[PATCH v4 4/5] Makefile: Support building with Clang and LLVM binutils

[Bin Meng]

On Sat, Jul 10, 2021 at 9:23 PM Bin Meng <bmeng.cn at gmail.com> wrote:
>
> On Sat, Jul 10, 2021 at 3:35 AM Jessica Clarke <jrtc27 at jrtc27.com> wrote:
> >
> > This is intended to mirror the Linux kernel. Building with CC=clang will
> > use Clang as the compiler but default to using the existing binutils.
> > Building with LLVM=1 will default to using Clang and LLVM binutils.
> >
> > Whilst GCC will accept the -N linker option and forward it on to the
> > linker, Clang will not, and so in order to support both compilers we
> > must use -Wl, to forward it to the linker as is required for most other
> > linker options.
> >
> > Signed-off-by: Jessica Clarke <jrtc27 at jrtc27.com>
> > ---
> >  Makefile  | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++-----
> >  README.md | 43 ++++++++++++++++++++++++++++++++++--
> >  2 files changed, 100 insertions(+), 8 deletions(-)
> >
>
> Here are test results:
>
> Building with "riscv64-linux-gcc",
>
> $ file build/platform/generic/firmware/fw_dynamic.elf
> build/platform/generic/firmware/fw_dynamic.elf: ELF 64-bit LSB
> executable, UCB RISC-V, version 1 (SYSV), dynamically linked, with
> debug_info, not stripped
>
> $ riscv64-linux-readelf -r build/platform/generic/firmware/fw_dynamic.elf
> Relocation section '.rela.dyn' at offset 0x140f8 contains 184 entries:
>   Offset          Info           Type           Sym. Value    Sym. Name + Addend
> 000080013090  000000000003 R_RISCV_RELATIVE                     80000db4
> ...
> 000080013628  000200000002 R_RISCV_64        0000000080013720
> fdt_serial_uart8250 + 0
> 000080013830  000d00000002 R_RISCV_64        00000000800138d8
> fdt_reset_sifive_test + 0
>
> Building with "LLVM=1",
>
> $ file build/platform/generic/firmware/fw_dynamic.elf
> build/platform/generic/firmware/fw_dynamic.elf: ELF 64-bit LSB shared
> object, UCB RISC-V, version 1 (SYSV), dynamically linked, with
> debug_info, not stripped
>
> $ riscv64-linux-readelf -r build/platform/generic/firmware/fw_dynamic.elf
> Relocation section '.rela.dyn' at offset 0x17d98 contains 188 entries:
>   Offset          Info           Type           Sym. Value    Sym. Name + Addend
> 000080017000  000000000003 R_RISCV_RELATIVE                     8000b680
> 000080017030  000000000003 R_RISCV_RELATIVE                     8001b1b8
> ...
> 000080017c90  000000000003 R_RISCV_RELATIVE                     80017628
>
> There are two differences:
>
> 1. LLVM toolchain generates a "shared object" firmware image, while
> GCC generates "executable".
> 2. LLVM one has 4 more entries in .rela.dyn than the GCC. All entries
> of LLVM have the R_RISCV_RELATIVE type, but GCC one has two R_RISCV_64
> entries.

Do you have any explanations on these 2 differences? Are these
possible toolchain bugs?

>
> I am not sure whether GCC / LLVM is doing things correctly for the
> above 2 differences. fw_dynamic image of both can boot to S-mode
> U-Boot on QEMU 'virt' though.
>
> Using clang and GNU binutils, fw_dynamic image does not boot on QEMU
> 'virt', as reported before.
> $ make CC=clang CROSS_COMPILE=riscv64-linux- PLATFORM=generic

Regards,
Bin