netdev - Re: [Intel-wired-lan] [PATCH iwl-next v2 5/5] ixgbe: drop unnecessary constant casts to u16

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Message-ID: <b3273f0c-c708-488e-88c0-853e4e8e5ed5@jacekk.info>
Date: Tue, 8 Jul 2025 11:34:48 +0200
From: Jacek Kowalski <jacek@...ekk.info>
To: "Loktionov, Aleksandr" <aleksandr.loktionov@...el.com>,
 "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>,
 "Kitszel, Przemyslaw" <przemyslaw.kitszel@...el.com>,
 Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>
Cc: "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [Intel-wired-lan] [PATCH iwl-next v2 5/5] ixgbe: drop unnecessary
 constant casts to u16

>> -	checksum = (u16)IXGBE_EEPROM_SUM - checksum;
>> +	checksum = IXGBE_EEPROM_SUM - checksum;
>>
> Can't lead to different results, especially when:
> checksum > IXGBE_EEPROM_SUM → the result becomes negative in int, and narrowing to u16 causes unexpected wraparound?
> 
> With this patch you are changing the semantics of the code - from explicit 16bit arithmetic to full int implicit promotion which can be error-prone or compiler-dependent /* for different targets */.

As far as I understand the C language does this by design - in the terms of C specification:

> If an int can represent all values of the original type (...), the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions. (see note)
> 
> (note) The integer promotions are applied only: as part of the usual arithmetic conversions, (...)

And subtraction semantics are:

> If both operands have arithmetic type, the usual arithmetic conversions are performed on them.

So there is no *16 bit arithmetic* - it is always done on integers (usually 32 bits).

Or have I missed something?

Additionally I've checked AMD64, ARM and MIPS assembly output from GCC and clang on https://godbolt.org/z/GPsMxrWfe - both of the following snippets compile to exactly the same assembly:

#define NVM_SUM 0xBABA
int test(int num) {
    volatile unsigned short test = 0xFFFF;
    unsigned short checksum = NVM_SUM - test;
    return checksum;
}

vs.:

#define NVM_SUM 0xBABA
int test(int num) {
    volatile unsigned short test = 0xFFFF;
    unsigned short checksum = ((unsigned short)NVM_SUM) - test;
    return checksum;
}

-- 
Best regards,
  Jacek Kowalski