linux-kernel - Re: [PATCH V2] LoongArch: Add unaligned access support

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Message-ID: <e58d7c6d-cf8a-017b-7e72-6ae579ce6be2@xen0n.name>
Date:   Tue, 18 Oct 2022 16:09:27 +0800
From:   WANG Xuerui <kernel@...0n.name>
To:     David Laight <David.Laight@...LAB.COM>,
        'Huacai Chen' <chenhuacai@...nel.org>
Cc:     Huacai Chen <chenhuacai@...ngson.cn>,
        "loongarch@...ts.linux.dev" <loongarch@...ts.linux.dev>,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Tiezhu Yang <yangtiezhu@...ngson.cn>,
        Guo Ren <guoren@...nel.org>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2] LoongArch: Add unaligned access support

On 2022/10/18 15:48, David Laight wrote:
> From: Huacai Chen
>> Sent: 18 October 2022 08:33
> ...
>>> What about my more structured approach in another reply that avoids the
>>> huge else-if conditions? Both the terrible line wraps and codegen could
>>> be avoided.
> ...
>> OK, let me try.
> 
> I suspect you can mask out some 'operand size' bits from the
> instructions - instead of checking each opcode.

Technically LoongArch instruction formats don't contain any "operand 
size bit", because most current opcodes seem to be simply sequentially 
allocated. While there seem to exist a certain pattern in e.g. encodings 
of {LD,ST,FLD,FST}.{B,H,W,D}, I believe it's just coincidence (e.g. bits 
23:22 of those instructions seem to represent "B/H/W/D"; but other 
instructions clearly don't follow such a pattern, not even the 
{LD,ST}.{BU,HU,WU} ones).

For now I'd personally prefer readability and maintainability over 
performance, because traps are already expensive enough that 
optimizations like this don't really matter.

-- 
WANG "xen0n" Xuerui

Linux/LoongArch mailing list: https://lore.kernel.org/loongarch/