nir: add lowering for idiv/udiv/umod

Based on the algo from NV50LegalizeSSA::handleDIV() and handleMOD().
See also trans_idiv() in freedreno/ir3/ir3_compiler.c (which was an
adaptation of the nv50 code from Ilia Mirkin).

A python/numpy script which implements the same algorithm (and is
possibly useful for debugging or analysis) can be found here:

  http://people.freedesktop.org/~robclark/div-lowering.py

I've tested this on i965 hacked up to insert the idiv lowering pass,
and on freedreno with NIR frontend.

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Tested-by: Eric Anholt <eric@anholt.net> (vc4)

src/glsl/Makefile.sources

@@ -33,6 +33,7 @@ NIR_FILES = \
 	nir/nir_lower_atomics.c \
 	nir/nir_lower_global_vars_to_local.c \
 	nir/nir_lower_locals_to_regs.c \
+	nir/nir_lower_idiv.c \
 	nir/nir_lower_io.c \
 	nir/nir_lower_phis_to_scalar.c \
 	nir/nir_lower_samplers.cpp \

src/glsl/nir/nir.h

@@ -1615,6 +1615,7 @@ void nir_lower_samplers(nir_shader *shader,
 
 void nir_lower_system_values(nir_shader *shader);
 void nir_lower_tex_projector(nir_shader *shader);
+void nir_lower_idiv(nir_shader *shader);
 
 void nir_lower_atomics(nir_shader *shader);
 void nir_lower_to_source_mods(nir_shader *shader);

src/glsl/nir/nir_lower_idiv.c

@@ -0,0 +1,157 @@
+/*
+ * Copyright © 2015 Red Hat
+ *
+ * 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.
+ *
+ * Authors:
+ *    Rob Clark <robclark@freedesktop.org>
+ */
+
+#include "nir.h"
+#include "nir_builder.h"
+
+/* Lowers idiv/udiv/umod
+ * Based on NV50LegalizeSSA::handleDIV()
+ *
+ * Note that this is probably not enough precision for compute shaders.
+ * Perhaps we want a second higher precision (looping) version of this?
+ * Or perhaps we assume if you can do compute shaders you can also
+ * branch out to a pre-optimized shader library routine..
+ */
+
+static void
+convert_instr(nir_builder *bld, nir_alu_instr *alu)
+{
+   nir_ssa_def *numer, *denom, *af, *bf, *a, *b, *q, *r;
+   nir_op op = alu->op;
+   bool is_signed;
+
+   if ((op != nir_op_idiv) &&
+       (op != nir_op_udiv) &&
+       (op != nir_op_umod))
+      return;
+
+   is_signed = (op == nir_op_idiv);
+
+   nir_builder_insert_before_instr(bld, &alu->instr);
+
+   numer = nir_ssa_for_src(bld, alu->src[0].src,
+                           nir_ssa_alu_instr_src_components(alu, 0));
+   denom = nir_ssa_for_src(bld, alu->src[1].src,
+                           nir_ssa_alu_instr_src_components(alu, 1));
+
+   if (is_signed) {
+      af = nir_i2f(bld, numer);
+      bf = nir_i2f(bld, denom);
+      af = nir_fabs(bld, af);
+      bf = nir_fabs(bld, bf);
+      a  = nir_iabs(bld, numer);
+      b  = nir_iabs(bld, denom);
+   } else {
+      af = nir_u2f(bld, numer);
+      bf = nir_u2f(bld, denom);
+      a  = numer;
+      b  = denom;
+   }
+
+   /* get first result: */
+   bf = nir_frcp(bld, bf);
+   bf = nir_isub(bld, bf, nir_imm_int(bld, 2));  /* yes, really */
+   q  = nir_fmul(bld, af, bf);
+
+   if (is_signed) {
+      q = nir_f2i(bld, q);
+   } else {
+      q = nir_f2u(bld, q);
+   }
+
+   /* get error of first result: */
+   r = nir_imul(bld, q, b);
+   r = nir_isub(bld, a, r);
+   r = nir_u2f(bld, r);
+   r = nir_fmul(bld, r, bf);
+   r = nir_f2u(bld, r);
+
+   /* add quotients: */
+   q = nir_iadd(bld, q, r);
+
+   /* correction: if modulus >= divisor, add 1 */
+   r = nir_imul(bld, q, b);
+   r = nir_isub(bld, a, r);
+
+   r = nir_ige(bld, r, b);
+   r = nir_b2i(bld, r);
+
+   q = nir_iadd(bld, q, r);
+   if (is_signed)  {
+      /* fix the sign: */
+      r = nir_ixor(bld, numer, denom);
+      r = nir_ushr(bld, r, nir_imm_int(bld, 31));
+      r = nir_i2b(bld, r);
+      b = nir_ineg(bld, q);
+      q = nir_bcsel(bld, r, b, q);
+   }
+
+   if (op == nir_op_umod) {
+      /* division result in q */
+      r = nir_imul(bld, q, b);
+      q = nir_isub(bld, a, r);
+   }
+
+   assert(alu->dest.dest.is_ssa);
+   nir_ssa_def_rewrite_uses(&alu->dest.dest.ssa,
+                            nir_src_for_ssa(q),
+                            ralloc_parent(alu));
+}
+
+static bool
+convert_block(nir_block *block, void *state)
+{
+   nir_builder *b = state;
+
+   nir_foreach_instr_safe(block, instr) {
+      if (instr->type == nir_instr_type_alu)
+         convert_instr(b, nir_instr_as_alu(instr));
+   }
+
+   return true;
+}
+
+static void
+convert_impl(nir_function_impl *impl)
+{
+   nir_builder b;
+   nir_builder_init(&b, impl);
+
+   nir_foreach_block(impl, convert_block, &b);
+   nir_metadata_preserve(impl, nir_metadata_block_index |
+                               nir_metadata_dominance);
+}
+
+void
+nir_lower_idiv(nir_shader *shader)
+{
+   nir_foreach_overload(shader, overload) {
+      if (overload->impl)
+         convert_impl(overload->impl);
+   }
+
+   exec_list_make_empty(&shader->system_values);
+}