> 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); _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.