Unnecessary GRF bank conflicts increase the issue time of ternary
instructions (the overwhelmingly most common of which is MAD) by
roughly 50%, leading to reduced ALU throughput. This pass attempts to
minimize the number of bank conflicts by rearranging the layout of the
GRF space post-register allocation. It's in general not possible to
eliminate all of them without introducing extra copies, which are
typically more expensive than the bank conflict itself.
In a shader-db run on SKL this helps roughly 46k shaders:
total conflicts in shared programs: 1008981 -> 600461 (-40.49%)
conflicts in affected programs: 816222 -> 407702 (-50.05%)
helped: 46234
HURT: 72
The running time of shader-db itself on SKL seems to be increased by
roughly 2.52%±1.13% with n=20 due to the additional work done by the
compiler back-end.
On earlier generations the pass is somewhat less effective in relative
terms because the hardware incurs a bank conflict anytime the last two
sources of the instruction are duplicate (e.g. while trying to square
a value using MAD), which is impossible to avoid without introducing
copies. E.g. for a shader-db run on SNB:
total conflicts in shared programs: 944636 -> 623185 (-34.03%)
conflicts in affected programs: 853258 -> 531807 (-37.67%)
helped: 31052
HURT: 19
And on BDW:
total conflicts in shared programs: 1418393 -> 987539 (-30.38%)
conflicts in affected programs: 1179787 -> 748933 (-36.52%)
helped: 47592
HURT: 70
On SKL GT4e this improves performance of GpuTest Volplosion by 3.64%
±0.33% with n=16.
NOTE: This patch intentionally disregards some i965 coding conventions
for the sake of reviewability. This is addressed by the next
squash patch which introduces an amount of (for the most part
boring) boilerplate that might distract reviewers from the
non-trivial algorithmic details of the pass.
---
src/intel/Makefile.sources | 1 +
src/intel/compiler/brw_fs.cpp | 2 +
src/intel/compiler/brw_fs.h | 1 +
src/intel/compiler/brw_fs_bank_conflicts.cpp | 791
+++++++++++++++++++++++++++
4 files changed, 795 insertions(+)
create mode 100644 src/intel/compiler/brw_fs_bank_conflicts.cpp
diff --git a/src/intel/Makefile.sources b/src/intel/Makefile.sources
index a877ff2..1b9799a 100644
--- a/src/intel/Makefile.sources
+++ b/src/intel/Makefile.sources
@@ -44,6 +44,7 @@ COMPILER_FILES = \
compiler/brw_eu_util.c \
compiler/brw_eu_validate.c \
compiler/brw_fs_builder.h \
+ compiler/brw_fs_bank_conflicts.cpp \
compiler/brw_fs_cmod_propagation.cpp \
compiler/brw_fs_combine_constants.cpp \
compiler/brw_fs_copy_propagation.cpp \
diff --git a/src/intel/compiler/brw_fs.cpp
b/src/intel/compiler/brw_fs.cpp
index 43b6e34..0a85c0c 100644
--- a/src/intel/compiler/brw_fs.cpp
+++ b/src/intel/compiler/brw_fs.cpp
@@ -5858,6 +5858,8 @@ fs_visitor::allocate_registers(bool
allow_spilling)
if (failed)
return;
+ opt_bank_conflicts();
+
schedule_instructions(SCHEDULE_POST);
if (last_scratch > 0) {
diff --git a/src/intel/compiler/brw_fs.h b/src/intel/compiler/brw_fs.h
index 6c8c027..b1fc7b3 100644
--- a/src/intel/compiler/brw_fs.h
+++ b/src/intel/compiler/brw_fs.h
@@ -141,6 +141,7 @@ public:
exec_list *acp);
bool opt_drop_redundant_mov_to_flags();
bool opt_register_renaming();
+ bool opt_bank_conflicts();
bool register_coalesce();
bool compute_to_mrf();
bool eliminate_find_live_channel();
diff --git a/src/intel/compiler/brw_fs_bank_conflicts.cpp
b/src/intel/compiler/brw_fs_bank_conflicts.cpp
new file mode 100644
index 0000000..0225c70
--- /dev/null
+++ b/src/intel/compiler/brw_fs_bank_conflicts.cpp
@@ -0,0 +1,791 @@
+/*
+ * Copyright © 2017 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.
