On 24/10/2024 16:35, Jonathan Wakely via Gcc wrote:
On Thu, 24 Oct 2024 at 15:00, Mateusz Guzik via Gcc <gcc@gcc.gnu.org> wrote:
I understand the stock behavior of pilling variables on may happen to
improve cache usage.
However, in a multicore setting it is a never-ending source of
unintentionally showing up and disappearing false-sharing depending on
unrelated variables being added or removed.
I'm aware one can manually add padding or explicitly tell the linker
to put a specific variable elsewhere, but in large codebases getting
to a point where everything is annotated is quite a challenge. To give
you one example of a project which did not get there despite literally
decades of multicore development: the Linux kernel (or in fact any
open-source kernel out there, last I checked).
In the meantime a great clutch would contain such problems to a
specific .o file.
...
For illustrative purposes, I don't care about the name:
-align-object-data-section=64
Thoughts?
Wouldn't that be a linker option, and so not part of GCC?
I would have thought it would be better as part of the compiler. For
each compilation unit, you generate one or more data sections depending
on the variable initialisations, compiler options and target (.bss,
.data, .rodata, .sbss, etc.). If the compiler has
"-align-object-data-section=64" in effect, then you could add ".align
64" to the start and the end of each section. As far as I can see, that
would give the effect the OP is looking for in a simple manner that can
also be different for different translation units in the same program
(which would be hard to do with a linker flag). And if this is a flag
in gcc, then it will work for any ld-compatible linker (gold, mold, etc.).
gcc already generates a ".align" linker command with the minimum
alignment required by the variables in the section. This new flag would
then do two things - one is set a minimum alignment for the data
sections (it can still increase if a variable has bigger alignment, such
as via an "_Alignas" specifier or an "aligned" attribute). Secondly, it
would add another ".align" command at the end of the section so that if
a section with a small alignment specifier is linked after one with a
specified large alignment, you'll get padding inserted.
mvh.,
David
