i965/fs: Add a local common subexpression elimination pass.

Total instructions: 18210 -> 17836
49/163 programs affected (30.1%)
12888 -> 12514 instructions in affected programs (2.9% reduction)

This reduces Lightsmark's "Scale down filter" shader from 395
instructions to 283, a whopping 28%.  It also reduces register pressure
significantly: the SIMD8 program now uses 29 registers instead of 101,
giving us more than enough room for a SIMD16 program.

v2: Add && !inst->conditional_mod to the "skip some instructions" check.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/drivers/dri/i965/Makefile.sources

@@ -113,6 +113,7 @@ i965_CXX_FILES = \
 	brw_cubemap_normalize.cpp \
 	brw_fs.cpp \
 	brw_fs_cfg.cpp \
+	brw_fs_cse.cpp \
 	brw_fs_emit.cpp \
 	brw_fs_live_variables.cpp \
 	brw_fs_visitor.cpp \

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -1681,6 +1681,7 @@ fs_visitor::run()
 
 	 progress = propagate_constants() || progress;
 	 progress = opt_algebraic() || progress;
+	 progress = opt_cse() || progress;
 	 progress = register_coalesce() || progress;
 	 progress = compute_to_mrf() || progress;
 	 progress = dead_code_eliminate() || progress;

src/mesa/drivers/dri/i965/brw_fs.h

@@ -49,6 +49,8 @@ extern "C" {
 #include "glsl/glsl_types.h"
 #include "glsl/ir.h"
 
+class fs_bblock;
+
 enum register_file {
    BAD_FILE,
    ARF,
@@ -507,6 +509,8 @@ public:
    void calculate_live_intervals();
    bool propagate_constants();
    bool opt_algebraic();
+   bool opt_cse();
+   bool opt_cse_local(fs_bblock *block, exec_list *aeb);
    bool register_coalesce();
    bool compute_to_mrf();
    bool dead_code_eliminate();

