i965/fs: Strip trailing constant zeroes in sample messages

If a send message is emitted with a message length that is less than
required for the message then the remaining parameters default to
zero. We can take advantage of this to save a register when a shader
passes constant zeroes as the final coordinates to the sample
function.

I think this might be useful for GLES applications that are using 2D
textures to simulate 1D textures.

On Skylake it will be useful for shaders that do
texelFetch(tex,something,0) which I think is fairly common. This helps
more on Skylake because in that case the order of the instruction
operands are u,v,lod,r which is good for 2D textures whereas before
they were u,lod,v,r which is only good for 1D textures.

On Haswell:
total instructions in shared programs: 8535730 -> 8533261 (-0.03%)
instructions in affected programs:     236968 -> 234499 (-1.04%)
helped:                                1174

On Skylake:
total instructions in shared programs: 10345646 -> 10341237 (-0.04%)
instructions in affected programs:     293011 -> 288602 (-1.50%)
helped:                                1218

Reviewed-by: Matt Turner <mattst88@gmail.com>
v2: Applied suggestions by Kenneth Graunke:
    - Only apply on Gen5+
    - Apply to all texture opcodes, not just TEX and TXF.
    Moved the optimisation into the loop as suggested by Matt Turner.
    Fix the array index when there is a header.

diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
index a6483230e527fd2b29ee1ade688d2f071d203f9f..ed86e0b291ca1464513605bd9b6a7d4c548d2b13 100644 (file)
--- a/src/mesa/drivers/dri/i965/brw_fs.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
@@ -2535,6 +2535,54 @@ fs_visitor::opt_algebraic()
    return progress;
 }
 
+/**
+ * Optimize sample messages that have constant zero values for the trailing
+ * texture coordinates. We can just reduce the message length for these
+ * instructions instead of reserving a register for it. Trailing parameters
+ * that aren't sent default to zero anyway. This will cause the dead code
+ * eliminator to remove the MOV instruction that would otherwise be emitted to
+ * set up the zero value.
+ */
+bool
+fs_visitor::opt_zero_samples()
+{
+   /* Gen4 infers the texturing opcode based on the message length so we can't
+    * change it.
+    */
+   if (devinfo->gen < 5)
+      return false;
+
+   bool progress = false;
+
+   foreach_block_and_inst(block, fs_inst, inst, cfg) {
+      if (!inst->is_tex())
+         continue;
+
+      fs_inst *load_payload = (fs_inst *) inst->prev;
+
+      if (load_payload->is_head_sentinel() ||
+          load_payload->opcode != SHADER_OPCODE_LOAD_PAYLOAD)
+         continue;
+
+      /* We don't want to remove the message header. Removing all of the
+       * parameters is avoided because it seems to cause a GPU hang but I
+       * can't find any documentation indicating that this is expected.
+       */
+      while (inst->mlen > inst->header_present + dispatch_width / 8 &&
+             load_payload->src[(inst->mlen - inst->header_present) /
+                               (dispatch_width / 8) +
+                               inst->header_present - 1].is_zero()) {
+         inst->mlen -= dispatch_width / 8;
+         progress = true;
+      }
+   }
+
+   if (progress)
+      invalidate_live_intervals();
+
+   return progress;
+}
+
 /**
  * Optimize sample messages which are followed by the final RT write.
  *
@@ -3818,6 +3866,7 @@ fs_visitor::optimize()
       OPT(opt_register_renaming);
       OPT(opt_redundant_discard_jumps);
       OPT(opt_saturate_propagation);
+      OPT(opt_zero_samples);
       OPT(register_coalesce);
       OPT(compute_to_mrf);
 

diff --git a/src/mesa/drivers/dri/i965/brw_fs.h b/src/mesa/drivers/dri/i965/brw_fs.h
index 4e17d4447476c88606e2fc92544bf0cfb22cce2d..6200deb72189585527bb6837441591383dc396fb 100644 (file)
--- a/src/mesa/drivers/dri/i965/brw_fs.h
+++ b/src/mesa/drivers/dri/i965/brw_fs.h
@@ -324,6 +324,7 @@ public:
    bool opt_peephole_predicated_break();
    bool opt_saturate_propagation();
    bool opt_cmod_propagation();
+   bool opt_zero_samples();
    void emit_bool_to_cond_code(ir_rvalue *condition);
    void emit_bool_to_cond_code_of_reg(ir_expression *expr, fs_reg op[3]);
    void emit_if_gen6(ir_if *ir);