ISA Zfinx extension

[Snorny]

Hello,

I'd like to ask you a question. I would like to use the Zfinx ISA extension to perform float calculations.
The only problem is that my code compiles whether I add zfinx to the compile flag or not.
(-march=rv32im_zfinx)

My question is how can I be sure that the float calculations are used by the hardware and not the software?

Thank you in advance for your answers.

[GideonZ]

The easiest way to check this is disassembling the generated code with riscv32-unknown-elf-objdump -d and check for the float operations. As you said you can compile twice, so you can make the dump twice and diff them.

[Snorny]

It seems that the operations are the same, so I deduce that I haven't activated the zfinx extension properly.

[stnolting]

You need to check first if your toolchain actually supports the Zfinx extension. I think support has been added with GCC version 12.something?!

Anyway, you can also "force" the compiler to infer Zfinx instructions using the provided intrinsics library. This is also possible even if the toolchain does not support Zfinx yet.

[Snorny]

You need to check first if your toolchain actually supports the Zfinx extension. I think support has been added with GCC version 12.something?!

Anyway, you can also "force" the compiler to infer Zfinx instructions using the provided intrinsics library. This is also possible even if the toolchain does not support Zfinx yet.

Thanks for those precision, how can i know if the toolchain support it ?

[stnolting]

Good question 😅

Unfortunately, it is quite hard to make GCC emit a list off all supported Z* sub-extensions... Better compare your version of GCC/binutils with the official release lists.

Or just try to compile a simple floating-point program with Zfinx in the march string and scan the resulting assembly code for any floating point instructions.

[Snorny]

I've tried looking, but it seems that my toolchain can't use the Zfinx extension, I'm working with RTEMS and the riscv-rtems6-gcc toolchain. I don't know if this is possible, but I need an answer and another opinion on my conclusion.

Other question, does the riscv32-unknown-elf-gcc can use this extension ?

[stnolting]

Other question, does the riscv32-unknown-elf-gcc can use this extension ?

No, not yet.

According to this list https://gcc.gnu.org/gcc-13/changes.html support for the Zfinx was introduced this year with GCC 13.

However, you can still utilize this extension with older GCC versions by using intrinsics.

[Snorny]

Thanks a lot for your help !

However, you can still utilize this extension with older GCC versions by using intrins

Yeah thanks for this precision ! It could be helpful.