mold refused to link GP-setup code for RISC-V

[piggynl]

Description

While building musl for riscv64 with mold as linker, mold refused to link the GP-setup code.

A minimal reproduce

This assembly snippet is created from arch/riscv64/crt_arch.h, which is included by ldso/dlstart.c. Save this snippet as dlstart.s.

.section .sdata,"aw"

.text
.global _dlstart
.type _dlstart,%function
_dlstart:
.weak __global_pointer$
.hidden __global_pointer$
.option push
.option norelax
        lla     gp, __global_pointer$
.option pop
        mv      a0, sp
.weak _DYNAMIC
.hidden _DYNAMIC
        lla     a1, _DYNAMIC
        andi    sp, sp, -16
        tail    _dlstart_c

.type _dlstart_c,%function
_dlstart_c:
        # The actual code here does not matter.
        ret

Then compile and link.

$ clang --target=riscv64 -march=rv64g dlstart.s -c -o dlstart.o
$ ld.lld -e _dlstart -shared dlstart.o -o fake_libc.so # no error
$ mold -shared -e _dlstart dlstart.o -o fake_libc.so
mold: error: dlstart.o:(.text): R_RISCV_PCREL_HI20 relocation at offset 0x0 against symbol `__global_pointer$' can not be used; recompile with -fno-PIC

Note that the GP-setup code in _dlstart do follows the recommended way to initialize the gp register in RISC-V ELF specification, and the PIC option for compiler doesn't affect the content of dlstart.o.

More information

With git bisect I found mold refuses to link such input after 5fb6091.

[rui314]

Thank you for your report. Maybe we should just revert the commit that broke your test case.