nir_imm_int(b, 1023)));
> +
> + ra = set_exponent(b, ra, new_exp);
> +
> + /* Do a few Newton-Raphson steps to improve precision.
> + *
> + * Each step doubles the precision, and we started off with around 24 bits,
> + * so we only need to do 2 steps to get to full precision. The step is:
> + *
> + * x_new = x * (2 - x*src)
> + *
> + * But we can re-arrange this to improve precision by using another fused
> + * multiply-add:
> + *
> + * x_new = x + x * (1 - x*src)
> + *
> + * See https://en.wikipedia.org/wiki/Division_algorithm for more details.
> + */
> +
> + 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);
> +
> + return fix_inv_result(b, ra, src, new_exp);
> +}
> +
> +static nir_ssa_def *
> +lower_sqrt_rsq(nir_builder *b, nir_ssa_def *src, bool sqrt)
> +{
> + /* We want to compute:
> + *
> + * 1/sqrt(m * 2^e)
> + *
> + * When the exponent is even, this is equivalent to:
> + *
> + * 1/sqrt(m) * 2^(-e/2)
> + *
> + * and then the exponent is odd, this is equal to:
> + *
> + * 1/sqrt(m * 2) * 2^(-(e - 1)/2)
> + *
> + * where the m * 2 is absorbed into the exponent. So we want the exponent
> + * inside the square root to be 1 if e is odd and 0 if e is even, and we
> + * want to subtract off e/2 from the final exponent, rounded to negative
> + * infinity. We can do the former by first computing the unbiased exponent,
> + * and then AND'ing it with 1 to get 0 or 1, and we can do the latter by
> + * shifting right by 1.
> + */
> +
> + nir_ssa_def *unbiased_exp = nir_isub(b, get_exponent(b, src),
> + nir_imm_int(b, 1023));
> + nir_ssa_def *even = nir_iand(b, unbiased_exp, nir_imm_int(b, 1));
> + nir_ssa_def *half = nir_ishr(b, unbiased_exp, nir_imm_int(b, 1));
> +
> + nir_ssa_def *src_norm = set_exponent(b, src,
> + nir_iadd(b, nir_imm_int(b, 1023),
> + even));
> +
> + nir_ssa_def *ra = nir_f2d(b, nir_frsq(b, nir_d2f(b, src_norm)));
> + nir_ssa_def *new_exp = nir_isub(b, get_exponent(b, ra), half);
> + ra = set_exponent(b, ra, new_exp);
> +
> + /*
> + * The following implements an iterative algorithm that's very similar
> + * between sqrt and rsqrt. We start with an iteration of Goldschmit's
> + * algorithm, which looks like:
> + *
> + * a = the source
> + * y_0 = initial (single-precision) rsqrt estimate
> + *
> + * h_0 = .5 * y_0
> + * g_0 = a * y_0
> + * r_0 = .5 - h_0 * g_0
> + * g_1 = g_0 * r_0 + g_0
> + * h_1 = h_0 * r_0 + h_0
> + *
> + * Now g_1 ~= sqrt(a), and h_1 ~= 1/(2 * sqrt(a)). We could continue
> + * applying another round of Goldschmit, but since we would never refer
> + * back to a (the original source), we would add too much rounding error.
> + * So instead, we do one last round of Newton-Raphson, which has better
> + * rounding characteristics, to get the final rounding correct. This is
> + * split into two cases:
> + *
> + * 1. sqrt
> + *
> + * Normally, doing a round of Newton-Raphson for sqrt involves taking a
> + * reciprocal of the original estimate, which is slow since it isn't
> + * supported in HW. But we can take advantage of the fact that we already
> + * computed a good estimate of 1/(2 * g_1) by rearranging it like so:
> + *
> + * g_2 = .5 * (g_1 + a / g_1)
> + * = g_1 + .5 * (a / g_1 - g_1)
> + * = g_1 + (.5 / g_1) * (a - g_1^2)
> + * = g_1 + h_1 * (a - g_1^2)
> + *
> + * The second term represents the error, and by splitting it out we can get
> + * better precision by computing it as part of a fused multiply-add. Since
> + * both Newton-Raphson and Goldschmit approximately double the precision of
> + * the result, these two steps should be enough.
> + *
> + * 2. rsqrt
> + *
> + * First off, note that the first round of the Goldschmit algorithm is
> + * really just a Newton-Raphson step in disguise:
> + *
> + * h_1 = h_0 * (.5 - h_0 * g_0) + h_0
> + * = h_0 * (1.5 - h_0 * g_0)
> + * = h_0 * (1.5 - .5 * a * y_0^2)
> + * = (.5 * y_0) * (1.5 - .5 * a * y_0^2)
> + *
> + * which is the standard formula multiplied by .5. Unlike in the sqrt case,
> + * we don't need the inverse to do a Newton-Raphson step; we just need h_1,
> + * so we can skip the calculation of g_1. Instead, we simply do another
> + * Newton-Raphson step:
> + *
> + * y_1 = 2 * h_1
> + * r_1 = .5 - h_1 * y_1 * a
> + * y_2 = y_1 * r_1 + y_1
> + *
> + * Where the difference from Goldschmit is that we calculate y_1 * a
> + * instead of using g_1. Doing it this way should be as fast as computing
> + * y_1 up front instead of h_1, and it lets us share the code for the
> + * initial Goldschmit step with the sqrt case.
