[Mesa-dev] [PATCH 1/6] i965: fix cycle estimates when there's a pipeline stall

[Connor Abbott]

The issue time for an instruction is how many cycles it takes to
actually put it into the pipeline. If there's a pipeline stall that
causes the instruction to be delayed, we should first take that into
account to figure out when the instruction would start executing and
*then* add the issue time. The old code had it backwards, and so we
would underestimate the total time whenever we thought there would be a
pipeline stall by up to the issue time of the instruction.

Signed-off-by: Connor Abbott <cwabbott0 at gmail.com>
---
 src/mesa/drivers/dri/i965/brw_schedule_instructions.cpp | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_schedule_instructions.cpp b/src/mesa/drivers/dri/i965/brw_schedule_instructions.cpp
index 4e43e5c..76d58e2 100644
--- a/src/mesa/drivers/dri/i965/brw_schedule_instructions.cpp
+++ b/src/mesa/drivers/dri/i965/brw_schedule_instructions.cpp
@@ -1405,18 +1405,19 @@ instruction_scheduler::schedule_instructions(bblock_t *block)
       instructions_to_schedule--;
       update_register_pressure(chosen->inst);
 
+      /* If we expected a delay for scheduling, then bump the clock to reflect
+       * that.  In reality, the hardware will switch to another hyperthread
+       * and may not return to dispatching our thread for a while even after
+       * we're unblocked.  After this, we have the time when the chosen
+       * instruction will start executing.
+       */
+      time = MAX2(time, chosen->unblocked_time);
+
       /* Update the clock for how soon an instruction could start after the
        * chosen one.
        */
       time += issue_time(chosen->inst);
 
-      /* If we expected a delay for scheduling, then bump the clock to reflect
-       * that as well.  In reality, the hardware will switch to another
-       * hyperthread and may not return to dispatching our thread for a while
-       * even after we're unblocked.
-       */
-      time = MAX2(time, chosen->unblocked_time);
-
       if (debug) {
          fprintf(stderr, "clock %4d, scheduled: ", time);
          bs->dump_instruction(chosen->inst);
-- 
2.1.0