[RISCV][lld?] Instruction relaxation information is not generated during partial linking with LTO

[vsukhoml]

When input object files are machine code compiled with -mrelax and containing RELAX records partial linking (lld --relocatable) produce expected result:
36: 97 00 00 00 auipc ra, 0
00000036: R_RISCV_CALL_PLT memcpy
00000036: R_RISCV_RELAX ABS
3a: e7 80 00 00 jalr ra
3e: 13 05 f4 02 addi a0, s0, 47

Same code when source is IR object file produced with -flto and compiled/linked as lld --relocatable:
36: 97 00 00 00 auipc ra, 0
00000036: R_RISCV_CALL_PLT memcpy
3a: e7 80 00 00 jalr ra
3e: 13 05 f4 02 addi a0, s0, 47

[MaskRay]

void bar();
void foo() {
  bar();
  bar();
}

If you use clang++ --target=riscv64-linux-gnu -flto -fuse-ld=lld a.cc -r -o a.o -Wl,-mllvm,-mattr=+c,-mllvm,-mattr=+relax, you'll get R_RISCV_RELAX. If you omit the -Wl, then there will be no R_RISCV_RELAX, and e_flags of the ELF header does not contain EF_RISCV_RVC. This is due to a target-features LTO issue which hasn't been resolved.

[llvmbot]

@llvm/issue-subscribers-backend-risc-v

[vsukhoml]

thanks for the suggestion of the workaround!

[nickdesaulniers]

This is due to a target-features LTO issue which hasn't been resolved.

Is there more info about this? re: https://lore.kernel.org/llvm/20231017-riscv-lto-v4-1-e7810b24e805@kernel.org/

[vsukhoml]

I had this issue with LLVM 15. But didn't yet check with LLVM16. The one in kernel seems to be different one. In my case I had LTO completed producing relocatable object (--relocatable) which had relaxation records, but when that object file was linked again, relaxations were not applied resulting in larger code size.

It also seems that RISC-V have some bugs in memory relaxation code like https://reviews.llvm.org/D149432

[EugeneZelenko]

@vsukhoml: Please try 17 which is only currently maintained release.