+ */
+
+/** @file brw_fs_bank_conflicts.cpp
+ *
+ * This file contains a GRF bank conflict mitigation pass. The pass is
+ * intended to be run after register allocation and works by
rearranging the
+ * layout of the GRF space (hopefully without altering the semantics
of the
+ * program) in a way that minimizes the number of GRF bank conflicts
incurred
+ * by ternary instructions.
+ *
+ * Unfortunately there is close to no information about bank
conflicts in the
+ * hardware spec, but experimentally on Gen7-Gen9 ternary
instructions seem to
+ * incur an average bank conflict penalty of one cycle per SIMD8 op
whenever
+ * the second and third source are stored in the same GRF bank (\sa
bank_of()
+ * for the exact bank layout) which cannot be fetched during the
same cycle by
+ * the EU, unless the EU logic manages to optimize out the read
cycle of a
+ * duplicate source register (\sa is_conflict_optimized_out()).
+ *
+ * The asymptotic run-time of the algorithm is dominated by the
+ * shader_conflict_weight_matrix() computation below, which is O(n)
on the
+ * number of instructions in the program, however for small and
medium-sized
+ * programs the run-time is likely to be dominated by
+ * optimize_reg_permutation() which is O(m^3) on the number of GRF
atoms of
+ * the program (\sa partitioning), which is bounded (since the
program uses a
+ * bounded number of registers post-regalloc) and of the order of
100. For
+ * that reason optimize_reg_permutation() is vectorized in order to
keep the
+ * cubic term within reasonable bounds for m close to its
theoretical maximum.
+ */
+
+#include "brw_fs.h"
+#include "brw_cfg.h"
+
+#include <vector>
+#include <array>
+
+#ifdef __SSE2__
+
+#include <emmintrin.h>
+
+/**
+ * Thin layer around vector intrinsics so they can be easily
replaced with
+ * e.g. the fall-back scalar path, an implementation with different
vector
+ * width or using different SIMD architectures (AVX-512?!).
+ *
+ * This implementation operates on pairs of independent SSE2 integer
vectors à
+ * la SIMD16 for somewhat improved throughput. SSE2 is supported by
virtually
+ * all platforms that care about bank conflicts, so this path should
almost
+ * always be available in practice.
+ */
+namespace {
+ /**
+ * SIMD integer vector data type.
+ */
+ typedef std::array<__m128i, 2> vector_type;
+
+ /**
+ * Scalar data type matching the representation of a single
component of \p
+ * vector_type.
+ */
+ typedef int16_t scalar_type;
+
+ /**
+ * Maximum integer value representable as a \p scalar_type.
+ */
+ const scalar_type max_scalar = INT16_MAX;
+
+ /**
+ * Number of components of a \p vector_type.
+ */
+ const unsigned vector_width = 2 * sizeof(vector_type::value_type) /
+ sizeof(scalar_type);
+
+ /**
+ * Set the i-th component of vector \p v to \p x.
+ */
+ void
+ set(vector_type &v, unsigned i, scalar_type x)
+ {
+ assert(i < vector_width);
+ memcpy((char *)v.data() + i * sizeof(x), &x, sizeof(x));
+ }
+
+ /**
+ * Get the i-th component of vector \p v.
+ */
+ scalar_type
+ get(const vector_type &v, unsigned i)
+ {
+ assert(i < vector_width);
+ scalar_type x;
+ memcpy(&x, (char *)v.data() + i * sizeof(x), sizeof(x));
+ return x;
+ }
+
+ /**
+ * Add two vectors with saturation.
