On 2023/2/6 下午9:30, Xi Ruoyao wrote:
On Mon, 2023-02-06 at 21:13 +0800, Jianmin Lv wrote:
(1) Is the difference contributed by a bad code generation of GCC? If
true, it's better to improve GCC before someone starts to build a distro
for LA264 as it would benefit the user space as well.
AFAIK, GCC builds to produce unaligned-access-enabled target binary by
default (without -mstrict-align) for improving user space performance
(small size and runtime high performance), which is also based the fact
that the vast majority of LoongArch CPUs support unaligned-access.
I mean: if someone starts to build a distro for a less-capable LoongArch
processor, (s)he will need an entire user space compiled with -mstrict-
align. So it would be better to start preparation now.
And it's likely (s)he will either submit a GCC patch to make GCC
enable/disable -mstrict-align based on the -march= (--with-arch at
configure time) value, or hack GCC to enable -mstrict-align by default
for the distro. So I think we'll also need:
+ifdef CONFIG_ARCH_STRICT_ALIGN may enable strict align by default.
# Don't emit unaligned accesses.
# Not all LoongArch cores support unaligned access, and as kernel we can't
# rely on others to provide emulation for these accesses.
KBUILD_CFLAGS += $(call cc-option,-mstrict-align)
+else
+# Distros designed for running on both kind of processors may disable
+# strict align by default, but the user may want a no-strict-align
+# kernel for his/her specific hardware.
KBUILD_CFLAGS += $(call cc-option,-mno-strict-align)
+endif
Thanks, Ruoyao, I think it's good suggestion. After talking about it
with GCC colleague, it's very likely make GCC enable/disable
-mstrict-align based on the -march= in future, just as you said.
