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constexpr function return is non-constant
In my humble try to compile DASH with all compilers we support, I also tried to use the latest Cray CCE to compile the DASH part. While this has not been successful so far (some symbols are missing during linking), the following warnings could be relevant nevertheless:
CC-12251 crayc++: WARNING File = /zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-development/dash/include/dash/pattern/BlockPattern1D.h, Line = 724
constexpr function return is non-constant
return global(_team->myid(), std::array<IndexType, 1> {{ l_index }})[0];
^
A template was detected during header processing.
The instantiation of "std::array<IndexType, 2UL>
dash::BlockPattern<1, Arrangement,
IndexType>::initialize_local_range(dash::BlockPattern<1,
Arrangement, IndexType>::SizeType) const [with
Arrangement=(dash::MemArrange)1,
IndexType=dash::default_index_t]" at line 319
The instantiation of "dash::BlockPattern<1, Arrangement,
IndexType>::BlockPattern(dash::BlockPattern<1,
Arrangement, IndexType>::SizeSpec_t,
dash::BlockPattern<1, Arrangement,
IndexType>::DistributionSpec_t, dash::Team &) [with
Arrangement=(dash::MemArrange)1,
IndexType=dash::default_index_t]" at line 769 of
"/zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-develop
ment/dash/include/dash/Array.h"
The instantiation of "dash::Array<ElementType, IndexType,
PatternType>::Array(dash::Team &) [with
ElementType=node_t, IndexType=dash::default_index_t,
PatternType=dash::BlockPattern<1, (dash::MemArrange)1,
dash::default_index_t>]" at line 85 of
"/zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-develop
ment/dash/examples/app.01.mindeg/main.cpp"
CC-12251 crayc++: WARNING File = /zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-development/dash/include/dash/iterator/GlobIter.h, Line = 613
constexpr function return is non-constant
return self_t(
^
A template was detected during header processing.
The instantiation of "bool dash::Array<ElementType, IndexType,
PatternType>::allocate(const PatternType &) [with
ElementType=node_t, IndexType=dash::default_index_t,
PatternType=dash::BlockPattern<1, (dash::MemArrange)1,
dash::default_index_t>]" at line 1204 of
"/zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-develop
ment/dash/include/dash/Array.h"
The instantiation of "bool dash::Array<ElementType, IndexType,
PatternType>::allocate(dash::Array<ElementType,
IndexType, PatternType>::size_type,
dash::DistributionSpec<1>, dash::Team &) [with
ElementType=node_t, IndexType=dash::default_index_t,
PatternType=dash::BlockPattern<1, (dash::MemArrange)1,
dash::default_index_t>]" at line 246 of
"/zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-develop
ment/dash/examples/app.01.mindeg/main.cpp"
CC-12251 crayc++: WARNING File = /zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-development/dash/include/dash/iterator/GlobIter.h, Line = 621
constexpr function return is non-constant
return self_t(
^
Detected during instantiation of "dash::GlobIter<ElementType,
PatternType,
dash::GlobMem<std::remove_const<ElementType>::type,
dash::allocator::CollectiveAllocator<std::remove_const<Elem
entType>::type>>, dash::GlobPtr<ElementType, PatternType>,
dash::GlobRef<ElementType>> dash::min_element(const
dash::GlobIter<ElementType, PatternType,
dash::GlobMem<std::remove_const<ElementType>::type,
dash::allocator::CollectiveAllocator<std::remove_const<Elem
entType>::type>>, dash::GlobPtr<ElementType, PatternType>,
dash::GlobRef<ElementType>> &, const
dash::GlobIter<ElementType, PatternType,
dash::GlobMem<std::remove_const<ElementType>::type,
dash::allocator::CollectiveAllocator<std::remove_const<Elem
entType>::type>>, dash::GlobPtr<ElementType, PatternType>,
dash::GlobRef<ElementType>> &, Compare) [with
ElementType=node_t, PatternType=dash::BlockPattern<1,
(dash::MemArrange)1, dash::default_index_t>,
Compare=std::less<const node_t &>]" at line 331 of
"/zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-developme
nt/dash/examples/app.01.mindeg/main.cpp".
The Intel compiler has reported similar warnings.
@devreal I fixed the code sections listed in the warnings you posted in PR #321. How did you obtain the constexpr warnings? I can't seem to generate them on my machines.