+ */
+ vector_type
+ adds(const vector_type &v, const vector_type &w)
+ {
+ const vector_type u = {
+ _mm_adds_epi16(v[0], w[0]),
+ _mm_adds_epi16(v[1], w[1])
+ };
+ return u;
+ }
+
+ /**
+ * Subtract two vectors with saturation.
+ */
+ vector_type
+ subs(const vector_type &v, const vector_type &w)
+ {
+ const vector_type u = {
+ _mm_subs_epi16(v[0], w[0]),
+ _mm_subs_epi16(v[1], w[1])
+ };
+ return u;
+ }
+
+ /**
+ * Compute the bitwise conjunction of two vectors.
+ */
+ vector_type
+ mask(const vector_type &v, const vector_type &w)
+ {
+ const vector_type u = {
+ _mm_and_si128(v[0], w[0]),
+ _mm_and_si128(v[1], w[1])
+ };
+ return u;
+ }
+
+ /**
+ * Reduce the components of a vector using saturating addition.
+ */
+ scalar_type
+ sums(const vector_type &v)
+ {
+ const __m128i v8 = _mm_adds_epi16(v[0], v[1]);
+ const __m128i v4 = _mm_adds_epi16(v8, _mm_shuffle_epi32(v8,
0x4e));
+ const __m128i v2 = _mm_adds_epi16(v4, _mm_shuffle_epi32(v4,
0xb1));
+ const __m128i v1 = _mm_adds_epi16(v2, _mm_shufflelo_epi16(v2,
0xb1));
+ return _mm_extract_epi16(v1, 0);
+ }
+}
+
+#else
+
+/**
+ * Thin layer around vector intrinsics so they can be easily
replaced with
+ * e.g. the fall-back scalar path, an implementation with different
vector
+ * width or using different SIMD architectures (AVX-512?!).
+ *
+ * This implementation operates on scalar values and doesn't rely on
+ * any vector extensions. This is mainly intended for debugging and
+ * to keep this file building on exotic platforms.
+ */
+namespace {
+ /**
+ * SIMD integer vector data type.
+ */
+ typedef int16_t vector_type;
+
+ /**
+ * Scalar data type matching the representation of a single
component of \p
+ * vector_type.
+ */
+ typedef int16_t scalar_type;
+
+ /**
+ * Maximum integer value representable as a \p scalar_type.
+ */
+ const scalar_type max_scalar = INT16_MAX;
+
+ /**
+ * Number of components of a \p vector_type.
+ */
+ const unsigned vector_width = 1;
+
+ /**
+ * Set the i-th component of vector \p v to \p x.
+ */
+ void
+ set(vector_type &v, unsigned i, scalar_type x)
+ {
+ assert(i < vector_width);
+ v = x;
+ }
+
+ /**
+ * Get the i-th component of vector \p v.
+ */
+ scalar_type
+ get(const vector_type &v, unsigned i)
+ {
+ assert(i < vector_width);
+ return v;
+ }
+
+ /**
+ * Add two vectors with saturation.
+ */
+ vector_type
+ adds(vector_type v, vector_type w)
+ {
+ return std::max(INT16_MIN, std::min(INT16_MAX, int(v) + w));
+ }
+
+ /**
+ * Substract two vectors with saturation.
+ */
+ vector_type
+ subs(vector_type v, vector_type w)
+ {
+ return std::max(INT16_MIN, std::min(INT16_MAX, int(v) - w));
+ }
+
+ /**
+ * Compute the bitwise conjunction of two vectors.
+ */
+ vector_type
+ mask(vector_type v, vector_type w)
+ {
+ return v & w;
+ }
+
+ /**
+ * Reduce the components of a vector using saturating addition.