> + *
> + * Putting it together, the computations are:
> + *
> + * h_0 = .5 * y_0
> + * g_0 = a * y_0
> + * r_0 = .5 - h_0 * g_0
> + * h_1 = h_0 * r_0 + h_0
> + * if sqrt:
> + * g_1 = g_0 * r_0 + g_0
> + * r_1 = a - g_1 * g_1
> + * g_2 = h_1 * r_1 + g_1
> + * else:
> + * y_1 = 2 * h_1
> + * r_1 = .5 - y_1 * (h_1 * a)
> + * y_2 = y_1 * r_1 + y_1
> + *
> + * For more on the ideas behind this, see "Software Division and Square
> + * Root Using Goldschmit's Algorithms" by Markstein and the Wikipedia page
> + * on square roots
> + * (https://en.wikipedia.org/wiki/Methods_of_computing_square_roots).
> + */
> +
> + 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);
> + nir_ssa_def *r_0 = nir_ffma(b, nir_fneg(b, h_0), g_0, one_half);
> + nir_ssa_def *h_1 = nir_ffma(b, h_0, r_0, h_0);
> + nir_ssa_def *res;
> + if (sqrt) {
> + nir_ssa_def *g_1 = nir_ffma(b, g_0, r_0, g_0);
> + nir_ssa_def *r_1 = nir_ffma(b, nir_fneg(b, g_1), g_1, src);
> + res = nir_ffma(b, h_1, r_1, g_1);
> + } else {
> + nir_ssa_def *y_1 = nir_fmul(b, nir_imm_double(b, 2.0), h_1);
> + nir_ssa_def *r_1 = nir_ffma(b, nir_fneg(b, y_1), nir_fmul(b, h_1, src),
> + one_half);
> + res = nir_ffma(b, y_1, r_1, y_1);
> + }
> +
> + if (sqrt) {
> + /* Here, the special cases we need to handle are
> + * 0 -> 0 and
> + * +inf -> +inf
> + */
> + res = nir_bcsel(b, nir_ior(b, nir_feq(b, src, nir_imm_double(b, 0.0)),
> + nir_feq(b, src, nir_imm_double(b, INFINITY))),
> + src, res);
> + } else {
> + res = fix_inv_result(b, res, src, new_exp);
> + }
> +
> + return res;
> +}
> +
> +static void
> +lower_doubles_instr(nir_alu_instr *instr, nir_lower_doubles_options options)
> +{
> + assert(instr->dest.dest.is_ssa);
> + if (instr->dest.dest.ssa.bit_size != 64)
> + return;
> +
> + switch (instr->op) {
> + case nir_op_frcp:
> + if (!(options & nir_lower_drcp))
> + return;
> + break;
> +
> + case nir_op_fsqrt:
> + if (!(options & nir_lower_dsqrt))
> + return;
> + break;
> +
> + case nir_op_frsq:
> + if (!(options & nir_lower_drsq))
> + return;
> + break;
> +
> + default:
> + return;
> + }
> +
> + nir_builder bld;
> + nir_builder_init(&bld, nir_cf_node_get_function(&instr->instr.block->cf_node));
> + bld.cursor = nir_before_instr(&instr->instr);
> +
> + nir_ssa_def *src = nir_fmov_alu(&bld, instr->src[0],
> + instr->dest.dest.ssa.num_components);
> +
> + nir_ssa_def *result;
> +
> + switch (instr->op) {
> + case nir_op_frcp:
> + result = lower_rcp(&bld, src);
> + break;
> + case nir_op_fsqrt:
> + result = lower_sqrt_rsq(&bld, src, true);
> + break;
> + case nir_op_frsq:
> + result = lower_sqrt_rsq(&bld, src, false);
> + break;
> + default:
> + unreachable("unhandled opcode");
> + }
> +
> + nir_ssa_def_rewrite_uses(&instr->dest.dest.ssa, nir_src_for_ssa(result));
> + nir_instr_remove(&instr->instr);
> +}
> +
> +static bool
> +lower_doubles_block(nir_block *block, void *ctx)
> +{
> + nir_lower_doubles_options options = *((nir_lower_doubles_options *) ctx);
> +
> + nir_foreach_instr_safe(block, instr) {
> + if (instr->type != nir_instr_type_alu)
> + continue;
> +
> + lower_doubles_instr(nir_instr_as_alu(instr), options);
> + }
> +
> + return true;
> +}
> +
> +static void
> +lower_doubles_impl(nir_function_impl *impl, nir_lower_doubles_options options)
> +{
> + nir_foreach_block(impl, lower_doubles_block, &options);
> +}
> +
> +void
> +nir_lower_doubles(nir_shader *shader, nir_lower_doubles_options options)
> +{
> + nir_foreach_function(shader, function) {
> + if (function->impl)
> + lower_doubles_impl(function->impl, options);
> + }
> +}
> --
> 2.5.0
>
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