Hi Vladimir,
Hi Jeff,
Richard and Alexander have reviewed this patch and [I assume] have no
further comments. OK to merge?
On Wed, 22 Nov 2023 at 15:14, Maxim Kuvyrkov <maxim.kuvyr...@linaro.org>
wrote:
> 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.
> ---
> gcc/sched-deps.cc | 48 +++++++++++++++++++++++++++++++++++++++++++----
> 1 file changed, 44 insertions(+), 4 deletions(-)
>
> diff --git a/gcc/sched-deps.cc b/gcc/sched-deps.cc
> index c23218890f3..005fc0f567e 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 +4873,11 @@ 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)
> --
> 2.34.1
>
>
--
Maxim Kuvyrkov
www.linaro.org