+ */
+ scalar_type
+ sums(vector_type v)
+ {
+ return v;
+ }
+}
+
+#endif
+
+namespace {
+ /**
+ * Variable-length vector type intended to represent cycle-count
costs for
+ * arbitrary atom-to-bank assignments. It's indexed by a pair of
integers
+ * (i, p), where i is an atom index and p in {0, 1} indicates the
parity of
+ * the conflict (respectively, whether the cost is incurred
whenever the
+ * atoms are assigned the same bank b or opposite-parity banks b
and b^1).
+ * \sa shader_conflict_weight_matrix()
+ */
+ typedef std::vector<vector_type> weight_vector_type;
+
+ /**
+ * Set the (i, p)-th component of weight vector \p v to \p x.
+ */
+ void
+ set(weight_vector_type &v, unsigned i, unsigned p, scalar_type x)
+ {
+ set(v[(2 * i + p) / vector_width], (2 * i + p) % vector_width,
x);
+ }
+
+ /**
+ * Get the (i, p)-th component of weight vector \p v.
+ */
+ scalar_type
+ get(const weight_vector_type &v, unsigned i, unsigned p)
+ {
+ return get(v[(2 * i + p) / vector_width], (2 * i + p) %
vector_width);
+ }
+
+ /**
+ * Swap the (i, p)-th and (j, q)-th components of weight vector
\p v.
+ */
+ void
+ swap(weight_vector_type &v,
+ unsigned i, unsigned p,
+ unsigned j, unsigned q)
+ {
+ const scalar_type tmp = get(v, i, p);
+ set(v, i, p, get(v, j, q));
+ set(v, j, q, tmp);
+ }
+}
+
+namespace {
+ /**
+ * Object that represents the partitioning of an arbitrary
register space
+ * into indivisible units (referred to as atoms below) that can
potentially
+ * be rearranged independently from other registers. The
partitioning is
+ * inferred from a number of contiguity requirements specified using
+ * require_contiguous(). This allows efficient look-up of the
atom index a
+ * given register address belongs to, or conversely the range of
register
+ * addresses that belong to a given atom.
+ */
+ struct partitioning {
+ /**
+ * Create a (for the moment unrestricted) partitioning of a
register
+ * file of size \p n. The units are arbitrary.
+ */
+ partitioning(unsigned n) {
+ for (unsigned i = 0; i < n + num_terminator_atoms; i++) {
+ offsets.push_back(i);
+ atoms.push_back(i);
+ }
+ }
+
+ /**
+ * Require register range [reg, reg + n[ to be considered part
of the
+ * same atom.
+ */
+ void
+ require_contiguous(unsigned reg, unsigned n)
+ {
+ unsigned r = atoms[reg];
+
+ /* Renumber atoms[reg...] = { r... } and their
offsets[r...] for the
+ * case that the specified contiguity requirement leads to
the fusion
+ * (yay) of one or more existing atoms.
+ */
+ for (unsigned reg1 = reg + 1; reg1 < atoms.size(); reg1++) {
+ if (offsets[atoms[reg1]] < reg + n) {
+ atoms[reg1] = r;
+ } else {
+ if (offsets[atoms[reg1 - 1]] != offsets[atoms[reg1]])
+ r++;
+
+ offsets[r] = offsets[atoms[reg1]];
+ atoms[reg1] = r;
+ }
+ }
+
+ /* Clean up the scraps if we ended up with less atoms than
we started
+ * with.
+ */
+ offsets.erase(offsets.begin() + r + 1, offsets.end());
+ }
+
+ /**
+ * Get the atom index register address \p reg belongs to.
+ */
+ unsigned
+ atom_of_reg(unsigned reg) const
+ {
+ return atoms[reg];
+ }
+
+ /**
+ * Get the base register address that belongs to atom \p r.
+ */
+ unsigned
+ reg_of_atom(unsigned r) const
+ {
+ return offsets[r];
+ }
+
+ /**
+ * Get the size of atom \p r in register address units.
