nir: Add a pass for lowering integer division by constants

It's a reasonably well-known fact in the world of compilers that integer
divisions by constants can be replaced by a multiply, an add, and some
shifts.  This commit adds such an optimization to NIR for easiest case
of udiv.  Other division operations will be added in following commits.
In order to provide some additional driver control, the pass takes a
minimum bit size to optimize.

Reviewed-by: Ian Romanick [email protected]

4 files changed, 219 insertions, 0 deletions

diff --git a/src/compiler/Makefile.sources b/src/compiler/Makefile.sources
index ae170f02e82..53c6da45c1e 100644
--- a/src/compiler/Makefile.sources
+++ b/src/compiler/Makefile.sources
@@ -279,6 +279,7 @@ NIR_FILES = \
 	nir/nir_opt_find_array_copies.c \
 	nir/nir_opt_gcm.c \
 	nir/nir_opt_global_to_local.c \
+	nir/nir_opt_idiv_const.c \
 	nir/nir_opt_if.c \
 	nir/nir_opt_intrinsics.c \
 	nir/nir_opt_loop_unroll.c \
diff --git a/src/compiler/nir/meson.build b/src/compiler/nir/meson.build
index b0ca27cb700..5eb1df2a68f 100644
--- a/src/compiler/nir/meson.build
+++ b/src/compiler/nir/meson.build
@@ -163,6 +163,7 @@ files_libnir = files(
   'nir_opt_find_array_copies.c',
   'nir_opt_gcm.c',
   'nir_opt_global_to_local.c',
+  'nir_opt_idiv_const.c',
   'nir_opt_if.c',
   'nir_opt_intrinsics.c',
   'nir_opt_large_constants.c',
diff --git a/src/compiler/nir/nir.h b/src/compiler/nir/nir.h
index a2c68d66aea..e8be2e101cc 100644
--- a/src/compiler/nir/nir.h
+++ b/src/compiler/nir/nir.h
@@ -3174,6 +3174,8 @@ bool nir_opt_find_array_copies(nir_shader *shader);
 
 bool nir_opt_gcm(nir_shader *shader, bool value_number);
 
+bool nir_opt_idiv_const(nir_shader *shader, unsigned min_bit_size);
+
 bool nir_opt_if(nir_shader *shader);
 
