Date: Thu, 29 Jun 2017 23:46:25 +0200 From: Dimitry Andric <dim@FreeBSD.org> To: Mark Millard <markmi@dsl-only.net> Cc: Konstantin Belousov <kostikbel@gmail.com>, Justin Hibbits <jhibbits@FreeBSD.org>, FreeBSD Toolchain <freebsd-toolchain@freebsd.org>, FreeBSD Current <freebsd-current@freebsd.org> Subject: Re: head -r320458 (e.g.) amd64 -> powerpc (32-bit) cross-buildworld fails for time libc++ static_asserts and overflow/underflow of long long (system clang/clang++ 4 based build) Message-ID: <C72D03A1-7977-4D18-A93F-3016D56BBA8B@FreeBSD.org> In-Reply-To: <22F5CFA1-E6A9-4C41-BD43-967105B44C49@dsl-only.net> References: <D8AB364D-9E4B-470A-A858-842E73530C3B@dsl-only.net> <5B12D9A6-C9A2-4B0D-B32D-D04D7DB1E3BC@dsl-only.net> <AFCE7DA0-E8D2-4FC7-9E2F-78011E54F1A0@dsl-only.net> <55A1694D-BC40-49AE-BEF7-CEE502126E96@FreeBSD.org> <20170629125407.GM1935@kib.kiev.ua> <22F5CFA1-E6A9-4C41-BD43-967105B44C49@dsl-only.net>
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[-- Attachment #1 --]
On 29 Jun 2017, at 19:16, Mark Millard <markmi@dsl-only.net> wrote:
>
> On 2017-Jun-29, at 5:54 AM, Konstantin Belousov <kostikbel at gmail.com> wrote:
>>
>> On Thu, Jun 29, 2017 at 12:47:10PM +0200, Dimitry Andric wrote:
>>> One nasty problem with this is that it is not possible to figure out at
>>> compile time what the size of time_t is. You always need some sort of
>>> configure-time test, and an external define.
>>
>> It is arguably possible, with constexpr.
>
> I took Dimitry's wording as probably referring to
> testing the size in the C/C++ preprocessor like
> the original code tests for __LP64__ being defined
> vs. not to control what it does: extending that to
> involve more preprocessor tests to pick from more
> code blocks. (But it is a guess given his wording.)
Yeah, what I meant is that the code does something like:
#if __LP64__
static_assert(whatever, "foo");
#else
static_assert(otherthing, "bar");
#endif
where __LP64__ was erroneously thought to determine whether time_t was
64-bit. You cannot replace this with something like the following C++11
construct, though:
constexpr bool time_t_64bit = sizeof(time_t) == 8;
if (time_t_64bit)
static_assert(true, "time_t is OK");
else
static_assert(false, "time_t is bad");
because both static assertions will be evaluated at compile time, and
one of them will fail.
In any case, Eric Fiselier already had some sort of patch lined up, but
I slacked off on trying it out. Sorry about that. :)
-Dimitry
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