[Mesa-dev] [PATCH 04/13] nir: add a pass to lower some double operations
Samuel Iglesias Gonsálvez
siglesias at igalia.com
Wed Apr 20 06:04:40 UTC 2016
On 19/04/16 23:52, Jason Ekstrand wrote:
> On Tue, Apr 12, 2016 at 1:05 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
>>
>
> We're not setting them to 1023, we're setting it to exp.
>
Right.
>
>> + */
>> + nir_ssa_def *new_hi = nir_bfi(b, nir_imm_uint(b, 0x7ff00000),
>> + exp, hi);
>>
>
> Does this really need a line-wrap? It looks shorter than the comment above.
>
Right. I will undo the line-wrap.
>
>> + /* 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);
>> +
>> + /* 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);
>>
>
> Just make this pack_double(b, nir_vec2(b, )). No need for the temporary.
>
> Other than that
>
> Reviewed-by: Jason Ekstrand <jason at jlekstrand.net>
>
OK, thanks!
Sam
>
>> +}
>> +
>> +/*
>> + * 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
>>
>> _______________________________________________
>> mesa-dev mailing list
>> mesa-dev at lists.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
>>
>
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