 bool nir_opt_intrinsics(nir_shader *shader);
diff --git a/src/compiler/nir/nir_opt_idiv_const.c b/src/compiler/nir/nir_opt_idiv_const.c
new file mode 100644
index 00000000000..7fa739161ba
--- /dev/null
+++ b/src/compiler/nir/nir_opt_idiv_const.c
@@ -0,0 +1,215 @@
+/*
+ * Copyright © 2018 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 "util/fast_idiv_by_const.h"
+#include "util/u_math.h"
+
+static nir_ssa_def *
+build_udiv(nir_builder *b, nir_ssa_def *n, uint64_t d)
+{
+   if (d == 0) {
+      return nir_imm_intN_t(b, 0, n->bit_size);
+   } else if (util_is_power_of_two_or_zero64(d)) {
+      return nir_ushr(b, n, nir_imm_int(b, util_logbase2_64(d)));
+   } else {
+      struct util_fast_udiv_info m =
+         util_compute_fast_udiv_info(d, n->bit_size, n->bit_size);
+
+      if (m.pre_shift)
+         n = nir_ushr(b, n, nir_imm_int(b, m.pre_shift));
+      if (m.increment)
+         n = nir_uadd_sat(b, n, nir_imm_intN_t(b, m.increment, n->bit_size));
+      n = nir_umul_high(b, n, nir_imm_intN_t(b, m.multiplier, n->bit_size));
+      if (m.post_shift)
+         n = nir_ushr(b, n, nir_imm_int(b, m.post_shift));
+
+      return n;
+   }
+}
+
+static nir_ssa_def *
+build_umod(nir_builder *b, nir_ssa_def *n, uint64_t d)
+{
+   if (d == 0) {
+      return nir_imm_intN_t(b, 0, n->bit_size);
+   } else if (util_is_power_of_two_or_zero64(d)) {
+      return nir_iand(b, n, nir_imm_intN_t(b, d - 1, n->bit_size));
+   } else {
+      return nir_isub(b, n, nir_imul(b, build_udiv(b, n, d),
+                                        nir_imm_intN_t(b, d, n->bit_size)));
+   }
+}
+
+static nir_ssa_def *
+build_idiv(nir_builder *b, nir_ssa_def *n, int64_t d)
+{
+   if (d == 0) {
+      return nir_imm_intN_t(b, 0, n->bit_size);
+   } else if (d == 1) {
+      return n;
+   } else if (d == -1) {
+      return nir_ineg(b, n);
+   } else if (util_is_power_of_two_or_zero64(d)) {
+      uint64_t abs_d = d < 0 ? -d : d;
+      nir_ssa_def *uq = nir_ishr(b, n, nir_imm_int(b, util_logbase2_64(abs_d)));
+      nir_ssa_def *n_neg = nir_ilt(b, n, nir_imm_intN_t(b, 0, n->bit_size));
+      nir_ssa_def *neg = d < 0 ? nir_inot(b, n_neg) : n_neg;
+      return nir_bcsel(b, neg, nir_ineg(b, uq), uq);
+   } else {
+      struct util_fast_sdiv_info m =
+         util_compute_fast_sdiv_info(d, n->bit_size);
+
+      nir_ssa_def *res =
+         nir_imul_high(b, n, nir_imm_intN_t(b, m.multiplier, n->bit_size));
+      if (d > 0 && m.multiplier < 0)
+         res = nir_iadd(b, res, n);
+      if (d < 0 && m.multiplier > 0)
+         res = nir_isub(b, res, n);
+      if (m.shift)
+         res = nir_ishr(b, res, nir_imm_int(b, m.shift));
+      res = nir_iadd(b, res, nir_ushr(b, res, nir_imm_int(b, n->bit_size - 1)));
+
+      return res;
+   }
+}
+
+static bool
+nir_opt_idiv_const_instr(nir_builder *b, nir_alu_instr *alu)
+{
+   assert(alu->dest.dest.is_ssa);
+   assert(alu->src[0].src.is_ssa && alu->src[1].src.is_ssa);
+
+   nir_const_value *const_denom = nir_src_as_const_value(alu->src[1].src);
+   if (!const_denom)
+      return false;
+
+   unsigned bit_size = alu->src[1].src.ssa->bit_size;
+
+   b->cursor = nir_before_instr(&alu->instr);
+
+   nir_ssa_def *q[4];
+   for (unsigned comp = 0; comp < alu->dest.dest.ssa.num_components; comp++) {
+      /* Get the numerator for the channel */
+      nir_ssa_def *n = nir_channel(b, alu->src[0].src.ssa,
+                                   alu->src[0].swizzle[comp]);
+
+      /* Get the denominator for the channel */
+      int64_t d;
+      switch (bit_size) {
+      case 8:
+         d = const_denom->i8[alu->src[1].swizzle[comp]];
+         break;
+      case 16:
+         d = const_denom->i16[alu->src[1].swizzle[comp]];
+         break;
+      case 32:
+         d = const_denom->i32[alu->src[1].swizzle[comp]];
+         break;
+      case 64:
+         d = const_denom->i64[alu->src[1].swizzle[comp]];
+         break;
+      default:
+         unreachable("Invalid bit size");
+      }
+
+      nir_alu_type d_type = nir_op_infos[alu->op].input_types[1];
+      if (nir_alu_type_get_base_type(d_type) == nir_type_uint) {
+         /* The code above sign-extended.  If we're lowering an unsigned op,
+          * we need to mask it off to the correct number of bits so that a
+          * cast to uint64_t will do the right thing.
+          */
+         if (bit_size < 64)
+            d &= (1ull << bit_size) - 1;
+      }
+
+      switch (alu->op) {
+      case nir_op_udiv:
+         q[comp] = build_udiv(b, n, d);
+         break;
+      case nir_op_idiv:
+         q[comp] = build_idiv(b, n, d);
+         break;
+      case nir_op_umod:
+         q[comp] = build_umod(b, n, d);
+         break;
+      default:
+         unreachable("Unknown integer division op");
+      }
+   }
+
+   nir_ssa_def *qvec = nir_vec(b, q, alu->dest.dest.ssa.num_components);
+   nir_ssa_def_rewrite_uses(&alu->dest.dest.ssa, nir_src_for_ssa(qvec));
+   nir_instr_remove(&alu->instr);
+
+   return true;
+}
+
+static bool
+nir_opt_idiv_const_impl(nir_function_impl *impl, unsigned min_bit_size)
+{
+   bool progress = false;
+
+   nir_builder b;
+   nir_builder_init(&b, impl);
+
+   nir_foreach_block(block, impl) {
+      nir_foreach_instr_safe(instr, block) {
+         if (instr->type != nir_instr_type_alu)
+            continue;
+
+         nir_alu_instr *alu = nir_instr_as_alu(instr);
+         if (alu->op != nir_op_udiv &&
+             alu->op != nir_op_idiv &&
+             alu->op != nir_op_umod)
+            continue;
+
+         assert(alu->dest.dest.is_ssa);
+         if (alu->dest.dest.ssa.bit_size < min_bit_size)
+            continue;
+
+         progress |= nir_opt_idiv_const_instr(&b, alu);
+      }
+   }
+
+   if (progress) {
+      nir_metadata_preserve(impl, nir_metadata_block_index |
+                                  nir_metadata_dominance);
+   }
+
+   return progress;
+}
+
+bool
+nir_opt_idiv_const(nir_shader *shader, unsigned min_bit_size)
+{
+   bool progress = false;
+
+   nir_foreach_function(function, shader) {
+      if (function->impl)
+         progress |= nir_opt_idiv_const_impl(function->impl, min_bit_size);
+   }
+
+   return progress;
+}