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From: "jmichae3 at yahoo dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/61121] -O2 -ftree-parallelize-loops=0 for maximum not accepted in 4.9.0
Date: Thu, 10 Jul 2014 20:56:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61121

--- Comment #8 from Jim Michaels <jmichae3 at yahoo dot com> ---
in the mingw-w64 gcc compiler at least, -ftree-parallize-loops=12 -O2 runs
single-threaded.

https://gcc.gnu.org/ml/gcc-help/2007-01/msg00165.html

I probably don't understand the compiler, but  I have been trying

with -static and without, using -Wall -Wextra -v -save-temps
-ftree-parallelize-loops=12 -O2 -std=c++11

I have also tried 0 on that number with same results.

I always get single-threaded programs, and my stuff contains a lot of vectors
and loops.

I have also tried with -floop-parallelize-all  -ftree-slp-vectorize
additionally with same results.

it's like that -ftree-parallize-loops is being ignored by the compiler. I did
what everybody said and used the posix/pthreads version. this has not helped.
auto-threading seems broken in 4.9.0 v3rev2.