[PATCH] Use 1U everywhere in elf/elf.h.

[Carlos O'Donell]

On 03/24/2015 05:03 PM, Petr Machata wrote:
> "Carlos O'Donell" <carlos@redhat.com> writes:
> 
>> On 03/06/2015 05:28 PM, Roland McGrath wrote:
>>>> I would like to write a test case for it, but I can't get anything to
>>>> trigger the original reported failure by readelf when running with
>>>> -fsanitize=undefined.
>>>>
>>>> Therefore I can't at present even get a reproducer nor write a regression
>>>> test, but I know what what we have for SHF_EXCLUDE is wrong.
>>>
>>> A regression test for a reported manifest issue is of course worthwhile.
>>> But I was talking about a generic test that verifies at compile time what
>>> the types actually are rather than some particular characteristic of behavior.
>>
>> How?
>>
>> I mean I could use C++ and the associated RTTI to do the comparison at
>> runtime and thus produce a detailed analysis of the types.
>>
>> Is that what you're thinking too?
> 
> Would something like the following be acceptable?  It builds on
> known-elf.h/known-elf.awk that I've earlier today proposed for
> contribution to elfutils.

Sure. 

> The way this would catch the original bug is that GCC refuses to
> compile-time evaluate 1 << 31.  I'm not sure, but it seems that's
> because it's UB--change it to 1U or 1L and GCC compile-time evaluates it
> happily.  So the compilation would fail, indicating that 1 << 31 is not
> a compile-time constant.
> 
> Porting back to C<11, the static assert can be emulated by the
> negative-sized char array thick.  Then the likes of 1 << 31 that are not
> compile-time constants lead to actual calls to log2, which we catch as
> link errors.  Not as nice as compile-time error, but also works.
> 
> If C++ is acceptable, it should be easy to replace the __builtin_log2
> and _Static_assert with a bit of (portable, if C++98 is considered that)
> template metaprogramming.

Any of your solutions looks good to me.

The test itself can be C or C++.

The test can include the header and test whatever it wants.

We support gcc 4.6 or newer for building glibc, so if that works great.

Cheers,
Carlos.