[Bug c/67435] Large performance drop on apparently unrelated changes (probable cause : strange inlining side-effect)

[yann.collet.73 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67435

--- Comment #6 from Yann Collet <yann.collet.73 at gmail dot com> ---
The issue seems in fact related to _instruction alignment_.
More precisely, to alignment of some critical loop.

That's basically why adding some code in the file would just "pushes" some
other code into another position, potentially into a less favorable path (hence
the appearance of "random impact").


The following GCC command saved the day :
-falign-loops=32

Note that -falign-loops=16 doesn't work.
I'm suspecting it might be the default value, but can't be sure.
I'm also suspecting that -falign-loops=32 is primarily useful for Broadwell
cpu.


Now, the problem is, `-falign-loops=32` is a gcc-only command line parameter.
It seems not possible to apply this optimization from within the source file,
such as using :
#pragma GCC optimize ("align-loops=32")
or the function targeted :
__attribute__((optimize("align-loops=32")))

None of these alternatives does work.