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:
     simple 1.265
     thread_local 33.219


            Arthur