+ */
+ unsigned
+ size_of_atom(unsigned r) const
+ {
+ assert(r < num_atoms());
+ return reg_of_atom(r + 1) - reg_of_atom(r);
+ }
+
+ /**
+ * Get the number of atoms the whole register space is
partitioned into.
+ */
+ unsigned
+ num_atoms() const
+ {
+ return offsets.size() - num_terminator_atoms;
+ }
+
+ private:
+ /**
+ * Number of trailing atoms inserted for convenience so among
other
+ * things we don't need to special-case the last element in
+ * size_of_atom().
+ */
+ static const unsigned num_terminator_atoms = 1;
+ std::vector<unsigned> offsets;
+ std::vector<unsigned> atoms;
+ };
+
+ /**
+ * Only GRF sources (whether they have been register-allocated or
not) can
+ * possibly incur bank conflicts.
+ */
+ bool
+ is_grf(const fs_reg &r)
+ {
+ return r.file == VGRF || r.file == FIXED_GRF;
+ }
+
+ /**
+ * Register offset of \p r in GRF units. Useful because the
representation
+ * of GRFs post-register allocation is somewhat inconsistent and
depends on
+ * whether the register already had a fixed GRF offset prior to
register
+ * allocation or whether it was part of a VGRF allocation.
+ */
+ unsigned
+ reg_of(const fs_reg &r)
+ {
+ assert(is_grf(r));
+ if (r.file == VGRF)
+ return r.nr + r.offset / REG_SIZE;
+ else
+ return reg_offset(r) / REG_SIZE;
+ }
+
+ /**
+ * Calculate the finest partitioning of the GRF space compatible
with the
+ * register contiguity requirements derived from all instructions
part of
+ * the program.
+ */
+ partitioning
+ shader_reg_partitioning(const fs_visitor *v)
+ {
+ partitioning p(BRW_MAX_GRF);
+
+ foreach_block_and_inst(block, fs_inst, inst, v->cfg) {
+ if (is_grf(inst->dst))
+ p.require_contiguous(reg_of(inst->dst),
regs_written(inst));
+
+ for (int i = 0; i < inst->sources; i++) {
+ if (is_grf(inst->src[i]))
+ p.require_contiguous(reg_of(inst->src[i]),
regs_read(inst, i));
+ }
+ }
+
+ return p;
+ }
+
+ /**
+ * Return the set of GRF atoms that should be left untouched at
their
+ * original location to avoid violating hardware or software
assumptions.
+ */
+ std::vector<bool>
+ shader_reg_constraints(const fs_visitor *v, const partitioning &p)
+ {
+ std::vector<bool> constrained(p.num_atoms());
+
+ /* These are read implicitly by some send-message instructions
without
+ * any indication at the IR level. Assume they are unsafe to
move
+ * around.
+ */
+ for (unsigned reg = 0; reg < 2; reg++)
+ constrained[p.atom_of_reg(reg)] = true;
+
+ /* Assume that anything referenced via fixed GRFs is baked
into the
+ * hardware's fixed-function logic and may be unsafe to move
around.
+ * Also take into account the source GRF restrictions of EOT
+ * send-message instructions.
+ */
+ foreach_block_and_inst(block, fs_inst, inst, v->cfg) {
+ if (inst->dst.file == FIXED_GRF)
+ constrained[p.atom_of_reg(reg_of(inst->dst))] = true;
+
+ for (int i = 0; i < inst->sources; i++) {
+ if (inst->src[i].file == FIXED_GRF ||
+ (is_grf(inst->src[i]) && inst->eot))
+ constrained[p.atom_of_reg(reg_of(inst->src[i]))] = true;
+ }
+ }
+
+ return constrained;
+ }
+
+ /**
+ * Return whether the hardware will be able to prevent a bank
conflict by
+ * optimizing out the read cycle of a source register. The
formula was
+ * found experimentally.
