linux-kernel - Re: [PATCH] LoongArch: Make -mstrict-align be configurable

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Message-ID: <85fb9d71-b956-3d3e-f279-1310eec8e5c9@loongson.cn>
Date:   Tue, 7 Feb 2023 09:27:25 +0800
From:   Jianmin Lv <lvjianmin@...ngson.cn>
To:     Xi Ruoyao <xry111@...111.site>, WANG Xuerui <kernel@...0n.name>,
        Huacai Chen <chenhuacai@...ngson.cn>,
        Arnd Bergmann <arnd@...db.de>,
        Huacai Chen <chenhuacai@...nel.org>
Cc:     loongarch@...ts.linux.dev, linux-arch@...r.kernel.org,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Guo Ren <guoren@...nel.org>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] LoongArch: Make -mstrict-align be configurable



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.