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sched-deps.cc (find_modifiable_mems): Avoid exponential behavior [PR96388]

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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]
===

[For simplicity, let's assume find_inc(backwards==true)].
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.
This commit is contained in:
Maxim Kuvyrkov 2023-11-19 08:43:05 +00:00
parent 25bb8a40ab
commit 0c42d1782e
1 changed files with 44 additions and 4 deletions
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48
gcc/sched-deps.cc
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@ -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 +4873,11 @@ 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));
/* Make sure that n_inc_deps above is consistent with dependencies
we create. */
gcc_assert (mii->inc_insn == inc_cand);
if (backwards)
{
FOR_EACH_DEP (mii->inc_insn, SD_LIST_BACK, sd_it, dep)