Mesa (master): i965: Add initial folding of constants into operand immediate slots.

[Eric Anholt]

Module: Mesa
Branch: master
Commit: 3bf8774e9c293fcad654d1bd67d4b43247b82f97
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=3bf8774e9c293fcad654d1bd67d4b43247b82f97

Author: Eric Anholt <eric at anholt.net>
Date:   Mon Oct  4 11:48:04 2010 -0700

i965: Add initial folding of constants into operand immediate slots.

We could try to detect this in expression handling and do it
proactively there, but it seems like less logic to do it in one
optional pass at the end.

---

 src/mesa/drivers/dri/i965/brw_fs.cpp |   90 ++++++++++++++++++++++++++++++++++
 1 files changed, 90 insertions(+), 0 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
index 472e6d0..5eaa998 100644
--- a/src/mesa/drivers/dri/i965/brw_fs.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
@@ -455,6 +455,7 @@ public:
    void assign_regs();
    void assign_regs_trivial();
    void calculate_live_intervals();
+   bool propagate_constants();
    bool dead_code_eliminate();
    bool virtual_grf_interferes(int a, int b);
    void generate_code();
@@ -2523,6 +2524,94 @@ fs_visitor::calculate_live_intervals()
 }
 
 /**
+ * Attempts to move immediate constants into the immediate
+ * constant slot of following instructions.
+ *
+ * Immediate constants are a bit tricky -- they have to be in the last
+ * operand slot, you can't do abs/negate on them,
+ */
+
+bool
+fs_visitor::propagate_constants()
+{
+   bool progress = false;
+
+   return false;
+
+   foreach_iter(exec_list_iterator, iter, this->instructions) {
+      fs_inst *inst = (fs_inst *)iter.get();
+
+      if (inst->opcode != BRW_OPCODE_MOV ||
+	  inst->predicated ||
+	  inst->dst.file != GRF || inst->src[0].file != IMM ||
+	  inst->dst.type != inst->src[0].type)
+	 continue;
+
+      /* Don't bother with cases where we should have had the
+       * operation on the constant folded in GLSL already.
+       */
+      if (inst->saturate)
+	 continue;
+
+      /* Found a move of a constant to a GRF.  Find anything else using the GRF
+       * before it's written, and replace it with the constant if we can.
+       */
+      exec_list_iterator scan_iter = iter;
+      scan_iter.next();
+      for (; scan_iter.has_next(); scan_iter.next()) {
+	 fs_inst *scan_inst = (fs_inst *)scan_iter.get();
+
+	 if (scan_inst->opcode == BRW_OPCODE_DO ||
+	     scan_inst->opcode == BRW_OPCODE_WHILE ||
+	     scan_inst->opcode == BRW_OPCODE_ELSE ||
+	     scan_inst->opcode == BRW_OPCODE_ENDIF) {
+	    break;
+	 }
+
+	 for (int i = 2; i >= 0; i--) {
+	    if (scan_inst->src[i].file != GRF ||
+		scan_inst->src[i].reg != inst->dst.reg ||
+		scan_inst->src[i].reg_offset != inst->dst.reg_offset)
+	       continue;
+
+	    /* Don't bother with cases where we should have had the
+	     * operation on the constant folded in GLSL already.
+	     */
+	    if (scan_inst->src[i].negate || scan_inst->src[i].abs)
+	       continue;
+
+	    switch (scan_inst->opcode) {
+	    case BRW_OPCODE_MOV:
+	       scan_inst->src[i] = inst->src[0];
+	       progress = true;
+	       break;
+
+	    case BRW_OPCODE_MUL:
+	    case BRW_OPCODE_ADD:
+	       if (i == 1) {
+		  scan_inst->src[i] = inst->src[0];
+		  progress = true;
+	       } else if (i == 0 && scan_inst->src[1].file != IMM) {
+		  /* Fit this constant in by commuting the operands */
+		  scan_inst->src[0] = scan_inst->src[1];
+		  scan_inst->src[1] = inst->src[0];
+	       }
+	       break;
+	    }
+	 }
+
+	 if (scan_inst->dst.file == GRF &&
+	     scan_inst->dst.reg == inst->dst.reg &&
+	     (scan_inst->dst.reg_offset == inst->dst.reg_offset ||
+	      scan_inst->opcode == FS_OPCODE_TEX)) {
+	    break;
+	 }
+      }
+   }
+
+   return progress;
+}
+/**
  * Must be called after calculate_live_intervales() to remove unused
  * writes to registers -- register allocation will fail otherwise
  * because something deffed but not used won't be considered to
@@ -2876,6 +2965,7 @@ brw_wm_fs_emit(struct brw_context *brw, struct brw_wm_compile *c)
 	 progress = false;
 
 	 v.calculate_live_intervals();
+	 progress = v.propagate_constants() || progress;
 	 progress = v.dead_code_eliminate() || progress;
       } while (progress);