This is the mail archive of the
gcc@gcc.gnu.org
mailing list for the GCC project.
Re: Massive performance regression from switching to gcc 4.5
On Fri, Jun 25, 2010 at 06:10:56AM -0700, Jan Hubicka wrote:
> When you compile with -Os, the inlining happens only when code size reduces.
> Thus we pretty much care about the code size metrics only. I suspect the
> problem here might be that normal C++ code needs some inlining to make
> abstraction penalty go away. GCC -Os implementation is generally tuned for
> CSiBE and it is somewhat C centric (that makes sense for embedded world). As a
> result we might get quite noticeable slowdowns on C++ apps compiled with -Os
> (and code size growth too since abstraction is never eliminated). It can be
> seen also at tramp3d (Pooma testcase) where -Os produces a lot bigger and a lot
> slower code.
One would think that in most of the abstraction-penalty cases, the inlined
code (often the direct reading or setting of a class data member) should
be both smaller and faster than the call, so -Os should inline. Perhaps
there are cases where the inlined version is, say, one or two instructions larger
than the version with a call, and this causes the degradation? If so,
maybe some heuristic could be produced that would inline anyway for
a small function?