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Date:	Mon, 6 May 2019 08:09:17 +0100 (GMT Summer Time)
From:	Arthur Norman <acn1 AT cam DOT ac DOT uk>
To:	cygwin AT cygwin DOT com
Subject:	thread_local performance using g++ for cygwin
Message-ID:	<alpine.WNT.2.00.1905052026250.14000@panamint>
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--2262781-27704-1557089932=:8112
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Content-ID: <alpine DOT WNT DOT 2 DOT 00 DOT 1905060751281 DOT 14296 AT panamint>

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
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