Re: [RFC] introduce --param max-lto-partition for having an upper bound on partition size

[Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>]

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On 5 April 2016 at 18:28, Richard Biener <rguenther@suse.de> wrote:
> On Tue, 5 Apr 2016, Prathamesh Kulkarni wrote:
>
>> On 5 April 2016 at 16:58, Richard Biener <rguenther@suse.de> wrote:
>> > On Tue, 5 Apr 2016, Prathamesh Kulkarni wrote:
>> >
>> >> On 4 April 2016 at 19:44, Jan Hubicka <hubicka@ucw.cz> 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.
>
> I'd do that separately.
>
> Please no default parameter for lto_balanced_map (), instead change
> all callers.
>
>> Is it OK after bootstrap+test ?
>
> Note that this is for stage1 only.  I'll leave approval to Honza
> (also verification of the default max param - not sure if for example
> chromium or firefox should/will be split to more than 32 partitions
> with the patch)
Removed default parameter in this version. I verified with the patch
for chromium LTO build:
n_lto_partitions == 32, ltrans_partitions.length() == 31

Thanks,
Prathamesh
>
> Richard.
>
>> 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 <rguenther@suse.de>
>> > SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)
>>
>
> --
> Richard Biener <rguenther@suse.de>
> SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)

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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<symtab_node *> &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;
@@ -511,6 +511,9 @@ 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);
@@ -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..a55594c 100644
--- a/gcc/lto/lto-partition.h
+++ b/gcc/lto/lto-partition.h
@@ -35,7 +35,7 @@ extern vec<ltrans_partition> 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); 
 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..ef2ce15 100644
--- a/gcc/lto/lto.c
+++ b/gcc/lto/lto.c
@@ -3117,9 +3118,9 @@ do_whole_program_analysis (void)
   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));
+    lto_balanced_map (PARAM_VALUE (PARAM_LTO_PARTITIONS), true);
   else
     gcc_unreachable ();
 
diff --git a/gcc/params.def b/gcc/params.def
index 9362c15..b5da384 100644
--- a/gcc/params.def
+++ b/gcc/params.def
@@ -1029,6 +1029,11 @@ DEFPARAM (MIN_PARTITION_SIZE,
 	  "Minimal size of a partition for LTO (in estimated instructions).",
 	  1000, 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.  */
 
 DEFPARAM (CXX_MAX_NAMESPACES_FOR_DIAGNOSTIC_HELP,

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2016-04-06  Prathamesh Kulkarni  <prathamesh.kulkarni@linaro.org>

	* params.def (MAX_PARTITION_SIZE): New param.
	* lto/lto-partition.h (lto_balanced_map): New parameter
	honor_max_partition. 
	* lto/lto-partition.c (lto_balanced_map): New parameter
	honor_max_partition.
	Check if partition size is greater than max-lto-partition.
	* lto/lto.c (do_whole_program_analysis): Adjust calls to
	lto_balanced_map() to pass 2nd argument.
	* invoke.texi: Document lto-max-partition.