intel/fs: fix scheduling of HALT instructions

With the following test :

dEQP-VK.spirv_assembly.instruction.terminate_invocation.terminate.no_out_of_bounds_load

There is a :

shader_start:
   ...                                 <- no control flow
   g0 = some_alu
   g1 = fbl
   g2 = broadcast g3, g1
   g4 = get_buffer_size g2
   ...                                 <- no control flow
   halt                                <- on some lanes
   g5 = send <surface>, g4

eliminate_find_live_channel will remove the fbl/broadcast because it
assumes lane0 is active at get_buffer_size :

shader_start:
   ...                                 <- no control flow
   g0 = some_alu
   g4 = get_buffer_size g0
   ...                                 <- no control flow
   halt                                <- on some lanes
   g5 = send <surface>, g4

But then the instruction scheduler will move the get_buffer_size after
the halt :

shader_start:
   ...                                 <- no control flow
   halt                                <- on some lanes
   g0 = some_alu
   g4 = get_buffer_size g0
   g5 = send <surface>, g4

get_buffer_size pulls the surface index from lane0 in g0 which could
have been turned off by the halt and we end up accessing an invalid
surface handle.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: mesa-stable
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20765>
(cherry picked from commit 9471ffa70a)

.pick_status.json

@@ -5974,7 +5974,7 @@
         "description": "intel/fs: fix scheduling of HALT instructions",
         "nominated": true,
         "nomination_type": 0,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": null,
         "notes": null

src/intel/compiler/brw_schedule_instructions.cpp

@@ -651,6 +651,7 @@ public:
       ralloc_free(this->mem_ctx);
    }
    void add_barrier_deps(schedule_node *n);
+   void add_cross_lane_deps(schedule_node *n);
    void add_dep(schedule_node *before, schedule_node *after, int latency);
    void add_dep(schedule_node *before, schedule_node *after);
 
@@ -1098,6 +1099,33 @@ is_scheduling_barrier(const backend_instruction *inst)
           inst->has_side_effects();
 }
 
+static bool
+has_cross_lane_access(const fs_inst *inst)
+{
+   /* FINISHME:
+    *
+    * This function is likely incomplete in terms of identify cross lane
+    * accesses.
+    */
+   if (inst->opcode == SHADER_OPCODE_BROADCAST ||
+       inst->opcode == SHADER_OPCODE_READ_SR_REG ||
+       inst->opcode == SHADER_OPCODE_CLUSTER_BROADCAST ||
+       inst->opcode == SHADER_OPCODE_SHUFFLE ||
+       inst->opcode == FS_OPCODE_LOAD_LIVE_CHANNELS ||
+       inst->opcode == SHADER_OPCODE_FIND_LAST_LIVE_CHANNEL ||
+       inst->opcode == SHADER_OPCODE_FIND_LIVE_CHANNEL)
+      return true;
+
+   for (unsigned s = 0; s < inst->sources; s++) {
+      if (inst->src[s].file == VGRF) {
+         if (inst->src[s].stride == 0)
+            return true;
+      }
+   }
+
+   return false;
+}
+
 /**
  * Sometimes we really want this node to execute after everything that
  * was before it and before everything that followed it.  This adds
@@ -1128,6 +1156,25 @@ instruction_scheduler::add_barrier_deps(schedule_node *n)
    }
 }
 
+/**
+ * Because some instructions like HALT can disable lanes, scheduling prior to
+ * a cross lane access should not be allowed, otherwise we could end up with
+ * later instructions accessing uninitialized data.
+ */
+void
+instruction_scheduler::add_cross_lane_deps(schedule_node *n)
+{
+   schedule_node *prev = (schedule_node *)n->prev;
+
+   if (prev) {
+      while (!prev->is_head_sentinel()) {
+         if (has_cross_lane_access((fs_inst *)prev->inst))
+            add_dep(prev, n, 0);
+         prev = (schedule_node *)prev->prev;
+      }
+   }
+}
+
 /* instruction scheduling needs to be aware of when an MRF write
  * actually writes 2 MRFs.
  */
@@ -1165,6 +1212,10 @@ fs_instruction_scheduler::calculate_deps()
       if (is_scheduling_barrier(inst))
          add_barrier_deps(n);
 
+      if (inst->opcode == BRW_OPCODE_HALT ||
+          inst->opcode == SHADER_OPCODE_HALT_TARGET)
+          add_cross_lane_deps(n);
+
       /* read-after-write deps. */
       for (int i = 0; i < inst->sources; i++) {
          if (inst->src[i].file == VGRF) {