+ */
+ bool
+ is_conflict_optimized_out(const gen_device_info *devinfo, const
fs_inst *inst)
+ {
+ return devinfo->gen >= 9 &&
+ ((is_grf(inst->src[0]) && (reg_of(inst->src[0]) ==
reg_of(inst->src[1]) ||
+ reg_of(inst->src[0]) ==
reg_of(inst->src[2]))) ||
+ reg_of(inst->src[1]) == reg_of(inst->src[2]));
+ }
+
+ /**
+ * Return a matrix that allows reasonably efficient computation
of the
+ * cycle-count cost of bank conflicts incurred throughout the
whole program
+ * for any given atom-to-bank assignment.
+ *
+ * More precisely, if C_r_s_p is the result of this function, the
total
+ * cost of all bank conflicts involving any given atom r can be
readily
+ * recovered as follows:
+ *
+ * S(B) = Sum_s_p(d_(p^B_r)_(B_s) * C_r_s_p)
+ *
+ * where d_i_j is the Kronecker delta, and B_r indicates the bank
+ * assignment of r. \sa delta_conflicts() for a vectorized
implementation
+ * of the expression above.
+ *
+ * FINISHME: Teach this about the Gen10+ bank conflict rules,
which are
+ * somewhat more relaxed than on previous generations.
In the
+ * meantime optimizing based on Gen9 weights is likely
to be more
+ * helpful than not optimizing at all.
+ */
+ std::vector<weight_vector_type>
+ shader_conflict_weight_matrix(const fs_visitor *v, const
partitioning &p)
+ {
+ std::vector<weight_vector_type> conflicts(p.num_atoms(),
+ weight_vector_type(DIV_ROUND_UP(2 * p.num_atoms(),
+ vector_width)));
+ /* Crude approximation of the number of times the current
basic block
+ * will be executed at run-time.
+ */
+ unsigned block_scale = 1;
+
+ foreach_block_and_inst(block, fs_inst, inst, v->cfg) {
+ if (inst->opcode == BRW_OPCODE_DO) {
+ block_scale *= 10;
+
+ } else if (inst->opcode == BRW_OPCODE_WHILE) {
+ block_scale /= 10;
+
+ } else if (inst->is_3src(v->devinfo) &&
+ is_grf(inst->src[1]) && is_grf(inst->src[2])) {
+ const unsigned r = p.atom_of_reg(reg_of(inst->src[1]));
+ const unsigned s = p.atom_of_reg(reg_of(inst->src[2]));
+
+ /* Estimate of the cycle-count cost of incurring a bank
conflict
+ * for this instruction. This is only true on the
average, for a
+ * sequence of back-to-back ternary instructions, since
the EU
+ * front-end only seems to be able to issue a new
instruction at
+ * an even cycle. The cost of a bank conflict incurred
by an
+ * isolated ternary instruction may be higher.
+ */
+ const unsigned exec_size =
inst->dst.component_size(inst->exec_size);
+ const unsigned cycle_scale = block_scale *
DIV_ROUND_UP(exec_size,
+
REG_SIZE);
+
+ /* Neglect same-atom conflicts (since they're either
trivial or
+ * impossible to avoid without splitting the atom), and
conflicts
+ * known to be optimized out by the hardware.
+ */
+ if (r != s && !is_conflict_optimized_out(v->devinfo,
inst)) {
+ /* Calculate the parity of the sources relative to
the start of
+ * their respective atoms. If their parity is the
same (and
+ * none of the atoms straddle the 2KB mark), the
instruction
+ * will incur a conflict iff both atoms are assigned
the same
+ * bank b. If their parity is opposite, the
instruction will
+ * incur a conflict iff they are assigned opposite
banks (b and
+ * b^1).
+ */
+ const bool p_r = 1 & (reg_of(inst->src[1]) -
p.reg_of_atom(r));
+ const bool p_s = 1 & (reg_of(inst->src[2]) -
p.reg_of_atom(s));
+ const unsigned p = p_r ^ p_s;
+
+ /* Calculate the updated cost of a hypothetical conflict
+ * between atoms r and s. Note that the weight
matrix is
+ * symmetric with respect to indices r and s by
construction.
