Re: [PATCH v2] aarch64: Add fallback if ARM_CC_FOR_TARGET not set (was: Re: [PATCH 5/5] gdb: native target invalid architecture detection)

[John Baldwin <jhb@FreeBSD.org>]

On 6/8/22 4:20 AM, Pedro Alves wrote:
> On 2022-06-08 11:12, Pedro Alves wrote:
>> On 2022-06-08 08:54, Luis Machado wrote:
> 
>> A 64-bit program can always exec a 32-bit program.  Likewise the other way around.  That should be
>> debuggable with a single gdb.
>>
>> As you say, it's supported.  So, better test it routinely.  Not testing it doesn't gain us anything,
>> other than leading to bitrot, as has happened.
>>
>>>>
>>>> Maybe it would be possible to come up with a way to default ARM_CC_FOR_TARGET to
>>>> something that works for most people when testing aarch64, somehow.
>>> The problem is that a compiler is not enough to make things run fine. If you have an aarch64
>>> Ubuntu, for example, you'd need to add armhf as an architecture option and install both the
>>> compiler and the libraries (libc6:armhf mostly).
>>
>> That is not an issue specific to aarch64.  On x86-64, you need to install extra runtime
>> packages to test -m32 too, just the compiler isn't enough, and those packages aren't installed
>> by default.
>>
>>>
>>> Another problem is that not every processor/kernel supports running a 32-bit process in a
>>> 64-bit environment. This makes things more complicated.
>>>
>>
>> Why "more complicated"?  If you're worried about FAILs in the testsuite, then we can e.g. make it run
>> the 32-bit program once outside gdb to check that running 32-bits works.  If that fails, then
>> we don't run the test.  Seems simple, not complicated.
> 
> Here's a new version of the patch, doing that.  This version also moves the ARM_CC_FOR_TARGET fallback
> discovery to lib/gdb.exp so it can be used by gdb.multi/multi-arch-exec.exp as well.  I've now tested
> this on gcc80 on the GCC compile farm (an aarch64 ubuntu 18.04 machine), and confirmed that it works.  As in,
> we pick the right compiler (turns out it is arm-linux-gnueabihf-gcc, the third fallback compiler, so the try-to-run
> code is exercised).  Of course, the testcases actually FAIL, due to 32-bit support bitrot in gdb itself.
> 
>  From 689671e40f88f1bf29af2433c7b6c7662c8f38c6 Mon Sep 17 00:00:00 2001
> From: Pedro Alves <pedro@palves.net>
> Date: Tue, 7 Jun 2022 20:11:32 +0100
> Subject: [PATCH] aarch64: Add fallback if ARM_CC_FOR_TARGET not set
> 
> On Aarch64, you can set ARM_CC_FOR_TARGET to point to the 32-bit
> compiler to use when testing gdb.multi/multi-arch.exp and
> gdb.multi/multi-arch-exec.exp.  If you don't set it, then those
> testcases don't run.
> 
> I guess that approximately nobody remembers to set ARM_CC_FOR_TARGET.
> 
> This commit adds a fallback.  If ARM_CC_FOR_TARGET is not set, and
> testing for Linux, try arm-linux-gnueabi-gcc,
> arm-none-linux-gnueabi-gcc, arm-linux-gnueabihf-gcc as 32-bit
> compilers, making sure that the produced executable runs on the target
> machine before claiming that the compiler produces useful executables.

Note that if the target compiler is clang, it will probably already
support 32-bit ARM via a -target option.  Not sure if you want to try
to handle that case here as well?

The other wrinkle perhaps is that just because you have a capable
compiler around may not mean that you have 32-bit libraries available
to link against?  I'm not sure if the compilers you list in your patch
ensure that you have an ARM libc around as well via dependencies.

-- 
John Baldwin