linux-kernel - Re: [PATCH 4/9] ARC: mm: do_page_fault refactor #3: tidyup vma access permission code

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Date:   Fri, 17 May 2019 22:23:40 +0000
From:   Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
To:     Vineet Gupta <Vineet.Gupta1@...opsys.com>
CC:     "paltsev@...opsys.com" <paltsev@...opsys.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
Subject: Re: [PATCH 4/9] ARC: mm: do_page_fault refactor #3: tidyup vma access
 permission code

Hmmm,

so load the bool variable from memory is converted to such asm code:

----------------->8------------------- 
ldb	r2,[some_bool_address]
extb_s	r2,r2
----------------->8-------------------

Could you please describe that the magic is going on there?

This extb_s instruction looks completely useless here, according on the LDB description from PRM:
----------------->8-------------------
LD LDH LDW LDB LDD:
The size of the requested data is specified by the data size field <.zz> and by default, data is zero
extended from the most-significant bit of the data to the most-significant bit of the destination
register.
----------------->8-------------------

Am I missing something?

On Thu, 2019-05-16 at 17:37 +0000, Vineet Gupta wrote:
> On 5/16/19 10:24 AM, Eugeniy Paltsev wrote:
> > > +    unsigned int write = 0, exec = 0, mask;
> > 
> > Probably it's better to use 'bool' type for 'write' and 'exec' as we really use them as a boolean variables.
> 
> Right those are semantics, but the generated code for "bool" is not ideal - given
> it is inherently a "char" it is promoted first to an int with an additional EXTB
> which I really dislike.
> Guess it is more of a style thing.
> 
> -Vineet
-- 
 Eugeniy Paltsev