[PATCH] RISC-V: Don't built 64-bit multilibs on 32-bit targets

[Palmer Dabbelt]

On Sun, 30 Apr 2017 16:38:35 PDT (-0700), joseph@codesourcery.com wrote:
> On Sat, 29 Apr 2017, Richard Biener wrote:
>> On April 29, 2017 3:59:27 AM GMT+02:00, Palmer Dabbelt <palmer@dabbelt.com> wrote:
>> >We've been telling people that "riscv32-*" and "riscv64-*" are exactly
>> >the same toolchain aside from defaults for "-march" and "-mabi", but it
>> >appears we were lying.  As far as I can tell, binutils doesn't support
>> >64-bit targets when it has been configured for a 32-bit target.  This
>> >seems to be an upstream limitation that we can't fix in the RISC-V
>> >port.
>> >This means that building the toolchain with "--with-arch=rv32i
>> >--enable-multilib" will fail when building the 64-bit multilibs.
>> >
>> >This patch adds two new multilib target fragments that don't add the
>> >64-bit multilibs.  This fixes the build, but has the disadvantage of
>> >making our 32-bit and 64-bit toolchains very different.
>>
>> PowerPC for 32bit can use powerpc64 as target and enable 32bit code-gen
>> by default at configure time.  Maybe this is an option for riscv as
>> well.

I think we already support this as well: you can build RISC-V with
"--target=riscv64-*" and "--with-isa=rv32g" which would build the 64-bit
toolchain that defaults to a 32-bit target.

We can (and pretty much always do, which is why this bug went unnoticed)
generate 32-bit objects using our 64-bit toolchain.  The bug here is that a LD
that's built with "--target=riscv32*" can't handle 64-bit ELFs

> Specifically, in such cases the GCC configure option is
> --enable-target=all to enable 64-bit multilibs for a default-32-bit
> target, and the binutils/GDB configure option is --enable-64-bit-bfd.  But
> you can also make a binutils/GDB target include 64-bit support
> unconditionally without requiring --enable-64-bit-bfd.

Do you happen to know how to do that (enable 64-bit BFDs in a 32-bit binutils
build)?  I'd prefer to have the riscv32 and riscv64 ports the same if possible,
but poked around a bit and couldn't find a way to do it.

If you just know some other port does this then I can probably figure it out
from there.  I'd hoped that submitting this patch would lead to someone telling
me a better way to do it :).