src/mesa/drivers/dri/i965/brw_fs_cse.cpp

@@ -0,0 +1,195 @@
+/*
+ * Copyright © 2012 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 "brw_fs.h"
+#include "brw_fs_cfg.h"
+
+/** @file brw_fs_cse.cpp
+ *
+ * Support for local common subexpression elimination.
+ *
+ * See Muchnik's Advanced Compiler Design and Implementation, section
+ * 13.1 (p378).
+ */
+
+namespace {
+struct aeb_entry : public exec_node {
+   /** The instruction that generates the expression value. */
+   fs_inst *generator;
+
+   /** The temporary where the value is stored. */
+   fs_reg tmp;
+};
+}
+
+static bool
+is_expression(const fs_inst *const inst)
+{
+   switch (inst->opcode) {
+   case BRW_OPCODE_SEL:
+   case BRW_OPCODE_NOT:
+   case BRW_OPCODE_AND:
+   case BRW_OPCODE_OR:
+   case BRW_OPCODE_XOR:
+   case BRW_OPCODE_SHR:
+   case BRW_OPCODE_SHL:
+   case BRW_OPCODE_RSR:
+   case BRW_OPCODE_RSL:
+   case BRW_OPCODE_ASR:
+   case BRW_OPCODE_ADD:
+   case BRW_OPCODE_MUL:
+   case BRW_OPCODE_FRC:
+   case BRW_OPCODE_RNDU:
+   case BRW_OPCODE_RNDD:
+   case BRW_OPCODE_RNDE:
+   case BRW_OPCODE_RNDZ:
+   case BRW_OPCODE_LINE:
+   case BRW_OPCODE_PLN:
+   case BRW_OPCODE_MAD:
+   case FS_OPCODE_CINTERP:
+   case FS_OPCODE_LINTERP:
+      return true;
+   default:
+      return false;
+   }
+}
+
+static bool
+operands_match(fs_reg *xs, fs_reg *ys)
+{
+   return xs[0].equals(ys[0]) && xs[1].equals(ys[1]) && xs[2].equals(ys[2]);
+}
+
+bool
+fs_visitor::opt_cse_local(fs_bblock *block, exec_list *aeb)
+{
+   bool progress = false;
+
+   void *mem_ctx = ralloc_context(this->mem_ctx);
+
+   for (fs_inst *inst = block->start;
+	inst != block->end->next;
+	inst = (fs_inst *) inst->next) {
+
+      /* Skip some cases. */
+      if (is_expression(inst) && !inst->predicated && inst->mlen == 0 &&
+          !inst->force_uncompressed && !inst->force_sechalf &&
+          !inst->conditional_mod)
+      {
+	 bool found = false;
+
+	 aeb_entry *entry;
+	 foreach_list(entry_node, aeb) {
+	    entry = (aeb_entry *) entry_node;
+
+	    /* Match current instruction's expression against those in AEB. */
+	    if (inst->opcode == entry->generator->opcode &&
+		inst->saturate == entry->generator->saturate &&
+		operands_match(entry->generator->src, inst->src)) {
+
+	       found = true;
+	       progress = true;
+	       break;
+	    }
+	 }
+
+	 if (!found) {
+	    /* Our first sighting of this expression.  Create an entry. */
+	    aeb_entry *entry = ralloc(mem_ctx, aeb_entry);
+	    entry->tmp = reg_undef;
+	    entry->generator = inst;
+	    aeb->push_tail(entry);
+	 } else {
+	    /* This is at least our second sighting of this expression.
+	     * If we don't have a temporary already, make one.
+	     */
+	    bool no_existing_temp = entry->tmp.file == BAD_FILE;
+	    if (no_existing_temp) {
+	       entry->tmp = fs_reg(this, glsl_type::float_type);
+	       entry->tmp.type = inst->dst.type;
+
+	       fs_inst *copy = new(ralloc_parent(inst))
+		  fs_inst(BRW_OPCODE_MOV, entry->generator->dst, entry->tmp);
+	       entry->generator->insert_after(copy);
+	       entry->generator->dst = entry->tmp;
+	    }
+
+	    /* dest <- temp */
+	    fs_inst *copy = new(ralloc_parent(inst))
+	       fs_inst(BRW_OPCODE_MOV, inst->dst, entry->tmp);
+	    inst->replace_with(copy);
+
+	    /* Appending an instruction may have changed our bblock end. */
+	    if (inst == block->end) {
+	       block->end = copy;
+	    }
+
+	    /* Continue iteration with copy->next */
+	    inst = copy;
+	 }
+      }
+
+      /* Kill all AEB entries that use the destination. */
+      int start_offset = inst->dst.reg_offset;
+      int end_offset = start_offset + inst->regs_written();
+
+      foreach_list_safe(entry_node, aeb) {
+	 aeb_entry *entry = (aeb_entry *)entry_node;
+
+	 for (int i = 0; i < 3; i++) {
+	    if (entry->generator->src[i].file == inst->dst.file &&
+		entry->generator->src[i].reg == inst->dst.reg &&
+		entry->generator->src[i].reg_offset >= start_offset &&
+		entry->generator->src[i].reg_offset < end_offset) {
+	       entry->remove();
+	       ralloc_free(entry);
+	       break;
+	    }
+	 }
+      }
+   }
+
+   ralloc_free(mem_ctx);
+
+   if (progress)
+      this->live_intervals_valid = false;
+
+   return progress;
+}
+
+bool
+fs_visitor::opt_cse()
+{
+   bool progress = false;
+
+   fs_cfg cfg(this);
+
+   for (int b = 0; b < cfg.num_blocks; b++) {
+      fs_bblock *block = cfg.blocks[b];
+      exec_list aeb;
+
+      progress = opt_cse_local(block, &aeb) || progress;
+   }
+
+   return progress;
+}