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Message-ID: <55641c2f-2745-ad92-c64c-0f8ee9603fbd@pobox.com>
Date: Sat, 4 Jul 2020 16:13:51 -0500
From: Daniel Santos <daniel.santos@...ox.com>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] Fix signed compare warning
On 6/26/20 1:26 AM, Jeffrey Walton wrote:
> On Fri, Jun 26, 2020 at 2:20 AM Daniel Santos <daniel@...t.us> wrote:
>> ...
>>>> if (at) {
>>>> - if (at->tv_nsec >= 1000000000UL) return EINVAL;
>>>> + if ((unsigned long)at->tv_nsec >= 1000000000UL) return EINVAL;
>>>> if (__clock_gettime(clk, &to)) return EINVAL;
>>>> to.tv_sec = at->tv_sec - to.tv_sec;
>>>> if ((to.tv_nsec = at->tv_nsec - to.tv_nsec) < 0) {
>>>>
>>> may be use < 0 || >= 1000000000L and avoid the cast.
>>> there is a similar issue in src/thread/pthread_cond_timedwait.c as well
>> Thank you for that. I'll resubmit changing both instances.
>>
>> In this case, the POSIX spec requires nt_nsec to be a long (
>> https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/time.h.html
>> ). Either way, a good optimizer should convert this into an unsigned
> I believe the C language says the signed value gets promoted to an
> unsigned value. I don't believe the optimizer has anything to do with
> it.
>
> That's why -1 is greater than 1 in C:
>
> int x = -1;
> unsigned int y = 1;
> if (x > y)
> printf("WTF???\n");
>
> Jeff
Yes, I was referring to Khem's suggestion:
may be use < 0 || >= 1000000000L and avoid the cast.
The optimizer should convert this into a single unsigned compare on just
about any modern processor (e.g., two's compliment).
I suppose the real solution is to not add -Wextra to CFLAGS unless you
add -Wno-sign-compare, as musl intentionally uses this promotion rule.
Thanks!
Daniel
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