intel/fs: Implement GRF bank conflict mitigation pass.

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.

The following patch is squashed in:

SQUASH: intel/fs/bank_conflicts: Roll back to the nineties.

Acked-by: Matt Turner <[email protected]>

5 files changed, 898 insertions, 0 deletions

diff --git a/src/intel/Makefile.sources b/src/intel/Makefile.sources
index cdb10ece35e..1c62bad816e 100644
--- a/src/intel/Makefile.sources
+++ b/src/intel/Makefile.sources
@@ -48,6 +48,7 @@ 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 93bb6b46732..c5d4f5634df 100644
--- a/src/intel/compiler/brw_fs.cpp
+++ b/src/intel/compiler/brw_fs.cpp
@@ -6065,6 +6065,8 @@ fs_visitor::allocate_registers(unsigned min_dispatch_width, 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 30557324d5a..0cec6fdcbad 100644
--- a/src/intel/compiler/brw_fs.h
+++ b/src/intel/compiler/brw_fs.h
@@ -145,6 +145,7 @@ 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 00000000000..b64a3d4a8a8
--- /dev/null
+++ b/src/intel/compiler/brw_fs_bank_conflicts.cpp
@@ -0,0 +1,893 @@
+/*
+ * 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 (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"
+
+#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.
+    */
+   struct vector_type {
+      __m128i v[2];
+   };
+
+   /**
+    * 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(__m128i) / 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.v + 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.v + 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.v[0], w.v[0]),
+            _mm_adds_epi16(v.v[1], w.v[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.v[0], w.v[0]),
+            _mm_subs_epi16(v.v[1], w.v[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.v[0], w.v[0]),
+            _mm_and_si128(v.v[1], w.v[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.v[0], v.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 MAX2(INT16_MIN, MIN2(INT16_MAX, int(v) + w));
+   }
+
+   /**
+    * Substract two vectors with saturation.
+    */
+   vector_type
+   subs(vector_type v, vector_type w)
+   {
+      return MAX2(INT16_MIN, MIN2(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
+
+/**
+ * Swap \p x and \p y.
+ */
+#define SWAP(x, y) do {                          \
+      __typeof(y) _swap_tmp = y;                 \
+      y = x;                                     \
+      x = _swap_tmp;                             \
+   } while (0)
+
+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()
+    */
+   struct weight_vector_type {
+      weight_vector_type() : v(NULL), size(0) {}
+
+      weight_vector_type(unsigned n) :
+         v(new vector_type[DIV_ROUND_UP(n, vector_width)]()),
+         size(n) {}
+
+      weight_vector_type(const weight_vector_type &u) :
+         v(new vector_type[DIV_ROUND_UP(u.size, vector_width)]()),
+         size(u.size)
+      {
+         memcpy(v, u.v,
+                DIV_ROUND_UP(u.size, vector_width) * sizeof(vector_type));
+      }
+
+      ~weight_vector_type()
+      {
+         delete[] v;
+      }
+
+      weight_vector_type &
+      operator=(weight_vector_type u)
+      {
+         SWAP(v, u.v);
+         SWAP(size, u.size);
+         return *this;
+      }
+
+      vector_type *v;
+      unsigned size;
+   };
+
+   /**
+    * 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.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.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) :
+         max_reg(n),
+         offsets(new unsigned[n + num_terminator_atoms]),
+         atoms(new unsigned[n + num_terminator_atoms])
+      {
+         for (unsigned i = 0; i < n + num_terminator_atoms; i++) {
+            offsets[i] = i;
+            atoms[i] = i;
+         }
+      }
+
+      partitioning(const partitioning &p) :
+         max_reg(p.max_reg),
+         offsets(new unsigned[p.num_atoms() + num_terminator_atoms]),
+         atoms(new unsigned[p.max_reg + num_terminator_atoms])
+      {
+         memcpy(offsets, p.offsets,
+                sizeof(unsigned) * (p.num_atoms() + num_terminator_atoms));
+         memcpy(atoms, p.atoms,
+                sizeof(unsigned) * (p.max_reg + num_terminator_atoms));
+      }
+
+      ~partitioning()
+      {
+         delete[] offsets;
+         delete[] atoms;
+      }
+
+      partitioning &
+      operator=(partitioning p)
+      {
+         SWAP(max_reg, p.max_reg);
+         SWAP(offsets, p.offsets);
+         SWAP(atoms, p.atoms);
+         return *this;
+      }
+
+      /**
+       * 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 <= max_reg; 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;
+            }
+         }
+      }
+
+      /**
+       * 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 atoms[max_reg];