I can't seem to produce them either with the Intel compiler (I was sure I saw them there). I threw the Cray compiler at your branch and it reported some more issues:
CC-12251 crayc++: WARNING File = /zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-development/dash/include/dash/pattern/BlockPattern1D.h, Line = 708
constexpr function return is non-constant
return global(_team->myid(), std::array<IndexType, 1> {{ l_index }})[0];
^
A template was detected during header processing.
The instantiation of "std::array<IndexType, 2UL>
dash::BlockPattern<1, Arrangement,
IndexType>::initialize_local_range(dash::BlockPattern<1,
Arrangement, IndexType>::SizeType) const [with
Arrangement=(dash::MemArrange)1, IndexType=long long]" at
line 319
The instantiation of "dash::BlockPattern<1, Arrangement,
IndexType>::BlockPattern(dash::BlockPattern<1,
Arrangement, IndexType>::SizeSpec_t,
dash::BlockPattern<1, Arrangement,
IndexType>::DistributionSpec_t, dash::Team &) [with
Arrangement=(dash::MemArrange)1, IndexType=long long]" at
line 793 of
"/zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-develop
ment/dash/include/dash/Array.h"
The instantiation of "dash::Array<ElementType, IndexType,
PatternType>::Array(dash::Array<ElementType, IndexType,
PatternType>::size_type, const dash::Array<ElementType,
IndexType, PatternType>::DistributionSpec_t &, dash::Team
&) [with ElementType=int, IndexType=long long,
PatternType=dash::BlockPattern<1, (dash::MemArrange)1,
long long>]" at line 810 of
"/zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-develop
ment/dash/include/dash/Array.h"
The instantiation of "dash::Array<ElementType, IndexType,
PatternType>::Array(dash::Array<ElementType, IndexType,
PatternType>::size_type, dash::Team &) [with
ElementType=int, IndexType=long long,
PatternType=dash::BlockPattern<1, (dash::MemArrange)1,
long long>]" at line 53 of
"/zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-develop
ment/dash/examples/bench.02.memhog/main.cpp"
The instantiation of "void test_array_alloc<T>(size_t) [with
T=int]" at line 19 of
"/zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-develop
ment/dash/examples/bench.02.memhog/main.cpp"
CC-12251 crayc++: WARNING File = /zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-development/dash/include/dash/pattern/BlockPattern1D.h, Line = 708
constexpr function return is non-constant
return global(_team->myid(), std::array<IndexType, 1> {{ l_index }})[0];
^
A template was detected during header processing.
The instantiation of "std::array<IndexType, 2UL>
dash::BlockPattern<1, Arrangement,
IndexType>::initialize_local_range(dash::BlockPattern<1,
Arrangement, IndexType>::SizeType) const [with
Arrangement=(dash::MemArrange)1,
IndexType=dash::default_index_t]" at line 319
The instantiation of "dash::BlockPattern<1, Arrangement,
IndexType>::BlockPattern(dash::BlockPattern<1,
Arrangement, IndexType>::SizeSpec_t,
dash::BlockPattern<1, Arrangement,
IndexType>::DistributionSpec_t, dash::Team &) [with
Arrangement=(dash::MemArrange)1,
IndexType=dash::default_index_t]" at line 793 of
"/zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-develop
ment/dash/include/dash/Array.h"
The instantiation of "dash::Array<ElementType, IndexType,
PatternType>::Array(dash::Array<ElementType, IndexType,
PatternType>::size_type, const dash::Array<ElementType,
IndexType, PatternType>::DistributionSpec_t &, dash::Team
&) [with ElementType=int,
IndexType=dash::default_index_t,
PatternType=dash::BlockPattern<1, (dash::MemArrange)1,
dash::default_index_t>]" at line 810 of
"/zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-develop
ment/dash/include/dash/Array.h"
The instantiation of "dash::Array<ElementType, IndexType,
PatternType>::Array(dash::Array<ElementType, IndexType,
PatternType>::size_type, dash::Team &) [with
ElementType=int, IndexType=dash::default_index_t,
PatternType=dash::BlockPattern<1, (dash::MemArrange)1,
dash::default_index_t>]" at line 54 of
"/zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-develop
ment/dash/examples/bench.02.meminit/main.cpp"
The instantiation of "bool init_array<T>(size_t) [with T=int]" at
line 32 of
"/zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-develop
ment/dash/examples/bench.02.meminit/main.cpp"
The instantiation of "void perform_test<T>(size_t, size_t) [with
T=int]" at line 20 of
"/zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-develop
ment/dash/examples/bench.02.meminit/main.cpp"
CC-12251 crayc++: WARNING File = /zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-development/dash/include/dash/pattern/BlockPattern1D.h, Line = 708
constexpr function return is non-constant
return global(_team->myid(), std::array<IndexType, 1> {{ l_index }})[0];
^
A template was detected during header processing.
