nir,microsoft: Move scale_fdiv into a common NIR pass

While we're at it, convert to nir_shader_instructions_pass() to get rid
of some boilerplate and get metadata correct.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15983>

src/compiler/nir/meson.build

@@ -263,6 +263,7 @@ files_libnir = files(
   'nir_range_analysis.h',
   'nir_remove_dead_variables.c',
   'nir_repair_ssa.c',
+  'nir_scale_fdiv.c',
   'nir_schedule.c',
   'nir_schedule.h',
   'nir_search.c',

src/compiler/nir/nir.h

@@ -4722,6 +4722,8 @@ bool nir_lower_alu(nir_shader *shader);
 bool nir_lower_flrp(nir_shader *shader, unsigned lowering_mask,
                     bool always_precise);
 
+bool nir_scale_fdiv(nir_shader *shader);
+
 bool nir_lower_alu_to_scalar(nir_shader *shader, nir_instr_filter_cb cb, const void *data);
 bool nir_lower_bool_to_bitsize(nir_shader *shader);
 bool nir_lower_bool_to_float(nir_shader *shader);

src/compiler/nir/nir_scale_fdiv.c

@@ -0,0 +1,75 @@
+/*
+ * Copyright © 2020 Microsoft 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"
+
+static bool
+nir_scale_fdiv_instr(nir_builder *b, nir_instr *instr, UNUSED void *_data)
+{
+   if (instr->type != nir_instr_type_alu)
+      return false;
+
+   nir_alu_instr *alu = nir_instr_as_alu(instr);
+   if (alu->op != nir_op_fdiv || alu->src[0].src.ssa->bit_size != 32)
+      return false;
+
+   b->cursor = nir_before_instr(&alu->instr);
+
+   nir_ssa_def *fabs = nir_fabs(b, alu->src[1].src.ssa);
+   nir_ssa_def *big = nir_flt(b, nir_imm_int(b, 0x7e800000), fabs);
+   nir_ssa_def *small = nir_flt(b, fabs, nir_imm_int(b, 0x00800000));
+
+   nir_ssa_def *scaled_down_a = nir_fmul_imm(b, alu->src[0].src.ssa, 0.25);
+   nir_ssa_def *scaled_down_b = nir_fmul_imm(b, alu->src[1].src.ssa, 0.25);
+   nir_ssa_def *scaled_up_a = nir_fmul_imm(b, alu->src[0].src.ssa, 16777216.0);
+   nir_ssa_def *scaled_up_b = nir_fmul_imm(b, alu->src[1].src.ssa, 16777216.0);
+
+   nir_ssa_def *final_a =
+      nir_bcsel(b, big, scaled_down_a,
+     (nir_bcsel(b, small, scaled_up_a, alu->src[0].src.ssa)));
+   nir_ssa_def *final_b =
+      nir_bcsel(b, big, scaled_down_b,
+     (nir_bcsel(b, small, scaled_up_b, alu->src[1].src.ssa)));
+
+   nir_instr_rewrite_src_ssa(instr, &alu->src[0].src, final_a);
+   nir_instr_rewrite_src_ssa(instr, &alu->src[1].src, final_b);
+
+   return true;
+}
+
+/** Scale both sides of an fdiv if needed to prevent denorm flushing
+ *
+ * This may be needed to satisfy the precision requirements of OpenCL.  When
+ * fdiv is lowered to frcp+fmul, denorm flushing may cause the frcp to return
+ * zero even for finite floats.  This multiplies both sides of an fdiv by a
+ * constant, if needed, to prevent such flushing.
+ */
+bool
+nir_scale_fdiv(nir_shader *shader)
+{
+   return nir_shader_instructions_pass(shader, nir_scale_fdiv_instr,
+                                       nir_metadata_block_index |
+                                       nir_metadata_dominance,
+                                       NULL);
+}