linux-kernel - Re: [PATCH] ext4: improve str2hashbuf by processing 4-byte chunks

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Message-ID: <aR9Ir6fdzD5_0Pkn@google.com>
Date: Fri, 21 Nov 2025 00:58:23 +0800
From: Kuan-Wei Chiu <visitorckw@...il.com>
To: Theodore Tso <tytso@....edu>
Cc: David Laight <david.laight.linux@...il.com>,
	Guan-Chun Wu <409411716@....tku.edu.tw>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ext4: improve str2hashbuf by processing 4-byte chunks

Hi Ted,

On Thu, Nov 20, 2025 at 10:58:16AM -0500, Theodore Tso wrote:
> On Sun, Nov 16, 2025 at 07:35:13PM +0000, David Laight wrote:
> > 
> > The (int) casts are unnecessary (throughout), 'char' is always promoted to
> > 'signed int' before any arithmetic.
> 
> nit: in this case the casts aren't necessary, but your comment is not
> correct in general, so I just wanted to make sure it's corrected in
> case someone later looks at the mail archive.
> 
> "char" is not always signed.  It can be signed or unsigned; the C
> specification allows either.  In this particular case, scp is a
> "signed char", not "char".
> 
> Secondly, it's not that a promotion happens before "any" arithmetic.
> If we add two 8-bit values together, promotion doesn't happen.  In
> this case, we are adding a signed char to an int, so the promotion
> will happen.
> 
I believe David was referring to the C11 spec 6.3.1.1:

If an int can represent all values of the original type (as restricted
by the width, for a bit-field), the value is converted to an int;
otherwise, it is converted to an unsigned int. These are called the
integer promotions. All other types are unchanged by the integer
promotions.

The spec explicitly mentions char + char in 5.1.2.3 example:

EXAMPLE 2 In executing the fragment
char c1, c2;
/* ... */
c1 = c1 + c2;
the ‘‘integer promotions’’ require that the abstract machine promote
the value of each variable to int size and then add the two ints and
truncate the sum. Provided the addition of two chars can be done
without overflow, or with overflow wrapping silently to produce the
correct result, the actual execution need only produce the same result,
possibly omitting the promotions.

So IIUC conceptually the promotion happens, even if the compiler
optimizes it out in the actual execution.

Link: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf

Regards,
Kuan-Wei