Bug in gcc and gas

[jacob navia <jacob@jacob.remcomp.fr>]

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Hi
Neither gcc nor gas recognize the Zbb and Zba instructions under linux.
You have to force gcc with:, "march=rv64gc_zbb »

This is an undocumented option.

The processor is an Sifive U74 and in the documentation of that processor those instructions are present.
star64:~/tiny-asm$ uname -a
Linux star64 5.15.115 #1 SMP Sat Jun 10 09:56:27 UTC 2023 riscv64 riscv64 riscv64 GNU/Linux

All versions of gcc up to 13 have this bug.

Maybe the problem is not with gcc but with linux itself:
star64:~/tiny-asm$ cat /proc/cpuinfo
processor	: 0
hart		: 1
isa		: rv64imafdc
mmu		: sv39
isa-ext		: 
uarch		: sifive,u74-mc

…


In the « Isa » entry, those extensions are absent.

This is a general problem with riscv. How can a user program know which CPU extensions are installed in the processor?
There is a dedicated register that would be possible to query the  register

M-mode instruction set architecture register (misa)

But this is possible only in M mode. Should the libc in riscv develop a special function that returns the value of this register? Would it be possible or that would need a device driver if there isn’t any system call that would do that?

Jacob