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Re: [PATCH] x86: don't have gcc over-align data
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 21 Jul 2025 18:05:22 +0200
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Chen Jiqian <Jiqian.Chen@xxxxxxx>
- Delivery-date: Mon, 21 Jul 2025 16:05:35 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 21.07.2025 16:48, Roger Pau Monné wrote:
> On Wed, Jun 25, 2025 at 11:04:14AM +0200, 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
>> 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>
>>
>> --- a/xen/arch/x86/arch.mk
>> +++ b/xen/arch/x86/arch.mk
>> @@ -8,6 +8,9 @@ CFLAGS += -DXEN_IMG_OFFSET=$(XEN_IMG_OFF
>> # Prevent floating-point variables from creeping into Xen.
>> CFLAGS += -msoft-float
>>
>> +# Don't needlessly over-align larger aggregates.
>> +CFLAGS-$(CONFIG_CC_IS_GCC) += -malign-data=abi
>
> Instead of using CONFIG_CC_IS_GCC should be just use cc-option-add to
> check for the option begin present, regardless of the underlying
> compiler?
We could do so, but why would we want to, when all gcc versions we support
know of the option and Clang has never had a need for it? cc-option-add
is more overhead, and I think we want to avoid such, even if each individual
instance contributes only a tiny bit to overall build time.
Jan
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