linux-kernel - Re: [PATCH 4/5] ARC: add support for DSP-enabled userspace applications

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Message-ID: <8f5af6bd-af31-8f2f-9565-f1f505e6ba11@synopsys.com>
Date:   Wed, 4 Mar 2020 17:58:04 +0000
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Alexey Brodkin" <Alexey.Brodkin@...opsys.com>
Subject: Re: [PATCH 4/5] ARC: add support for DSP-enabled userspace
 applications

On 3/4/20 3:51 AM, Eugeniy Paltsev wrote:
>>> +                "r" (_saveto),                                       \
>>> +                "r" (_readfrom),                                     \
>>> +                "I" (_aux),                                          \
>>> +                "I" (_offt)                                          \
>>> +             :                                                       \
>>
>> AEX with "I" constraint will likely be an 8 byte instructions always. Best to give
>> compiler wiggle room with "Ir"
>
> Could you please explain how "Ir" will work in this case?
> Does this mean that compiler can pass the value either as constant ('I') or
> via register ('r')?

Right. If the same compile-time const is used multiple times, say SR of same value
to a reg, if can decide to hoist the value in a register, if you provide it "r" or
"Ir", leading to shorter overall code.

-Vineet