linux-kernel - Re: Can we drop upstream Linux x32 support?

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Message-ID: <dfefa4b4-f008-9e7a-491d-af3dc324d718@petrovitsch.priv.at>
Date:   Fri, 14 Dec 2018 17:29:51 +0100
From:   Bernd Petrovitsch <bernd@...rovitsch.priv.at>
To:     Rich Felker <dalias@...c.org>
Cc:     John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Florian Weimer <fweimer@...hat.com>,
        Mike Frysinger <vapier@...too.org>,
        "H. J. Lu" <hjl.tools@...il.com>, x32@...ldd.debian.org,
        Arnd Bergmann <arnd@...db.de>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Can we drop upstream Linux x32 support?

On 14/12/2018 17:17, Rich Felker wrote:
> On Fri, Dec 14, 2018 at 03:13:10PM +0100, Bernd Petrovitsch wrote:
[..]
>> FWIW I have
>> ----  snip  ----
>> #if defined __x86_64__
>> # if defined __ILP32__		// x32
>> #  define PRI_time_t	"lld"	// for time_t
>> #  define PRI_nsec_t	"lld"	// for tv_nsec in struct timespec
>> # else				// x86_64
>> #  define PRI_time_t	"ld"	// for time_t
>> #  define PRI_nsec_t	"ld"	// for tv_nsec in struct timespec
>> # endif
>> #else				// i[3-6]68
>> # define PRI_time_t	"ld"	// for time_t
>> # define PRI_nsec_t	"ld"	// for tv_nsec in struct timespec
>> #endif
>> ----  snip  ----
>> in my userspace code for printf() and friends - I don't know how libc's
>> react to such a patch (and I don't care for the name of the macros as
>> long it's obviously clear for which type they are).
>> I assume/fear we won't get additional modifiers into the relevant
>> standards for libc types (as they are far more like pid_t, uid_t etc.).
>> And casting to u/intmaxptr_t to get a defined printf()-modifier doesn't
>> look appealing to me to "solve" such issues.
> 
> This is all useless (and wrong since tv_nsec is required to have type
> long as part of C and POSIX, regardless of ILP32-vs-LP64; that's a bug

Thanks. OK, I didn't know that - and 32bit is enough to represent 1E9
(as a second won't have more nanosecs).
Hmm, can we fix that in the x32 world?
Sry, I'm not the expert on glibc vs ABI va syscall interface vs breakage
there though.

> in glibc's x32). Just do:
> 
> 	printf("%jd", (intmax_t)t);
> 
> Saving 2 or 3 insns (for sign or zero extension) around a call to
> printf is not going to make any measurable difference to performance

Until someone comes up with hardware with ASIC support for 1k bit int's
and (ab)uses intmax_t for that. SCNR ....

> or any significant difference to size, and it's immeasurably more
> readable than the awful PRI* macros and the
> adjacent-string-concatenation they rely on.

One gets used to the PRI_* macros over time (and there no calculated
format strings in my world) - and type casts are not better in my eyes ...

MfG,
	Bernd
-- 
"I dislike type abstraction if it has no real reason. And saving
on typing is not a good reason - if your typing speed is the main
issue when you're coding, you're doing something seriously wrong."
    - Linus Torvalds

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