Re: Why is VC++ STL so slow?
P.J. Plauger wrote:
"David Alasiga" <dialectics@yahoo.com> wrote in message
news:kkjTg.182$If3.172@trnddc07...
I have a program using a map to insert 10,000 item and to do some
searching.
I compiled the program with VC++ 7.1 (VS2003), g++ (3.4.4)
under Cygwin and g++ (3.4.4) under linux. g++ linux version
runs 80% fast than VC++ version and g++ Cygwin version
(running on windows) runs 50% fast than VC++ version.
The VC++ and g++ Cygwin run in the same windows box and
linux box is the same machine as windows box. Why is VC++
STL so slow? Is the VC++ 7.1 STL implemented by
Dinkwumware?
Yes, we've supplied the Standard C++ library to Microsoft
since 1996. The only significant difference I can think of
between the libstdc++ (g++) version of map and ours *might* be
in the choice of allocators. They favor allocators that
optimize for speed, often at the expense of wasting or even
failing to release storage. We provide by default a simple
allocator that calls new and delete for each map node. (Our
add-on library, available at our web site, comes with an
assortment of allocators to choose among if you don't like the
default.) It's also possible that the underlying heap manager
may contribute, but I doubt it.
As for the insert and find machinery, they're too similar to
account for the difference you report.
Another major difference is the compiler which compiled them.
Since the code in question is templates, it is compiled with the
options the user code is compiled with. The original poster
didn't say anything about the options used, so we can only
speculate. I think VC++ compiles in a certain number of runtime
checks by default, but I'm not sure. If they're doing a stack
check at each function call, for example, and g++ is inlining
the function, that would definitly make a difference. Also,
both g++ and VC++ (at least 2005) have debugging versions of the
implementation, which check things like iterator validity and
such. You definitly need special options to get it with g++; if
it is turned on by default in VC++, that could also account for
a difference.
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