Re: question about inlining

[Martin Sebor <msebor at gmail dot com>]

On 12/06/2017 12:11 PM, Richard Biener wrote:
> On December 6, 2017 6:38:11 PM GMT+01:00, Martin Sebor <msebor@gmail.com> wrote:
> > While testing a libstdc++ patch that relies on inlining to
> > expose a GCC limitation I noticed that the same member function
> > of a class template is inlined into one function in the test but
> > not into the other, even though it is inlined into each if each
> > is compiled separately (i.e., in a file on its own).
> >
> > I wasn't aware that inlining decisions made in one function could
> > affect those in another, or in the whole file.  Is that expected?
> > And if yes, what's the rationale?
> >
> > Here's a simplified test case.  When compiled with -O2 or -O3
> > and either just -DFOO or just -DBAR, the call to vector::resize()
> > and all the functions called from it, including (crucially)
> > vector::_M_default_append, are inlined.  But when compiled with
> > -DFOO -DBAR _M_default_append is not inlined.  With a somewhat
> > more involved test case I've also seen the first call inlined
> > but not the second, which was also surprising to me.
>
> There are unit and function growth limits that can be hit.

I see, thank you for reminding me.

Nothing brings the implications into sharp focus like two virtually
identical functions optimized differently as a result of exceeding
some size limit.  It would make perfect sense to me if I were using
-Os but I can't help but wonder how useful this heuristic is at -O3.

It also makes me wonder if users are aware of how this impacts
their decisions to structure code.  If adding a new function to
a file containing carefully optimized code can result in outlining
what was previously inlined, it's probably best to define one
function per file.  I've seen C libraries laid out like that but
never a C++ program.

Martin

> >   #include <vector>
> >
> >   void sink (std::vector<int>&);
> >
> >   #if FOO
> >   void foo (unsigned n)
> >   {
> >     std::vector<int> a;
> >     a.resize (n);
> >     sink (a);
> >   }
> >   #endif
> >
> >   #if BAR
> >   void bar (unsigned long n)
> >   {
> >     std::vector<int> a;
> >     a.resize (n);
> >     sink (a);
> >   }
> >   #endif
> >
> > Thanks
> > Martin