linux-kernel - Re: [PATCH] LoongArch: Improve memory ops

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Date:   Mon, 10 Apr 2023 22:22:39 +0800
From:   Rui Wang <wangrui@...ngson.cn>
To:     Xi Ruoyao <xry111@...111.site>
Cc:     Huacai Chen <chenhuacai@...nel.org>,
        WANG Xuerui <kernel@...0n.name>, loongarch@...ts.linux.dev,
        linux-kernel@...r.kernel.org, loongson-kernel@...ts.loongnix.cn
Subject: Re: [PATCH] LoongArch: Improve memory ops

On Mon, Apr 10, 2023 at 8:54 PM Xi Ruoyao <xry111@...111.site> wrote:
> Regarding these functions: we have -ffreestanding which is preventing
> the compiler from optimizing for e.g. "memcpy(a, b, 8);" into a simple
> ld.d/st.d pair.  A explicit compiler built-in usage like
>
>   #define memcpy(a, b, c) __builtin_memcpy(a, b, c)
>
> would allow the compiler to do such kind of optimization.   Will this
> improve the performance?

That's a good question. IIUC, the current compiler generates
inefficient code for constant-length memcpy, which may sacrifice
performance for compatibility, as not all hardware supports unaligned
memory access. We need a runtime CPU feature dispatch mechanism
similar to alternatives to improve the compiler. This is indeed a
problem that needs to be addressed.

Regards,
Rui