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From: "mattreecebentley at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/96750] 10-12% performance decrease in benchmark
 going from GCC8 to GCC9/GCC10
Date: Sun, 27 Sep 2020 23:34:42 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96750

--- Comment #5 from Matt Bentley <mattreecebentley at gmail dot com> ---
If anyone out there is interested in working on this,=20
I found the smallest change possible to create the same performance as GCC8=
-=20
it is literally eliminating one branch possibility in one function
(move-insert).

The branch in question questions whether there are existing memory blocks to
re-use or if we need to create a new memory block. For example, if the
reserve() function has been called there will be existing memory blocks to
re-use.

However the performance drop occurs whether or not there are memory blocks =
to
reuse. The actual if statement is irrelevant. I have tested and can remove =
all
instances of memory block storage (reserve(), erase()) and problem still ex=
ists
if this one branch is still in insert.

I've attached the source files to demonstrate this above, including one
plf_colony.h with the branch removed (renamed to plf_colony_fast.h), so you=
 can
see what difference there is.
This code is all zlib license, free to share, but is early beta so don't
redistribute please.

Thanks,
Matt

ps. For consistency I've also removed the non-move-insert and emplace insta=
nces
of this branch statement, even though they won't be called by the benchmark
code in a C++11-compliant compiler.=