Mesa (master): nir: add a pass to lower some double operations
Samuel Iglesias Gonsálvez
samuelig at kemper.freedesktop.org
Thu Apr 28 10:02:38 UTC 2016
Module: Mesa
Branch: master
Commit: 2ea3649c63f39f07b9cebda1ef0569b7347ada93
URL: http://cgit.freedesktop.org/mesa/mesa/commit/?id=2ea3649c63f39f07b9cebda1ef0569b7347ada93
Author: Connor Abbott <connor.w.abbott at intel.com>
Date: Fri Jul 31 11:57:48 2015 -0700
nir: add a pass to lower some double operations
v2: Move to compiler/nir (Iago)
v3: Use nir_imm_int() to load the constants (Sam)
v4 (Sam):
- Undo line-wrap (Jason).
- Fix comment (Jason).
- Improve generated code for get_signed_inf() function (Connor).
Signed-off-by: Samuel Iglesias Gonsálvez <siglesias at igalia.com>
Reviewed-by: Jason Ekstrand <jason at jlekstrand.net>
---
src/compiler/Makefile.sources | 1 +
src/compiler/nir/nir.h | 7 +
src/compiler/nir/nir_lower_double_ops.c | 384 ++++++++++++++++++++++++++++++++
3 files changed, 392 insertions(+)
diff --git a/src/compiler/Makefile.sources b/src/compiler/Makefile.sources
index 1973533..8915943 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 b23130e..cbbf47e 100644
--- a/src/compiler/nir/nir.h
+++ b/src/compiler/nir/nir.h
@@ -2413,6 +2413,13 @@ 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..e22e822
--- /dev/null
+++ b/src/compiler/nir/nir_lower_double_ops.c
@@ -0,0 +1,384 @@
+/*
+ * 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 the exponent
+ * to 1023
+ */
+ nir_ssa_def *new_hi = nir_bfi(b, nir_imm_int(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_hi = nir_unpack_double_2x32_split_y(b, zero);
+
+ /* 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_int(b, 0x7ff00000), zero_hi);
+ return nir_pack_double_2x32_split(b, nir_imm_int(b, 0), inf_hi);
+}
+
+/*
+ * 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_call(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);
+ }
+}
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