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[PATCH] x86: don't have gcc over-align data
- To: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 25 Jun 2025 11:04:14 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Chen Jiqian <Jiqian.Chen@xxxxxxx>
- Delivery-date: Wed, 25 Jun 2025 09:04:32 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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
+
$(call cc-options-add,CFLAGS,CC,$(EMBEDDED_EXTRA_CFLAGS))
$(call cc-option-add,CFLAGS,CC,-Wnested-externs)
$(call as-option-add,CFLAGS,CC,".equ \"x\"$(comma)1",-DHAVE_AS_QUOTED_SYM)
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