MySQL Bugs: #87365: Preference of newer gcc atomic built ins over sync

Bug #87365	Preference of newer gcc atomic built ins over sync
Submitted:	10 Aug 2017 4:49	Modified:	30 Aug 2017 14:37
Reporter:	Debayan Ghosh (OCA)	Email Updates:
Status:	Verified	Impact on me:	None
Category:	MySQL Server: InnoDB storage engine	Severity:	S4 (Feature request)
Version:	5.7	OS:	Any
Assigned to:		CPU Architecture:	Any
Tags:	aarch64, atomic

Description:
In os0atomic.h , preference given to older sync atomic apis over newer atomic apics.

#if defined(HAVE_GCC_SYNC_BUILTINS)
  <sync>
#else
  <atomics>

This results in the older sync apis to be used in place of newer atomics if available.

How to repeat:

File: storage/innobase/include/os0atomic.h

Hi!

Would you be so kind to let us know which GCC version, exactly, has faster atomics then sync ???

Thanks in advance.

The atomic apis were introduced quite some time back in GCC 4.7.1

Release Notes:
https://gcc.gnu.org/gcc-4.7/changes.html

And also as the gcc docs mentions below:
[https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html]

"New code should always use the ‘__atomic’ builtins rather than the ‘__sync’ builtins."

Hi!

We have not updated all platforms on 4.7.*, so we can not use that universally. However, since upgrades will occur, I consider that this is a fully justified feature request.

Verified as a feature request.

Thanks Sinisa .

My suggestion was to change the order from currently as below

#if defined(HAVE_GCC_SYNC_BUILTINS)
  <sync>
#else
  <atomics>

To something like:

#if defined (HAVE_GCC_ATOMIC_BUILTINS)
 <atomics>
#else if defined (HAVE_GCC_SYNC_BUILTINS)
 <sync>
..

This should work for earlier versions as well where sync will be used and also gives preference to atomics for the newer versions.

It will be researched. Very carefully.