From 4a7f0aa2e034c7f82cb143367efadb0e3eeca08e Mon Sep 17 00:00:00 2001
From: Jason Ekstrand <jason.ekstrand@intel.com>
Date: Tue, 1 Dec 2020 09:48:30 -0600
Subject: [PATCH] intel/fs: Remove unnecessary HALT_TARGET in
 opt_redundant_halt()
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This means the pass has to walk all the instructions but it was doing
that in a bunch of cases anyway when it didn't have a HALT_TARGET.
However, removing HALT_TARGET frees up the scheduler a bit because
HALT_TARGET is considered a scheduling barrier.  The shader-db results
are kind-of a wash but we're about to add HALT_TARGET unconditionally so
we want to be able to get rid of it.

Shader-db results on Ice Lake:

    total instructions in shared programs: 19935623 -> 19935623 (0.00%)
    instructions in affected programs: 0 -> 0
    helped: 0
    HURT: 0

    total cycles in shared programs: 976758472 -> 976766135 (<.01%)
    cycles in affected programs: 11097707 -> 11105370 (0.07%)
    helped: 1750
    HURT: 875
    helped stats (abs) min: 1 max: 866 x̄: 26.39 x̃: 4
    helped stats (rel) min: <.01% max: 39.24% x̄: 1.25% x̃: 0.46%
    HURT stats (abs)   min: 1 max: 1678 x̄: 61.54 x̃: 10
    HURT stats (rel)   min: <.01% max: 65.69% x̄: 1.86% x̃: 0.42%
    95% mean confidence interval for cycles value: -2.48 8.32
    95% mean confidence interval for cycles %-change: -0.40% -0.03%
    Inconclusive result (value mean confidence interval includes 0).

    LOST:   62
    GAINED: 46

All of the lost/gained programs are SIMD32 fragment shaders.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5071>
---
 src/intel/compiler/brw_fs.cpp | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/src/intel/compiler/brw_fs.cpp b/src/intel/compiler/brw_fs.cpp
index 15362c7c79b..87dd28f23cc 100644
--- a/src/intel/compiler/brw_fs.cpp
+++ b/src/intel/compiler/brw_fs.cpp
@@ -3033,24 +3033,36 @@ fs_visitor::opt_redundant_halt()
 {
    bool progress = false;
 
-   bblock_t *last_bblock = cfg->blocks[cfg->num_blocks - 1];
-
+   unsigned halt_count = 0;
    fs_inst *halt_target = NULL;
-   foreach_inst_in_block_reverse(fs_inst, inst, last_bblock) {
+   bblock_t *halt_target_block = NULL;
+   foreach_block_and_inst(block, fs_inst, inst, cfg) {
+      if (inst->opcode == BRW_OPCODE_HALT)
+         halt_count++;
+
       if (inst->opcode == SHADER_OPCODE_HALT_TARGET) {
          halt_target = inst;
+         halt_target_block = block;
          break;
       }
    }
 
-   if (!halt_target)
+   if (!halt_target) {
+      assert(halt_count == 0);
       return false;
+   }
 
    /* Delete any HALTs immediately before the halt target. */
    for (fs_inst *prev = (fs_inst *) halt_target->prev;
         !prev->is_head_sentinel() && prev->opcode == BRW_OPCODE_HALT;
         prev = (fs_inst *) halt_target->prev) {
-      prev->remove(last_bblock);
+      prev->remove(halt_target_block);
+      halt_count--;
+      progress = true;
+   }
+
+   if (halt_count == 0) {
+      halt_target->remove(halt_target_block);
       progress = true;
    }