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Re: [PATCH] x86: don't have gcc over-align data
- To: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 27 Jun 2025 08:09:33 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Chen Jiqian <Jiqian.Chen@xxxxxxx>
- Delivery-date: Fri, 27 Jun 2025 06:09:48 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 25.06.2025 11:04, Jan Beulich wrote:
> For (aiui) backwards compatibility reasons, gcc defaults to a mode that
> was the exclusive one up to gcc4.8, establishing 32-byte alignment for
Correction - it's 16- or 32-byte alignment, depending on size.
> aggregates larger than a certain size. We don't rely on such, and hence
> we can do with the psABI-compliant 16-byte alignment.
>
> Savings in the build I'm looking at:
> - .data.ro_after_init 344 bytes
> - .rodata + .data.rel.ro 1904 bytes
> - .init.*data.cf_clobber 232 bytes
> - .init (overall) 688 bytes
> - .data.read_mostly 864 bytes
> - .data 600 bytes
> - .bss 1472 bytes
>
> Overall xen-syms' _end happens to move down there by 2 pages.
>
> Clang doesn't support the option, presumably because they never over-
> aligned data.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Looks like this is becoming a necessary pre-req to "vpci: Refactor
REGISTER_VPCI_INIT" [1], unless we want to use undesirable workarounds
or hackery there [2]. Hence may I ask for feedback here?
Thanks, Jan
[1] https://lists.xen.org/archives/html/xen-devel/2025-06/msg00840.html
[2] https://lists.xen.org/archives/html/xen-devel/2025-06/msg01760.html
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