+ */
+ const scalar_type w = std::min(unsigned(max_scalar),
+ get(conflicts[r], s, p) + cycle_scale);
+ set(conflicts[r], s, p, w);
+ set(conflicts[s], r, p, w);
+ }
+ }
+ }
+
+ return conflicts;
+ }
+
+ /**
+ * Return the set of GRF atoms that could potentially lead to bank
+ * conflicts if laid out unfavorably in the GRF space according to
+ * the specified \p conflicts matrix (\sa
+ * shader_conflict_weight_matrix()).
+ */
+ std::vector<bool>
+ have_any_conflicts(const std::vector<weight_vector_type> &conflicts)
+ {
+ std::vector<bool> any_conflicts(conflicts.size());
+
+ for (unsigned r = 0; r < conflicts.size(); r++) {
+ for (unsigned s = 0; s < conflicts[r].size(); s++)
+ any_conflicts[r] = any_conflicts[r] ||
sums(conflicts[r][s]);
+ }
+
+ return any_conflicts;
+ }
+
+ /**
+ * Calculate the difference between two S(B) cost estimates as
defined
+ * above (\sa shader_conflict_weight_matrix()). This represents the
+ * (partial) cycle-count benefit from moving an atom r from bank
p to n.
+ * The respective bank assignments Bp and Bn are encoded as the \p
+ * bank_mask_p and \p bank_mask_n bitmasks for efficient
computation,
+ * according to the formula:
+ *
+ * bank_mask(B)_s_p = -d_(p^B_r)_(B_s)
+ *
+ * Notice the similarity with the delta function in the S(B)
expression
+ * above, and how bank_mask(B) can be precomputed for every possible
+ * selection of r since bank_mask(B) only depends on it via B_r
that may
+ * only assume one of four different values, so the caller can
keep every
+ * possible bank_mask(B) vector in memory without much hassle (\sa
+ * bank_characteristics()).
+ */
+ int
+ delta_conflicts(const weight_vector_type &bank_mask_p,
+ const weight_vector_type &bank_mask_n,
+ const weight_vector_type &conflicts)
+ {
+ vector_type s_p = {}, s_n = {};
+
+ for (unsigned r = 0; r < conflicts.size(); r++) {
+ s_p = adds(s_p, mask(bank_mask_p[r], conflicts[r]));
+ s_n = adds(s_n, mask(bank_mask_n[r], conflicts[r]));
+ }
+
+ return sums(subs(s_p, s_n));
+ }
+
+ /**
+ * Return an identity permutation of GRF atoms, represented as
the start GRF
+ * offset each atom is mapped into.
+ */
+ std::vector<unsigned>
+ identity_reg_permutation(const partitioning &p)
+ {
+ std::vector<unsigned> map(p.num_atoms());
+
+ for (unsigned r = 0; r < map.size(); r++)
+ map[r] = p.reg_of_atom(r);
+
+ return map;
+ }
+
+ /**
+ * Return the bank index of GRF address \p reg, numbered
according to the
+ * table:
+ * Even Odd
+ * Lo 0 1
+ * Hi 2 3
+ */
+ unsigned
+ bank_of(unsigned reg)
+ {
+ return (reg & 0x40) >> 5 | (reg & 1);
+ }
+
+ /**
+ * Return bitmasks suitable for use as bank mask arguments for the
+ * delta_conflicts() computation. Note that this is just the
(negative)
+ * characteristic function of each bank, if you regard it as a set
+ * containing all atoms assigned to it according to the \p map
array.
