Re: [PATCH] x86: don't have gcc over-align data

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Jul 21, 2025 at 06:05:22PM +0200, Jan Beulich wrote:
> 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.

IIRC we only evaluate those once now, so the overhead would be
minimal.

Regardless:

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks, Roger.