RFR: 8333697: C2: Hit MemLimit in PhaseCFG::global_code_motion [v2]

[Roland Westrelin]

On Mon, 3 Feb 2025 14:43:30 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> I investigated the failure from the `Test.java` that's attached to the
>> bug. The failure with this test is only reproducible up to 8334060
>> (Implementation of Late Barrier Expansion for G1) so experiments I
>> describe here are from the source code for the commit right before it.
>> 
>> Peak malloc memory usage reported by NMT is: 1.3GB
>> 
>> `PhaseCFG::global_code_motion()`, when `OptoRegScheduling` is true,
>> creates a `PhaseIFG` that's, when initialized, allocates `_adjs`: a
>> `maxlrg` array of `IndexSet`s that can contain up to `maxlrg`.
>> 
>> `maxlrg` in this case is 122839. An `IndexSet` is an array of pointers
>> to a 256 bit bitset: one `IndexSet` array needs: 
>> 
>> 
>> 122839 / 256 * 8 = 3832
>> 
>> 
>> and there are of 122839:
>> 
>> 
>> 3832 * 122839 = ~470 MB
>> 
>> 
>> It turns out the `PhaseIFG` object when used from
>> `PhaseCFG::global_code_motion()` doesn't even use the `_adjs`
>> array. So a patch like:
>> 
>> 
>> diff --git a/src/hotspot/share/opto/chaitin.hpp b/src/hotspot/share/opto/chaitin.hpp
>> index cf02deb6019..4e5333bf181 100644
>> --- a/src/hotspot/share/opto/chaitin.hpp
>> +++ b/src/hotspot/share/opto/chaitin.hpp
>> @@ -258,7 +258,7 @@ class PhaseIFG : public Phase {
>>    VectorSet *_yanked;
>>  
>>    PhaseIFG( Arena *arena );
>> -  void init( uint maxlrg );
>> +  void init( uint maxlrg, bool no_adjs = false );
>>  
>>    // Add edge between a and b.  Returns true if actually added.
>>    int add_edge( uint a, uint b );
>> diff --git a/src/hotspot/share/opto/gcm.cpp b/src/hotspot/share/opto/gcm.cpp
>> index ebdefe597ff..fefd75a88c5 100644
>> --- a/src/hotspot/share/opto/gcm.cpp
>> +++ b/src/hotspot/share/opto/gcm.cpp
>> @@ -1704,7 +1704,9 @@ void PhaseCFG::global_code_motion() {
>>      rm_live.reset_to_mark();           // Reclaim working storage
>>      IndexSet::reset_memory(C, &live_arena);
>>      uint node_size = regalloc._lrg_map.max_lrg_id();
>> -    ifg.init(node_size); // Empty IFG
>> +    ifg.init(node_size, true); // Empty IFG
>>      regalloc.set_ifg(ifg);
>>      regalloc.set_live(live);
>>      regalloc.gather_lrg_masks(false);    // Collect LRG masks
>> diff --git a/src/hotspot/share/opto/ifg.cpp b/src/hotspot/share/opto/ifg.cpp
>> index d12698121b9..e42121c2254 100644
>> --- a/src/hotspot/share/opto/ifg.cpp
>> +++ b/src/hotspot/share/opto/ifg.cpp
>> @@ -42,18 +42,24 @@
>>  PhaseIFG::PhaseIFG( Arena *arena ) : Phase(Interference_Graph), _arena(arena) {
>>  }
>>  
>> -void PhaseIFG::init( uint maxlrg ) {
>> +void PhaseIFG::init( uint maxlrg, bool no_adjs ) {
>>  ...
>
> Roland Westrelin has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
> 
>  - review
>  - Merge branch 'master' into JDK-8333697
>  - fix

Thanks for reviewing this.

> Could you please merge with master?

Done.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/23075#issuecomment-2631227463