+ */
+ std::array<weight_vector_type, 4>
+ bank_characteristics(const std::vector<unsigned> &map)
+ {
+ std::array<weight_vector_type, 4> banks;
+
+ for (unsigned b = 0; b < banks.size(); b++) {
+ banks[b].resize(DIV_ROUND_UP(2 * map.size(), vector_width));
+
+ for (unsigned j = 0; j < map.size(); j++) {
+ for (unsigned p = 0; p < 2; p++)
+ set(banks[b], j, p,
+ (b ^ p) == bank_of(map[j]) ? -1 : 0);
+ }
+ }
+
+ return banks;
+ }
+
+ /**
+ * Return an improved permutation of GRF atoms based on \p map
attempting
+ * to reduce the total cycle-count cost of bank conflicts greedily.
+ *
+ * Note that this doesn't attempt to merge multiple atoms into
one, which
+ * may allow it to do a better job in some cases -- It simply
reorders
+ * existing atoms in the GRF space without affecting their identity.
+ */
+ std::vector<unsigned>
+ optimize_reg_permutation(const partitioning &p,
+ const std::vector<bool> &constrained,
+ const std::vector<weight_vector_type>
&conflicts,
+ std::vector<unsigned> map)
+ {
+ const std::vector<bool> any_conflicts =
have_any_conflicts(conflicts);
+ std::array<weight_vector_type, 4> banks =
bank_characteristics(map);
+
+ for (unsigned r = 0; r < map.size(); r++) {
+ const unsigned bank_r = bank_of(map[r]);
+
+ if (!constrained[r]) {
+ unsigned best_s = r;
+ int best_benefit = 0;
+
+ for (unsigned s = 0; s < map.size(); s++) {
+ const unsigned bank_s = bank_of(map[s]);
+
+ if (bank_r != bank_s && !constrained[s] &&
+ p.size_of_atom(r) == p.size_of_atom(s) &&
+ (any_conflicts[r] || any_conflicts[s])) {
+ const int benefit =
+ delta_conflicts(banks[bank_r], banks[bank_s],
conflicts[r]) +
+ delta_conflicts(banks[bank_s], banks[bank_r],
conflicts[s]);
+
+ if (benefit > best_benefit) {
+ best_s = s;
+ best_benefit = benefit;
+ }
+ }
+ }
+
+ if (best_s != r) {
+ for (unsigned b = 0; b < banks.size(); b++) {
+ for (unsigned p = 0; p < 2; p++)
+ swap(banks[b], r, p, best_s, p);
+ }
+
+ std::swap(map[r], map[best_s]);
+ }
+ }
+ }
+
+ return map;
+ }
+
+ /**
+ * Apply the GRF atom permutation given by \p map to register \p
r and
+ * return the result.
+ */
+ fs_reg
+ transform(const partitioning &p, const std::vector<unsigned> &map,
+ fs_reg r)
+ {
+ if (r.file == VGRF) {
+ const unsigned reg = reg_of(r);
+ const unsigned s = p.atom_of_reg(reg);
+ r.nr = map[s] + reg - p.reg_of_atom(s);
+ r.offset = r.offset % REG_SIZE;
+ }
+
+ return r;
+ }
+}
+
+bool
+fs_visitor::opt_bank_conflicts()
+{
+ assert(grf_used || !"Must be called after register allocation");
+
+ /* No ternary instructions -- No bank conflicts. */
+ if (devinfo->gen < 6)
+ return false;
+
+ const partitioning p = shader_reg_partitioning(this);
+ const std::vector<bool> constrained =
shader_reg_constraints(this, p);
+ const std::vector<weight_vector_type> conflicts =
+ shader_conflict_weight_matrix(this, p);
+ const std::vector<unsigned> map =
+ optimize_reg_permutation(p, constrained, conflicts,
+ identity_reg_permutation(p));
+
+ foreach_block_and_inst(block, fs_inst, inst, cfg) {
+ inst->dst = transform(p, map, inst->dst);
+
+ for (int i = 0; i < inst->sources; i++)
+ inst->src[i] = transform(p, map, inst->src[i]);
+ }
+
+ return true;
+}
--
2.10.2
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