thread_local performance using g++ for cygwin

[Brian Inglis]

On 2019-05-06 01:09, Arthur Norman wrote:
> The attached code tried two loops each of which just calls a function that
> increments an integer variable. One loop is a simple variable, the other has the
> thread_local qualifier. I put in ugly annotations to prevent g++ from inlining
> the functions even though I compile with -O3, but in real cases separate
> compilation forces each TL access to be independent.
> The timing as between the two cases is EXTREME on cygwin (both 32 and 64-bit)
> however g++ on Linux and the Microsoft compiler on Windows both manage to keep
> the base of thread-local regions in a segment register in such a way that the
> thread_local overhead is minimal. The cygwin thread_local overhead is large
> enough to be very visible in my code as a whole. I can see that changing to use
> a segment register might be a painful API change even if it was feasible, but
> has there been any consideration of it?
> Note that x86_64-w64-mingw32-g++ and clang also do not use the segment register
> so suffer the significant speed penalty, so maybe it would be hard to match what
> Microsoft manage?
> 
> Sample output:
> Â Â Â  simple 1.265
> Â Â Â  thread_local 33.219

See:
https://cygwin.com/git/gitweb.cgi?p=newlib-cygwin.git;f=winsup/cygwin/how-cygtls-works.txt;a=blob

and you may want to compare the gcc default options and -S assembler output for
your test case on Linux and Cygwin, and perhaps also any glibc and newlib TLS
support functions called: running your Linux tests under some WSL distro will
even out OS kernel differences.

My own tests on Win 10.0.17763.437 1809 are worse than yours:

$ g++ -O3 -o tltime.{bin,cpp}
$ ./tltime.bin
simple 1.60938
thread_local 1.95312
$ uname -srvmo
Linux 4.4.0-17763-Microsoft #379-Microsoft Wed Mar 06 19:16:00 PST 2019 x86_64
GNU/Linux
$ head /etc/os-release
PRETTY_NAME="Debian GNU/Linux 9 (stretch)"
NAME="Debian GNU/Linux"
VERSION_ID="9"
VERSION="9 (stretch)"
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"

$ g++ -O3 -o tltime.{exe,cpp}
$ ./tltime.exe
simple 1.608
thread_local 53.25
$ uname -srvmo
CYGWIN_NT-10.0 3.0.7(0.338/5/3) 2019-04-30 18:08 x86_64 Cygwin
$ head /etc/os-release
PRETTY_NAME="Cygwin 64 3.0.7 2019-04-30"
NAME=Cygwin
ID=cygwin
ID_LIKE=msys mingw
VARIANT="64"
VARIANT_ID="x86_64"
VERSION="3.0.7 (0.338/5/3) 2019-04-30 18:08"
VERSION_ID="3.0.7"
BUILD_ID="0.338/5/3 2019-04-30 18:08"
CPE_NAME="cpe:/a:cygwin:cygwin:3.0.7::~~~~x64~Windows%3e%3d6.0"

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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