[1/2] Improve compute_control_dep_chain documentation
Commit Message
In the quest to understand how compute_control_dep_chain works I've
produced the following two changes, documenting PR106754 on the
way.
Bootstrap and regtest is running on x86_64-unknown-linux-gnu,
the changes should be no-ops but hopefully improve understanding
of the code.
--
The following refactors compute_control_dep_chain slightly by
inlining is_loop_exit and factoring the check on the loop
invariant condition. It also adds a comment as of how I
understand the code and it's current problem.
* gimple-predicate-analysis.cc (compute_control_dep_chain):
Inline is_loop_exit and refactor, add comment about
loop exits.
---
gcc/gimple-predicate-analysis.cc | 27 +++++++++++++++++++++++----
1 file changed, 23 insertions(+), 4 deletions(-)
Comments
On 8/26/2022 6:34 AM, Richard Biener via Gcc-patches wrote:
> In the quest to understand how compute_control_dep_chain works I've
> produced the following two changes, documenting PR106754 on the
> way.
>
> Bootstrap and regtest is running on x86_64-unknown-linux-gnu,
> the changes should be no-ops but hopefully improve understanding
> of the code.
Thanks so much for this. Every time I've had to get into that code it
makes my head hurt -- anything that clarifies is an improvement.
Jeff
@@ -1110,6 +1110,10 @@ compute_control_dep_chain (basic_block dom_bb, const_basic_block dep_bb,
vec<edge> &cur_cd_chain, unsigned *num_calls,
unsigned in_region = 0, unsigned depth = 0)
{
+ /* In our recursive calls this doesn't happen. */
+ if (single_succ_p (dom_bb))
+ return false;
+
if (*num_calls > (unsigned)param_uninit_control_dep_attempts)
{
if (dump_file)
@@ -1167,7 +1171,21 @@ compute_control_dep_chain (basic_block dom_bb, const_basic_block dep_bb,
basic_block cd_bb = e->dest;
cur_cd_chain.safe_push (e);
while (!dominated_by_p (CDI_POST_DOMINATORS, dom_bb, cd_bb)
- || is_loop_exit (dom_bb, cd_bb))
+ /* We want to stop when the CFG merges back from the
+ branch in dom_bb. The post-dominance check alone
+ falls foul of the case of a loop exit test branch
+ where the path on the loop exit post-dominates
+ the branch block.
+ The following catches this but will not allow
+ exploring the post-dom path further. For the
+ outermost recursion this means we will fail to
+ reach dep_bb while for others it means at least
+ dropping the loop exit predicate from the path
+ which is problematic as it increases the domain
+ spanned by the resulting predicate.
+ See gcc.dg/uninit-pred-11.c for the first case
+ and PR106754 for the second. */
+ || single_pred_p (cd_bb))
{
if (cd_bb == dep_bb)
{
@@ -1187,9 +1205,10 @@ compute_control_dep_chain (basic_block dom_bb, const_basic_block dep_bb,
break;
/* Check if DEP_BB is indirectly control-dependent on DOM_BB. */
- if (compute_control_dep_chain (cd_bb, dep_bb, cd_chains,
- num_chains, cur_cd_chain,
- num_calls, in_region, depth + 1))
+ if (!single_succ_p (cd_bb)
+ && compute_control_dep_chain (cd_bb, dep_bb, cd_chains,
+ num_chains, cur_cd_chain,
+ num_calls, in_region, depth + 1))
{
found_cd_chain = true;
break;