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Tue, 5 Apr 2016 05:54:51 -0700 (PDT) In-Reply-To: References: <20160404120030.GD14122@kam.mff.cuni.cz> <20160404141436.GB95176@kam.mff.cuni.cz> Date: Tue, 5 Apr 2016 18:24:51 +0530 Message-ID: Subject: Re: [RFC] introduce --param max-lto-partition for having an upper bound on partition size From: Prathamesh Kulkarni To: Richard Biener Cc: Jan Hubicka , gcc Patches , Ramana Radhakrishnan X-IsSubscribed: yes On 5 April 2016 at 16:58, Richard Biener wrote: > On Tue, 5 Apr 2016, Prathamesh Kulkarni wrote: > >> On 4 April 2016 at 19:44, Jan Hubicka wrote: >> > >> >> diff --git a/gcc/lto/lto-partition.c b/gcc/lto/lto-partition.c >> >> index 9eb63c2..bc0c612 100644 >> >> --- a/gcc/lto/lto-partition.c >> >> +++ b/gcc/lto/lto-partition.c >> >> @@ -511,9 +511,20 @@ lto_balanced_map (int n_lto_partitions) >> >> varpool_order.qsort (varpool_node_cmp); >> >> >> >> /* Compute partition size and create the first partition. */ >> >> + if (PARAM_VALUE (MIN_PARTITION_SIZE) > PARAM_VALUE (MAX_PARTITION_SIZE)) >> >> + fatal_error (input_location, "min partition size cannot be greater than max partition size"); >> >> + >> >> partition_size = total_size / n_lto_partitions; >> >> if (partition_size < PARAM_VALUE (MIN_PARTITION_SIZE)) >> >> partition_size = PARAM_VALUE (MIN_PARTITION_SIZE); >> >> + else if (partition_size > PARAM_VALUE (MAX_PARTITION_SIZE)) >> >> + { >> >> + n_lto_partitions = total_size / PARAM_VALUE (MAX_PARTITION_SIZE); >> >> + if (total_size % PARAM_VALUE (MAX_PARTITION_SIZE)) >> >> + n_lto_partitions++; >> >> + partition_size = total_size / n_lto_partitions; >> >> + } >> > >> > lto_balanced_map actually works in a way that looks for cheapest cutpoint in range >> > 3/4*parittion_size to 2*partition_size and picks the cheapest range. >> > Setting partition_size to this value will thus not cause partitioner to produce smaller >> > partitions only. I suppose modify the conditional: >> > >> > /* Partition is too large, unwind into step when best cost was reached and >> > start new partition. */ >> > if (partition->insns > 2 * partition_size) >> > >> > and/or in the code above set the partition_size to half of total_size/max_size. >> > >> > I know this is somewhat sloppy. This was really just first cut implementation >> > many years ago. I expected to reimplement it marter soon, but then there was >> > never really a need for it (I am trying to avoid late IPA optimizations so the >> > partitioning decisions should mostly affect compile time performance only). >> > If ARM is more sensitive for partitining, perhaps it would make sense to try to >> > look for something smarter. >> > >> >> + >> >> npartitions = 1; >> >> partition = new_partition (""); >> >> if (symtab->dump_file) >> >> diff --git a/gcc/lto/lto.c b/gcc/lto/lto.c >> >> index 9dd513f..294b8a4 100644 >> >> --- a/gcc/lto/lto.c >> >> +++ b/gcc/lto/lto.c >> >> @@ -3112,6 +3112,12 @@ do_whole_program_analysis (void) >> >> timevar_pop (TV_WHOPR_WPA); >> >> >> >> timevar_push (TV_WHOPR_PARTITIONING); >> >> + >> >> + if (flag_lto_partition != LTO_PARTITION_BALANCED >> >> + && PARAM_VALUE (MAX_PARTITION_SIZE) != INT_MAX) >> >> + fatal_error (input_location, "--param max-lto-partition should only" >> >> + " be used with balanced partitioning\n"); >> >> + >> > >> > I think we should wire in resonable MAX_PARTITION_SIZE default. THe value you >> > found experimentally may be a good start. For that reason we can't really >> > refuse a value when !LTO_PARTITION_BALANCED. Just document it as parameter for >> > balanced partitioning only and add a parameter to lto_balanced_map specifying whether >> > this param should be honored (because the same path is used for partitioning to one partition) >> > >> > Otherwise the patch looks good to me modulo missing documentation. >> Thanks for the review. I have updated the patch. >> Does this version look OK ? >> I had randomly chosen 10000, not sure if that's an appropriate value >> for default. > > I think it's way too small. This is roughly the number of GIMPLE stmts > (thus roughly the number of instructions). So with say a 8 byte > instruction format it is on the order of 80kB. You'd want to have a > default of at least several ten times of large-unit-insns (also 10000). > I'd choose sth like 1000000 (one million). I find the lto-min-partition > number quite small as well (and up it by a factor of 10). Done in this version. Is it OK after bootstrap+test ? Thanks, Prathamesh > > Richard. > >> I have a silly question about partitioning: Does it hamper >> transformations on ipa optimizations if caller and >> callee get placed in separate partitions ? For instance if callee is >> supposed to be inlined >> into caller, would inlining still take place if callee and caller get >> placed in separate partitions ? >> I tried with a trivial example with -flto-partition=max >> which created 3 partitions for 3 functions (bar, foo and main), and it was >> able to inline bar into foo and foo into main. I am not sure how that happens. >> I thought ltrans can perform transformations on functions only within >> a single partition >> and not across partitions ? >> >> Thanks, >> Prathamesh >> > >> > Honza >> > > -- > Richard Biener > SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg) diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi index 9e54bb7..f0de7ec 100644 --- a/gcc/doc/invoke.texi +++ b/gcc/doc/invoke.texi @@ -9477,6 +9477,11 @@ Size of minimal partition for WHOPR (in estimated instructions). This prevents expenses of splitting very small programs into too many partitions. +@item lto-max-partition +Size of max partition for WHOPR (in estimated instructions). +to provide an upper bound for individual size of partition. +Meant to be used only with balanced partitioning. + @item cxx-max-namespaces-for-diagnostic-help The maximum number of namespaces to consult for suggestions when C++ name lookup fails for an identifier. The default is 1000. diff --git a/gcc/lto/lto-partition.c b/gcc/lto/lto-partition.c index 9eb63c2..d385dd9 100644 --- a/gcc/lto/lto-partition.c +++ b/gcc/lto/lto-partition.c @@ -447,7 +447,7 @@ add_sorted_nodes (vec &next_nodes, ltrans_partition partition) and in-partition calls was reached. */ void -lto_balanced_map (int n_lto_partitions) +lto_balanced_map (int n_lto_partitions, bool honor_max_partition) { int n_nodes = 0; int n_varpool_nodes = 0, varpool_pos = 0, best_varpool_pos = 0; @@ -511,9 +511,13 @@ lto_balanced_map (int n_lto_partitions) varpool_order.qsort (varpool_node_cmp); /* Compute partition size and create the first partition. */ + if (PARAM_VALUE (MIN_PARTITION_SIZE) > PARAM_VALUE (MAX_PARTITION_SIZE)) + fatal_error (input_location, "min partition size cannot be greater than max partition size"); + partition_size = total_size / n_lto_partitions; if (partition_size < PARAM_VALUE (MIN_PARTITION_SIZE)) partition_size = PARAM_VALUE (MIN_PARTITION_SIZE); + npartitions = 1; partition = new_partition (""); if (symtab->dump_file) @@ -719,7 +723,9 @@ lto_balanced_map (int n_lto_partitions) best_cost, best_internal, best_i); /* Partition is too large, unwind into step when best cost was reached and start new partition. */ - if (partition->insns > 2 * partition_size) + if (partition->insns > 2 * partition_size + || (honor_max_partition + && partition->insns > PARAM_VALUE (MAX_PARTITION_SIZE))) { if (best_i != i) { diff --git a/gcc/lto/lto-partition.h b/gcc/lto/lto-partition.h index 31e3764..2992bee 100644 --- a/gcc/lto/lto-partition.h +++ b/gcc/lto/lto-partition.h @@ -35,7 +35,7 @@ extern vec ltrans_partitions; void lto_1_to_1_map (void); void lto_max_map (void); -void lto_balanced_map (int); +void lto_balanced_map (int, bool honor_max_partition = true); void lto_promote_cross_file_statics (void); void free_ltrans_partitions (void); void lto_promote_statics_nonwpa (void); diff --git a/gcc/lto/lto.c b/gcc/lto/lto.c index 9dd513f..82bd9b3 100644 --- a/gcc/lto/lto.c +++ b/gcc/lto/lto.c @@ -3112,12 +3112,13 @@ do_whole_program_analysis (void) timevar_pop (TV_WHOPR_WPA); timevar_push (TV_WHOPR_PARTITIONING); + if (flag_lto_partition == LTO_PARTITION_1TO1) lto_1_to_1_map (); else if (flag_lto_partition == LTO_PARTITION_MAX) lto_max_map (); else if (flag_lto_partition == LTO_PARTITION_ONE) - lto_balanced_map (1); + lto_balanced_map (1, false); else if (flag_lto_partition == LTO_PARTITION_BALANCED) lto_balanced_map (PARAM_VALUE (PARAM_LTO_PARTITIONS)); else diff --git a/gcc/params.def b/gcc/params.def index 9362c15..97e41aa 100644 --- a/gcc/params.def +++ b/gcc/params.def @@ -1027,7 +1027,12 @@ DEFPARAM (PARAM_LTO_PARTITIONS, DEFPARAM (MIN_PARTITION_SIZE, "lto-min-partition", "Minimal size of a partition for LTO (in estimated instructions).", - 1000, 0, 0) + 10000, 0, 0) + +DEFPARAM (MAX_PARTITION_SIZE, + "lto-max-partition", + "Maximal size of a partition for LTO (in estimated instructions).", + 1000000, 0, INT_MAX) /* Diagnostic parameters. */