Re: libstdc++: Speed up push_back

[Jonathan Wakely <jwakely@redhat.com>]

On Thu, 23 Nov 2023 at 15:34, Jan Hubicka <hubicka@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.
>