riscv-non-isa/riscv-asm-manual

Will the "tail" pseudo instruction always get transformed to the "jalr" instruction?

SaiVK opened this issue · 9 comments

SaiVK commented

Hello Everyone
I compiled a C program with -O2 flag enabled. On inspecting the assembly file generated by the compiler (-S flag) I noticed a couple of tail pseudo instruction. But in the object dump file generated, I noticed that the tail pseudo instruction gets converted to j instruction.
My doubt is, in the explanation given in the manual, it is specified that tail pseudo instruction gets transformed to jalr. Is this not followed always?

Thank you so much
Sai

aswaterman commented
SaiVK commented

Hello @aswaterman
Oh ok, so only when a far-away subroutine is being invoked, the tail pseudo instruction transforms to an auipc + jalr instruction. Else, it can transform to j or c.j instruction.
Did I understand it properly?

Thank you so much
Sai

aswaterman commented

Yes. Note the linker performs that transformation, so the object code will always show auipc/jalr.

SaiVK commented

@aswaterman
Got it.
Is it possible to modify the linker such that all tail pseudo instructions transform only to jalr instruction and not to j or c.j instruction?

Thank you so much
Sai

aswaterman commented

I think you can pass the --no-relax flag to the linker.

SaiVK commented

@aswaterman
Oh ok. I will try using that flag.
Are there any downsides in using that?

Thank you so much
Sai

aswaterman commented

Yes - increased code size and lower performance

SaiVK commented

Oh ok. Got it.

Thank you so much @aswaterman for taking your time and explaining it to me.
Sai

aswaterman commented

You're welcome!