The instantiation of "std::array<IndexType, 2UL>
dash::BlockPattern<1, Arrangement,
IndexType>::initialize_local_range(dash::BlockPattern<1,
Arrangement, IndexType>::SizeType) const [with
Arrangement=(dash::MemArrange)1,
IndexType=dash::default_index_t]" at line 319
The instantiation of "dash::BlockPattern<1, Arrangement,
IndexType>::BlockPattern(dash::BlockPattern<1,
Arrangement, IndexType>::SizeSpec_t,
dash::BlockPattern<1, Arrangement,
IndexType>::DistributionSpec_t, dash::Team &) [with
Arrangement=(dash::MemArrange)1,
IndexType=dash::default_index_t]" at line 769 of
"/zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-develop
ment/dash/include/dash/Array.h"
The instantiation of "dash::Array<ElementType, IndexType,
PatternType>::Array(dash::Team &) [with
ElementType=node_t, IndexType=dash::default_index_t,
PatternType=dash::BlockPattern<1, (dash::MemArrange)1,
dash::default_index_t>]" at line 85 of
"/zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-develop
ment/dash/examples/app.01.mindeg/main.cpp"
CC-12251 crayc++: WARNING File = /zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-development/dash/include/dash/pattern/CSRPattern.h, Line = 751
constexpr function return is non-constant
return global(_team->myid(), std::array<IndexType, 1> {{ l_index }})[0];
^
A template was detected during header processing.
The instantiation of "void dash::CSRPattern<1, Arrangement,
IndexType>::initialize_local_range() [with
Arrangement=(dash::MemArrange)1, IndexType=int]" at line
378
The instantiation of "dash::CSRPattern<1, Arrangement,
IndexType>::CSRPattern(const
std::vector<dash::CSRPattern<1, Arrangement,
IndexType>::size_type, std::allocator<dash::CSRPattern<1,
Arrangement, IndexType>::size_type>> &, dash::Team &)
[with Arrangement=(dash::MemArrange)1, IndexType=int]" at
line 133 of
"/zhome/academic/HLRS/hlrs/hpcjschu/src/dash/dash-develop
ment/dash/examples/bench.01.igups/main.cpp"
These warnings are caused by Team.myid() which is not constexpr and probably never will.
The value of _team->myid() could be stored in a member, but pattern constructors would then not be constexpr any more.
I will try some workarounds, but it's not a bug, just a missed opportunity for compile-time optimization.
The value of
_team->myid()could be stored in a member, but pattern constructors would then not beconstexprany more.
Most (all?) pattern constructor are not constexpr anyway afaics. The value of team->myid() could be cached upon team creation so that should allow constexpr there as well.
@devreal ... so ... what you are saying is that team ids are not cached at team creation? Why?
Well, the team-relative unit ID is not cached from the start but queried on first request and then cached. The reason is that the unit ID in dash::Team::All() cannot be initialized in the constructor so this initialization is deferred. This is what makes dash::Team::myid() non-constexpr. A workaround could be to set the unit ID for dash::Team::All() during initialization in dash::Init(). The same holds true for dash::Team::size().
I started implementing this but I'm stuck with the check for dash::is_initialized() in dash::Team::myid(). My understanding is that this check was required in case an application called dash::myid() before dash::Init() has been called. IMO, calling any DASH functionality (dash::myid() in this case) in a uninitialized DASH environment (before dash::Init() or after dash::Finalize()) is UB so the return value of dash::myid() is either -1 or the global unit ID that remained from the previous initialization.
OK, so I think I slightly misunderstood the meaning of constexpr. The C++11 standard (https://github.com/cplusplus/draft/blob/master/papers/N3376.pdf) says in 7.1.5:
For a constexpr function, if no function argument values exist such that the function invocation sub- stitution would produce a constant expression (5.19), the program is ill-formed; no diagnostic required.
My understanding is that any constexpr member function accessing a member variable in a class that has no constexpr constructor is ill-formed (not only a missed chance for optimization). The constructors of dash::Team cannot be made constexpr since there is no constexpr c'tor for std::list.
We may need to revise our usage of constexpr, e.g., BlockPattern has no constexpr c'tor but constexpr member functions accessing member variables (local_at for example).
Or am I missing something here?