No support for -mno-pic-data-is-text-relative -msingle-pic-base options?

[yf13]

Hello there,

I am porting a Cortex-m4 project to rv32, the project uses position independent code for downloadable app modules. The project uses GNU arm toolchain with -fpic -fno-plt -mno-pic-data-is-text-relative -msingle-pic-base options for compilation.

I noticed that [issue#1521] and alike alredy explains that RiscV elf toolchain doesn't support PIC code generation and RiscV linux toolchain should be used instead.

Then I tried both riscv32-unknown-linux-gnu-gcc and riscv64-unknown-linux-musl-gcc but found that they both:

• accept the -fpic -fno-plt options
• reject the -mno-pic-data-is-text-relative -msingle-pic-base options.

I guess the first two are general GCC options and the last two are ARM specific options.
I am unsure if RiscV linux toolchain:

• has same function for -fpic -fno-plt options like ARM toolchain?
• has any equivalent options for -mno-pic-data-is-text-relative -msingle-pic-base ARM toolchain? if no such options, then what is RiscV toolchain's behavior regarding these two options?

[TommyMurphyTM1234]

I guess the first two are general GCC options

Correct:

• https://gcc.gnu.org/onlinedocs/gcc-14.2.0/gcc/Code-Gen-Options.html

and the last two are ARM specific options.

Correct:

• https://gcc.gnu.org/onlinedocs/gcc-14.2.0/gcc/ARM-Options.html

I am unsure if RiscV linux toolchain: ...
has same function for -fpic -fno-plt options like ARM toolchain?

It does.

has any equivalent options for -mno-pic-data-is-text-relative -msingle-pic-base ARM toolchain?

It doesn't seem to:

• https://gcc.gnu.org/onlinedocs/gcc-14.2.0/gcc/RISC-V-Options.html

what is RiscV toolchain's behavior regarding these two options?

It doesn't accept the Arm specific options. And it doesn't seem to have any equivalent capability. Not all GCC target architectures support the same compiler options.

Why not simply try compiling without those Arm specific options?

[yf13]

Thank you @TommyMurphyTM1234 for the prompt reply.

So I will take that RiscV linux toolchain support -fpic -fno-plt but not -mno-pic-data-is-text-relative -msingle-pic-base, meaning the RiscV PIC feature has the assumption of data segments are relative to text segment.

[TommyMurphyTM1234]

So I will take that RiscV linux toolchain support -fpic -fno-plt but not -mno-pic-data-is-text-relative -msingle-pic-base,

Yes, that's what the GCC documentation says.

meaning the RiscV PIC feature has the assumption of data segments are relative to text segment.

I don't know. You might want to ask about this on the gcc-help mailing list.

• https://gcc.gnu.org/lists.html

[TommyMurphyTM1234]

In case these are of any use...

• Support for -mno-pic-data-is-text-relative? riscvarchive/riscv-gcc#158
• ROPI/RWPI Specification (Embedded PIC) riscv-non-isa/riscv-elf-psabi-doc#128
• https://book.tockos.org/doc/compilation
• https://maskray.me/blog/2024-02-20-mmu-less-systems-and-fdpic

[aswaterman]

The existing RISC-V PIC scheme requires the GOT and local data be at a fixed offset from text.
IIRC, people have made proposals for an ABI that supports your use case, but they haven't yet been accepted, let alone implemented.

[TommyMurphyTM1234]

The existing RISC-V PIC scheme requires the GOT and local data be at a fixed offset from text. IIRC, people have made proposals for an ABI that supports your use case, but they haven't yet been accepted, let alone implemented.

Thanks for the clarification @aswaterman.
If/when you get a chance, would you happen to remember where such discussions took place and if they are captured anywhere that could be linked to, please?
Thanks again.

[TommyMurphyTM1234]

Closing this since it is something that needs to be dealt with in the relevant RISC-V ABI specification forum - here I think?

• https://lf-riscv.atlassian.net/wiki/spaces/HOME/pages/16154769/RISC-V+Technical+Specifications
  • https://lists.riscv.org/g/apps-tools-software
  • https://github.com/riscv-non-isa/riscv-elf-psabi-doc

And then any concomitant changes implied by the ABI changes/enhancements implemented in the upstream toolchain projects.

[aswaterman]

Sorry for the delay in responding. Yes, the RISC-V ELF psABI spec is where these issues are discussed.

[TommyMurphyTM1234]

Sorry for the delay in responding. Yes, the RISC-V ELF psABI spec is where these issues are discussed.

Thanks a lot @aswaterman. 👍