[Bug libstdc++/45574] cin.getline() is extremely slow

["tstarling at wikimedia dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #24 from tstarling at wikimedia dot org  2010-09-10 15:25 -------
Created an attachment (id=21766)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=21766&action=view)
dynamic_cast<> hack

The attached patch uses a dynamic_cast<> hack to avoid the need to break the
ABI. I added *_unlocked functions to cstdio, I'm not sure if this is necessary,
but it's easy enough to remove that part if not. I also added some
lightly-tested autoconf stuff. I'm an autoconf newbie so that part should
probably be reviewed carefully. 

stdio_sync_filebuf<wchar_t>::_M_getline() is currently unreachable, since I
only edited basic_istream<char>::getline() and not
basic_istream<wchar_t>::getline(). It would be easy enough to fix that. I
haven't used getwc_unlocked() because it's a GNU extension, POSIX only has
non-wide unlocked I/O. 

The timings for 1M lines with 500 bytes per line, user time only, are:

Old library: 26.7s
New library: 1.65s
fgets: 0.280s

So it's better, but not perfect.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45574