RISC-V: _init and _fini vs array variants

[Palmer Dabbelt]

On Fri, 22 Jun 2018 11:25:48 PDT (-0700), Jim Wilson wrote:
> On 06/22/2018 06:35 AM, Florian Weimer wrote:
>> RISC-V doesn't have _init and _fini, it seems.  At least the glibc code
>> avoids calling these functions for main programs even if they are defined.
>>
>> Are they simply undefined, or are they defined and repeated in the array?
>>
>> Could we use a weak reference in generic code to simplify things?
>>
>> I also notice that the dynamic loader, for shared objects, still looks
>> for DT_INIT and DT_FINI.  Presumably, this is harmless because they are
>> never present on RISC-V?
>
> I don't know the history of this particular decision in the RISC-V port,
> but init_array has been preferred over init for a long time.  You can
> stack unwind/throw exceptions through init_array, but not init for
> instance.  GCC switched from init to init_array a while ago for this and
> other reasons.

The rationale is pretty much as you guessed: we don't have any legacy init/fini 
based systems so we just don't provide init/fini.

> As a new target, RISC-V has no old compilers that need init, and hence
> there should be no need for glibc to support it.  Supporting init means
> some extra code in the crt files, and some extra cycles at program
> startup running the extra code.  It would be nice if we can avoid that.
>
> The no-init glibc support was added in 2013
>      https://sourceware.org/ml/libc-alpha/2013-02/msg00604.html
>      https://sourceware.org/ml/libc-alpha/2013-03/msg00322.html
> Looking at the patches, I see that they agree with what I said, new
> targets should not need the overhead of supporting init.
>
> The fact that DT_INIT and DT_FINI are still used looks like a harmless
> oversight in the original patch adding this support.  I'd suggest filing
> a glibc bug report for that.
>
> It does look like RISC-V is the only target using this support, which
> means we will be finding code that has unnecessary dependencies on the
> obsolete _init.
>
> Do you have something specific that requires _init?  The Fedora distro
> has gotten pretty far along in their builds, and I haven't seen any
> report of problems with the missing _init.  Likewise, Debian and OpenSuse.