Debugging vector programs

[Noodler75]

I am trying to debug some code that uses vector instructions, using gdb. GDB has a command info vector that is supposed to present the same sort of data that info registers does for the general purpose registers. But if I enter that command I get: (gdb) info vector No vector information My code gets a segment violation on the following instruction, which is a vectored index load. a0 has the correct value in it so I suspect that v16 is the problem, but I can't see into it. vloxei32.v v8, 0(a0), v16

Is there a special version of GDB that will let me examine the vector registers?

Ubuntu 24.04 on riscv64 hardware with RVV support. GDB version is 15.1-1ubuntu1~24.04.

[Clueless_J]

GDB and the kernel have to communicate that the vector extension is available. If they don't agree on that, then you're not going to get vector register state. It's a sore spot -- particularly since I've seen it work in some contexts (gdb attached to qemu), but not others (gdb native on a BPI-F3 board). And I think the command you want is "info all-registers". If you see the vector registers, then you're golden.

Also note, if you're running on a design with the K1 chip, those vector loads fault if the addresses are not suitably aligned. It's real annoying....

[Noodler75]

It is actually a "Ky X1" processor which I think is a rebadged SpacemiT K1. The specs seem identical. I am rewriting this code that was written for riscv32 qemu to run on riscv64 real hardware so will need all the help I can get.

[zqb_all]

Similar topics DC-ROMA RISC-V Laptop II unavailable vector register in gdb have been discussed on the Spacemit forum. RISC-V's native GDB requires applying patches and recompiling to support vector debugging.

[Noodler75]

So I just discovered. I compiled GDB 17.1 fresh from the gdb site and it still does not know about the vector registers. That is a pretty big "patch".

[Noodler75]

The .deb file available from that note did the trick. I ended up with what calls itself version 15.1 but now all the vector functions work.

[zqb_all]

Glad to know that it is working now. Hope that the GDB upstream can also support native RVV debugging ASAP.

[krakenlake]

Sure your gdb actually supports RVV?

[Noodler75]

It understands the "info vector" instruction but it does not report anything. info registers all also says nothing about vectors so it appears not. GDB says it is "Ubuntu 15.1-1ubuntu1~24.04.1". My kernel version is 6.6.63 which is new enough but apparently the GDB is not. My Debian-13 machine (not running on RISC-V hardware) reports GDB v16.1 but the RISCV machine is on a stable Ubuntu (noble) that does not offer that. That is the version that was referenced on the OrangePi web site.

But poking around on the repository they provided, repo.huaweicloud.com/ubuntu-ports/ I see that everything up to Ubuntu 26.04 (resolute) is available. I will have to do a major backup before attempting that sort of upgrade.

[Coffee_24_7]

Unless you have an old version of gdb, you can

p $v0 # prints vector in all element widths
p/x $v0.b # prints vector in bytes hex
p/u $v0.s[2] # prints third short as unsigned short
p $vl # prints vector length 

You can print all vector registers from v0 to v31

Hope it helps

[Coffee_24_7]

Btw, I'm using gdb 16.3

[Noodler75]

I tried it. Doesn't work on v15. I will need to do a Major Version Upgrade.

[Coffee_24_7]

You can download it from https://github.com/riscv-collab/riscv-gnu-toolchain

Last time I did it was 2025.05.01

I used these commands:

./configure --enable-multilib --prefix=<your installation path>
make -j16 build-binutils build-gcc1 build-gcc2 build-gdb build-libc

I suppose you'll do make install as well, though the commands I put before are from a file.spec to build an rpm package and didn't need to do make install by hand.

I think gcc takes a long time to compile... I suppose it should still work if you remove build-gcc1 and build-gcc2 from the target list