Big Endian support as multi-ABI (was Re: [PATCH v4 02/15] ARC: ABI Implementation)

From: Vineet Gupta <Vineet.Gupta1@synopsys.com>
To: Joseph Myers <joseph@codesourcery.com>
Cc: "linux-snps-arc@lists.infradead.org"
	<linux-snps-arc@lists.infradead.org>,
	"libc-alpha@sourceware.org" <libc-alpha@sourceware.org>
Subject: Big Endian support as multi-ABI (was Re: [PATCH v4 02/15] ARC: ABI Implementation)
Date: Tue, 31 Mar 2020 21:08:57 +0000	[thread overview]
Message-ID: <ddb994a4-f3f2-e789-7cc0-5e284076315f@synopsys.com> (raw)
In-Reply-To: <alpine.DEB.2.21.2003261843120.24611@digraph.polyomino.org.uk>

On 3/26/20 11:48 AM, Joseph Myers wrote:
> Yes, if you want to support BE then it should be documented as supported, 
> it should have its own dynamic linker name (with consequent GCC change 
> required to use that name) and it should have its own build in 
> build-many-glibcs.py.

So I booted the reworked BE support as proper multi-ABI (so abi variants in
sysdeps makefile, configure and ld versions in arc sysdeps ldconfig.h and
shlib-versions

However I'm a bit unsure about other elaborations needed per your comments in [1].
It seems they make sense when disparate ABIs can co-exit. Quoting some lines from [1]

| "support simultaneous presence of
| libraries for two different ABIs on a single system (hard-float and
| soft-float ABIs)..."

| "In cases where libraries for multiple ABIs may be present at once, there
| should be flags defined in sysdeps/generic/ldconfig.h..."

But a BE system can't possibly be mixed with a LE, its not really a multilib case
IMHO and a totally separate software stack build. So things like rewriting ldd to
be able to detect either ABI don't really apply as we need a seperate ldd built
for LE/BE for it to be even exec'ed by Linux kernel.

Going back to the list of needed parts in [1]

> sysdeps ldconfig.h file defining SYSDEP_KNOWN_INTERPRETER_NAMES with all
> the supported dynamic linker names.

Added entries for ARC LE/BE

> flags defined in sysdeps/generic/ldconfig.h to allow such libraries to be
> distinguished in ld.so.cache.

I don't see any other BE entry.

> elf/cache.c should include support for printing a corresponding description
> for those flags.

Ditto.

> sysdeps readelflib.c file that includes code to identify the flags to
> associate with a given shared library.

> sysdeps dl-cache.h defining _dl_cache_check_flags and (given that you
> use directories other than plain lib) add_system_dir.

They seem to be used for 32/64 support and/or different soft/hard float
implementation, but not for BE

> ldd_rewrite_script setting in a sysdeps configure.ac file, pointing to a
> sed script that edits ldd so that ldd installed by glibc build for any ABI
> is able to work properly for binaries of any other ABI that can execute on
> that system.

I don't see the point as it will never need to run.

Again the intent is not to reduce work for myself :-) but to do things right.

Thx,
-Vineet

[1] https://sourceware.org/legacy-ml/libc-alpha/2018-01/msg00008.html
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