+      }
+
+   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;
+      unsigned max_reg;
+      unsigned *offsets;
+      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.
+    */
+   bool *
+   shader_reg_constraints(const fs_visitor *v, const partitioning &p)
+   {
+      bool *constrained = new bool[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.
+    */
+   weight_vector_type *
+   shader_conflict_weight_matrix(const fs_visitor *v, const partitioning &p)
+   {
+      weight_vector_type *conflicts = new weight_vector_type[p.num_atoms()];
+      for (unsigned r = 0; r < p.num_atoms(); r++)
+         conflicts[r] = weight_vector_type(2 * p.num_atoms());
+
+      /* 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 = MIN2(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()).
+    */
+   bool *
+   have_any_conflicts(const partitioning &p,
+                      const weight_vector_type *conflicts)
+   {
+      bool *any_conflicts = new bool[p.num_atoms()]();
+
+      for (unsigned r = 0; r < p.num_atoms(); r++) {
+         const unsigned m = DIV_ROUND_UP(conflicts[r].size, vector_width);
+         for (unsigned s = 0; s < m; s++)
+            any_conflicts[r] |= sums(conflicts[r].v[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)
+   {
+      const unsigned m = DIV_ROUND_UP(conflicts.size, vector_width);
+      vector_type s_p = {}, s_n = {};
+
+      for (unsigned r = 0; r < m; r++) {
+         s_p = adds(s_p, mask(bank_mask_p.v[r], conflicts.v[r]));
+         s_n = adds(s_n, mask(bank_mask_n.v[r], conflicts.v[r]));
+      }
+
+      return sums(subs(s_p, s_n));
+   }
+
+   /**
+    * Register atom permutation, represented as the start GRF offset each atom
+    * is mapped into.
+    */
+   struct permutation {
+      permutation() : v(NULL), size(0) {}
+
+      permutation(unsigned n) :
+         v(new unsigned[n]()), size(n) {}
+
+      permutation(const permutation &p) :
+         v(new unsigned[p.size]), size(p.size)
+      {
+         memcpy(v, p.v, p.size * sizeof(unsigned));
+      }
+
+      ~permutation()
+      {
+         delete[] v;
+      }
+
+      permutation &
+      operator=(permutation p)
+      {
+         SWAP(v, p.v);
+         SWAP(size, p.size);
+         return *this;
+      }
+
+      unsigned *v;
+      unsigned size;
+   };
+
+   /**
+    * Return an identity permutation of GRF atoms.
+    */
+   permutation
+   identity_reg_permutation(const partitioning &p)
+   {
+      permutation map(p.num_atoms());
+
+      for (unsigned r = 0; r < map.size; r++)
+         map.v[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.
+    */
+   weight_vector_type *
+   bank_characteristics(const permutation &map)
+   {
+      weight_vector_type *banks = new weight_vector_type[4];
+
+      for (unsigned b = 0; b < 4; b++) {
+         banks[b] = weight_vector_type(2 * map.size);
+
+         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.v[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.
+    */
+   permutation
+   optimize_reg_permutation(const partitioning &p,
+                            const bool *constrained,
+                            const weight_vector_type *conflicts,
+                            permutation map)
+   {
+      const bool *any_conflicts = have_any_conflicts(p, conflicts);
+      weight_vector_type *banks = bank_characteristics(map);
+
+      for (unsigned r = 0; r < map.size; r++) {
+         const unsigned bank_r = bank_of(map.v[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.v[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 < 4; b++) {
+                  for (unsigned p = 0; p < 2; p++)
+                     swap(banks[b], r, p, best_s, p);
+               }
+
+               SWAP(map.v[r], map.v[best_s]);
+            }
+         }
+      }
+
+      delete[] banks;
+      delete[] any_conflicts;
+      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 permutation &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.v[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 bool *constrained = shader_reg_constraints(this, p);
+   const weight_vector_type *conflicts =
+      shader_conflict_weight_matrix(this, p);
+   const permutation 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]);
+   }
+
+   delete[] conflicts;
+   delete[] constrained;
+   return true;
+}
diff --git a/src/intel/compiler/meson.build b/src/intel/compiler/meson.build
index fe0a1f6e8ac..91127bf35a9 100644
--- a/src/intel/compiler/meson.build
+++ b/src/intel/compiler/meson.build
@@ -43,6 +43,7 @@ libintel_compiler_files = files(
   'brw_eu.h',
   'brw_eu_util.c',
   'brw_eu_validate.c',
+  'brw_fs_bank_conflicts.cpp',
   'brw_fs_builder.h',
   'brw_fs_cmod_propagation.cpp',
   'brw_fs_combine_constants.cpp',