[Mesa-dev] [PATCH] intel/ir: Fix CFG corruption in opt_predicated_break().

Francisco Jerez currojerez at riseup.net
Wed Jul 24 00:56:01 UTC 2019

Specifically the optimization of a conditional BREAK + WHILE sequence
into a conditional WHILE seems pretty broken.  The list of successors
of "earlier_block" (where the conditional BREAK was found) is emptied
and then re-created with the same edges for no apparent reason.  On
top of that the list of predecessors of the block immediately after
the WHILE loop is emptied, but only one of the original edges will be
added back, which means that potentially several blocks that still
have it on their list of successors won't be on its list of
predecessors anymore, causing all sorts of hilarity due to the
inconsistency in the control flow graph.

The solution is to remove the code that's removing valid edges from
the CFG.  cfg_t::remove_block() will already clean up after itself.
The assert in bblock_t::combine_with() also needs to be removed since
we will be merging a block with multiple children into the first one
of them.

Found the issue on a hardware enabling branch originally, but
apparently somebody reproduced the same problem independently on
master in the meantime.

Fixes: d13bcdb3a9f ("i965/fs: Extend predicated break pass to predicate WHILE.")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111009
Cc: jiradet.jd at gmail.com
Cc: Sergii Romantsov <sergii.romantsov at globallogic.com>
Cc: Matt Turner <mattst88 at gmail.com>
Cc: mesa-stable at lists.freedesktop.org
 src/intel/compiler/brw_cfg.cpp              | 3 ---
 src/intel/compiler/brw_predicated_break.cpp | 6 ------
 2 files changed, 9 deletions(-)

diff --git a/src/intel/compiler/brw_cfg.cpp b/src/intel/compiler/brw_cfg.cpp
index 600b428a492..6c40889088d 100644
--- a/src/intel/compiler/brw_cfg.cpp
+++ b/src/intel/compiler/brw_cfg.cpp
@@ -128,9 +128,6 @@ void
 bblock_t::combine_with(bblock_t *that)
-   foreach_list_typed (bblock_link, link, link, &this->children) {
-      assert(link->block == that);
-   }
    foreach_list_typed (bblock_link, link, link, &that->parents) {
       assert(link->block == this);
diff --git a/src/intel/compiler/brw_predicated_break.cpp b/src/intel/compiler/brw_predicated_break.cpp
index 607715dace4..e60052f3608 100644
--- a/src/intel/compiler/brw_predicated_break.cpp
+++ b/src/intel/compiler/brw_predicated_break.cpp
@@ -128,14 +128,8 @@ opt_predicated_break(backend_shader *s)
          while_inst->predicate = jump_inst->predicate;
          while_inst->predicate_inverse = !jump_inst->predicate_inverse;
-         earlier_block->children.make_empty();
-         earlier_block->add_successor(s->cfg->mem_ctx, while_block);
-         earlier_block->next()->parents.make_empty();
-         earlier_block->add_successor(s->cfg->mem_ctx, earlier_block->next());
       progress = true;

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