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

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

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?

Thanks, Roger.