This patch avoids sched-deps.cc:find_inc() creating exponential number
of dependencies, which become memory and compilation time hogs.
Consider example (simplified from PR96388) ...
===
sp=sp-4 // sp_insnA
mem_insnA1[sp+A1]
...
mem_insnAN[sp+AN]
sp=sp-4 // sp_insnB
mem_insnB1[sp+B1]
...
mem_insnBM[sp+BM]
===
... in this example find_modifiable_mems() will arrange for mem_insnA*
to be able to pass sp_insnA, and, while doing this, will create
dependencies between all mem_insnA*s and sp_insnB -- because sp_insnB
is a consumer of sp_insnA. After this sp_insnB will have N new
backward dependencies.
Then find_modifiable_mems() gets to mem_insnB*s and starts to create
N new dependencies for _every_ mem_insnB*. This gets us N*M new
dependencies.
In PR96833's testcase N and M are 10k-15k, which causes RAM usage of
30GB and compilation time of 30 minutes, with sched2 accounting for
95% of both metrics. After this patch the RAM usage is down to 1GB
and compilation time is down to 3-4 minutes, with sched2 no longer
standing out on -ftime-report or memory usage.
gcc/ChangeLog:
PR rtl-optimization/96388
PR rtl-optimization/111554
* sched-deps.cc (find_inc): Avoid exponential behavior.
---
gcc/sched-deps.cc | 45 +++++++++++++++++++++++++++++++++++++++++----
1 file changed, 41 insertions(+), 4 deletions(-)
diff --git a/gcc/sched-deps.cc b/gcc/sched-deps.cc
index c23218890f3..a286ff319e2 100644
--- a/gcc/sched-deps.cc
+++ b/gcc/sched-deps.cc
@@ -4779,24 +4779,59 @@ parse_add_or_inc (struct mem_inc_info *mii, rtx_insn
*insn, bool before_mem)
/* Once a suitable mem reference has been found and the corresponding data
in MII has been filled in, this function is called to find a suitable
add or inc insn involving the register we found in the memory
- reference. */
+ reference.
+ If successful, this function will create additional dependencies between
+ - mii->inc_insn's producers and mii->mem_insn as a consumer (if backwards)
+ - mii->inc_insn's consumers and mii->mem_insn as a producer (if !backwards).
+*/
static bool
find_inc (struct mem_inc_info *mii, bool backwards)
{
sd_iterator_def sd_it;
dep_t dep;
+ sd_list_types_def mem_deps = backwards ? SD_LIST_HARD_BACK : SD_LIST_FORW;
+ int n_mem_deps = sd_lists_size (mii->mem_insn, mem_deps);
- sd_it = sd_iterator_start (mii->mem_insn,
- backwards ? SD_LIST_HARD_BACK : SD_LIST_FORW);
+ sd_it = sd_iterator_start (mii->mem_insn, mem_deps);
while (sd_iterator_cond (&sd_it, &dep))
{
dep_node_t node = DEP_LINK_NODE (*sd_it.linkp);
rtx_insn *pro = DEP_PRO (dep);
rtx_insn *con = DEP_CON (dep);
- rtx_insn *inc_cand = backwards ? pro : con;
+ rtx_insn *inc_cand;
+ int n_inc_deps;
+
if (DEP_NONREG (dep) || DEP_MULTIPLE (dep))
goto next;
+
+ if (backwards)
+ {
+ inc_cand = pro;
+ n_inc_deps = sd_lists_size (inc_cand, SD_LIST_BACK);
+ }
+ else
+ {
+ inc_cand = con;
+ n_inc_deps = sd_lists_size (inc_cand, SD_LIST_FORW);
+ }
+
+ /* In the FOR_EACH_DEP loop below we will create additional n_inc_deps
+ for mem_insn. This by itself is not a problem, since each mem_insn
+ will have only a few inc_insns associated with it. However, if
+ we consider that a single inc_insn may have a lot of mem_insns, AND,
+ on top of that, a few other inc_insns associated with it --
+ those _other inc_insns_ will get (n_mem_deps * number of MEM insns)
+ dependencies created for them. This may cause an exponential
+ growth of memory usage and scheduling time.
+ See PR96388 for details.
+ We [heuristically] use n_inc_deps as a proxy for the number of MEM
+ insns, and drop opportunities for breaking modifiable_mem dependencies
+ when dependency lists grow beyond reasonable size. */
+ if (n_mem_deps * n_inc_deps
+ >= param_max_pending_list_length * param_max_pending_list_length)
+ goto next;
+
if (parse_add_or_inc (mii, inc_cand, backwards))
{
struct dep_replacement *desc;
@@ -4838,6 +4873,8 @@ find_inc (struct mem_inc_info *mii, bool backwards)
desc->insn = mii->mem_insn;
move_dep_link (DEP_NODE_BACK (node), INSN_HARD_BACK_DEPS (con),
INSN_SPEC_BACK_DEPS (con));
+
+ gcc_assert (mii->inc_insn == inc_cand);
if (backwards)
{
FOR_EACH_DEP (mii->inc_insn, SD_LIST_BACK, sd_it, dep)
--
2.34.1
