[Mesa-dev] [PATCH 2/2] i965/fs: Register allocator shoudn't use grf127 for sends dest (v2)

[Jose Maria Casanova Crespo]

Since Gen8+ Intel PRM states that "r127 must not be used for return
address when there is a src and dest overlap in send instruction."

This patch implements this restriction creating new grf127_send_hack_node
at the register allocator. This node has a fixed assignation to grf127.

For vgrf that are used as destination of send messages we create node
interfereces with the grf127_send_hack_node. So the register allocator
will never assign to these vgrf a register that involves grf127.

If dispatch_width > 8 we don't create these interferences to the because
all instructions have node interferences between sources and destination.
That is enough to avoid the r127 restriction.

This fixes CTS tests that raised this issue as they were executed as SIMD8:
  dEQP-VK.spirv_assembly.instruction.graphics.16bit_storage.uniform_16struct_to_32struct.uniform_buffer_block_vert
  dEQP-VK.spirv_assembly.instruction.graphics.16bit_storage.uniform_16struct_to_32struct.uniform_buffer_block_tessc

Shader-db results on Skylake:
   total instructions in shared programs: 7686798 -> 7686797 (<.01%)
   instructions in affected programs: 301 -> 300 (-0.33%)
   helped: 1
   HURT: 0

   total cycles in shared programs: 337092322 -> 337091919 (<.01%)
   cycles in affected programs: 22420415 -> 22420012 (<.01%)
   helped: 712
   HURT: 588

Shader-db results on Broadwell:

   total instructions in shared programs: 7658574 -> 7658625 (<.01%)
   instructions in affected programs: 19610 -> 19661 (0.26%)
   helped: 3
   HURT: 4

   total cycles in shared programs: 340694553 -> 340676378 (<.01%)
   cycles in affected programs: 24724915 -> 24706740 (-0.07%)
   helped: 998
   HURT: 916

   total spills in shared programs: 4300 -> 4311 (0.26%)
   spills in affected programs: 333 -> 344 (3.30%)
   helped: 1
   HURT: 3

   total fills in shared programs: 5370 -> 5378 (0.15%)
   fills in affected programs: 274 -> 282 (2.92%)
   helped: 1
   HURT: 3

v2: Avoid duplicating register classes without grf127. Let's use a node
    with a fixed assignation to grf127 and create interferences to send
    message vgrf destinations. (Eric Anholt)
---
 src/intel/compiler/brw_fs_reg_allocate.cpp | 25 ++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/src/intel/compiler/brw_fs_reg_allocate.cpp b/src/intel/compiler/brw_fs_reg_allocate.cpp
index ec8e116cb38..59e047483c0 100644
--- a/src/intel/compiler/brw_fs_reg_allocate.cpp
+++ b/src/intel/compiler/brw_fs_reg_allocate.cpp
@@ -548,6 +548,9 @@ fs_visitor::assign_regs(bool allow_spilling, bool spill_all)
    int first_mrf_hack_node = node_count;
    if (devinfo->gen >= 7)
       node_count += BRW_MAX_GRF - GEN7_MRF_HACK_START;
+   int grf127_send_hack_node = node_count;
+   if (devinfo->gen >= 8 && dispatch_width == 8)
+      node_count ++;
    struct ra_graph *g =
       ra_alloc_interference_graph(compiler->fs_reg_sets[rsi].regs, node_count);
 
@@ -653,6 +656,28 @@ fs_visitor::assign_regs(bool allow_spilling, bool spill_all)
       }
    }
 
+   if (devinfo->gen >= 8 && dispatch_width == 8) {
+      /* At Intel Broadwell PRM, vol 07, section "Instruction Set Reference",
+       * subsection "EUISA Instructions", Send Message (page 990):
+       *
+       * "r127 must not be used for return address when there is a src and
+       * dest overlap in send instruction."
+       *
+       * We are avoiding using grf127 as part of the destination of send
+       * messages adding a node interference to the grf127_send_hack_node.
+       * This node has a fixed asignment to grf127.
+       *
+       * We don't apply it to SIMD16 because previous code avoids any register
+       * overlap between sources and destination.
+       */
+      ra_set_node_reg(g, grf127_send_hack_node, 127);
+      foreach_block_and_inst(block, fs_inst, inst, cfg) {
+         if (inst->is_send_from_grf() && inst->dst.file == VGRF) {
+            ra_add_node_interference(g, inst->dst.nr, grf127_send_hack_node);
+         }
+      }
+   }
+
    /* Debug of register spilling: Go spill everything. */
    if (unlikely(spill_all)) {
       int reg = choose_spill_reg(g);
-- 
2.17.0