On Thu, 23 Nov 2023 at 15:34, Jan Hubicka <hubi...@ucw.cz> wrote: > > > > On Sunday, 19 November 2023 22:53:37 CET Jan Hubicka wrote: > > > > Sadly it is really hard to work out this > > > > from IPA passes, since we basically care whether the iterator points to > > > > the same place as the end pointer, which are both passed by reference. > > > > This is inter-procedural value numbering that is quite out of reach. > > > > > > I've done a fair share of branching on __builtin_constant_p in > > > std::experimental::simd to improve code-gen. It's powerful! But maybe we > > > also need the other side of the story to tell the optimizer: "I know you > > > can't const-prop everything; but this variable / expression, even if you > > > need to put in a lot of effort, the performance difference will be worth > > > it." > > > > > > For std::vector, the remaining capacity could be such a value. The > > > functions f() and g() are equivalent (their code-gen isn't https:// > > > compiler-explorer.com/z/r44ejK1qz): > > > > > > #include <vector> > > > > > > auto > > > f() > > > { > > > std::vector<int> x; > > > x.reserve(10); > > > for (int i = 0; i < 10; ++i) > > > x.push_back(0); > > > return x; > > > } > > > auto > > > g() > > > { return std::vector<int>(10, 0); } > > > > With my changes at -O3 we now inline push_back, so we could optimize the > > first loop to the second. However with > > ~/trunk-install/bin/gcc -O3 auto.C -S -fdump-tree-all-details > > -fno-exceptions -fno-store-merging -fno-tree-slp-vectorize > > the fist problem is right at the begining: > > > > <bb 2> [local count: 97603128]: > > MEM[(struct _Vector_impl_data *)x_4(D)]._M_start = 0B; > > MEM[(struct _Vector_impl_data *)x_4(D)]._M_finish = 0B; > > MEM[(struct _Vector_impl_data *)x_4(D)]._M_end_of_storage = 0B; > > _37 = operator new (40); > > I also wonder, if default operator new and malloc can be handled as not > reading/modifying anything visible to the user code.
No, there's no way to know if the default operator new is being used. A replacement operator new could be provided at link-time. That's why we need -fsane-operator-new > That would help > us to propagate here even if we lose track of points-to information. > > We have: > > /* If the call is to a replaceable operator delete and results > from a delete expression as opposed to a direct call to > such operator, then we can treat it as free. */ > if (fndecl > && DECL_IS_OPERATOR_DELETE_P (fndecl) > && DECL_IS_REPLACEABLE_OPERATOR (fndecl) > && gimple_call_from_new_or_delete (stmt)) > return ". o "; > /* Similarly operator new can be treated as malloc. */ > if (fndecl > && DECL_IS_REPLACEABLE_OPERATOR_NEW_P (fndecl) > && gimple_call_from_new_or_delete (stmt)) > return "m "; > Which informs alias analysis that new returns pointer to memory > not aliasing with anything and that free is not reading anything > from its parameter (but it is modelled as a write to make it clear > that the memory dies). But this only applies to new T[n] not to operator new(n * sizeof(T)). So it's not relevant to std::vector. > stmt_kills_ref_p special cases BUILT_IN_FREE but not OPERATOR delete > to make it clear that everything pointed to by it dies. This is needed > because 'o' only means that some data may be overwritten, but it does > not make it clear that all data dies. > > Not handling operator delete seems like an omision, but maybe it is not > too critical since we emit clobbers around destructors that are usually > right before call to delete. Also ipa-modref kill analysis does not > understand BUILT_IN_FREE nor delete and could. > > I wonder if we can handle both as const except for side-effects > described. > > Honza > > _22 = x_4(D)->D.26019._M_impl.D.25320._M_finish; > > _23 = x_4(D)->D.26019._M_impl.D.25320._M_start; > > _24 = _22 - _23; > > if (_24 > 0) > > goto <bb 3>; [41.48%] > > else > > goto <bb 4>; [58.52%] > > > > So the vector is fist initialized with _M_start=_M_finish=0, but after > > call to new we already are not able to propagate this. > > > > This is because x is returned and PTA considers it escaping. This is > > problem discussed in > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112653 > > Which shows that it is likely worthwhile to fix PTA to handle this > > correctly. >