Re: [PATCH 5/5] RISC-V: Add configure support riscv*-linux*.

[Jim Wilson <jimw@sifive.com>]

On Wed, Aug 8, 2018 at 7:36 PM, Eli Zaretskii <eliz@gnu.org> wrote:
>> From: Tom Tromey <tom@tromey.com>
>> Cc: tom@tromey.com,  Jim Wilson <jimw@sifive.com>,  gdb-patches@sourceware.org
>> Date: Wed, 08 Aug 2018 17:25:59 -0600
>>
>> My main concern is really whether --disable-64-bit-bfd still works.
>> I don't know if people still use this, but as long as it exists it seems
>> like it should continue to work.  If RISC-V works in this setup, then it
>> is totally fine by me for gdb to follow, assuming it works as well.
>>
>> Maybe to really test it has to be a 32 bit build with
>> --disable-64-bit-bfd and --enable-targets=all.
>
> Btw, can you or someone else explain what exactly does
> "--enable-64-bit-bfd" do and what practical effects should it have on
> GDB?  I frequently wonder whether I want that switch when building
> GDB, and I still don't know the answer.

Since I'm looking at this at the moment...

BFD will automatically enable 64-bit support if you configure on a
host with 64-bit support, or configure for a target with 64-bit
support.  It will also enable 64-bit support if you use
--enable-64-bit-bfd.  64-bit support means you can use 64-bit integer
types such as long long on a 32-bit host or long on a 64-bit LP64
host.  If 64-bit support is enabled, and your compiler doesn't support
it, then you get a configure error.

If you use --disable-64-bit-bfd, then bfd will disallow use of 64-bit
integer types.  If you configure for a 64-bit target, you will get a
configure error.

As a practical matter, I think --enable/disable-64-bit-bfd doesn't
really do anything useful unless you are configuring with
--enable-targets=all.  In this case, 64-bit targets will be enabled by
default, and will fail to build on a 32-bit host with a compiler that
doesn't support long long.  You can work around that problem by using
--disable-64-bit-bfd which will get you all of the 32-bit targets, and
none of the 64-bit targets, which will allow the build to complete.
Of course, nowadays, finding a 32-bit compiler without long long
support would be hard, so this option combination is unlikely to be
useful to anyone.

I've been looking at the riscv{32,64} bfd support.  Since configuring
for riscv32 enables riscv64 support also, and riscv64 requires 64-bit
support, this means that there is no riscv bfd support at all when
using --disable-64-bit-bfd --enable-targets=all.  It looks like the
gdb riscv support is broken even without my patch, as we can't build a
riscv32-elf gdb if we don't have bfd.  I've got a build running to see
if I can force a build failure.

Jim