On Mon, Jan 16, 2017 at 03:47:55PM +0000, Pedro Alves wrote:

On 01/16/2017 03:11 PM, Yao Qi wrote:

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gdb must already re-detect the vector length on stop, since the target
could have called the prctl() in the meantime.

Yes, gdb assumes the vector length may be changed, so it re-detects on
every stop, but I don't see the need for gdb to change the vector length.

Do we need to consider inferior function calls here?

Say the program is stopped in code that assumes "vector length N", and
the user does "print some_function_that_assumes_some_other_vector_length ()".

Is that a use case we need to cover?

If so, to make it work correctly, the debugger needs to be able to change the
vector length to the length assumed by that called function, and then
restore it back after the call completes (or is aborted).

I have no idea whether the debugger will be able to figure
out a function's assumed vector length from debug info or some such.

I think the proposed ptrace interface can support this -- i.e., it
should provide everything needed to save off the VL and register state,
switch VL, do something else, then restore the VL and state (if not,
that's a bug).

My current position is that determining what vector length is
required by what function or binary is a 100% userspace problem, though.

ELF/DWARF could have annotations about this, though it wouldn't
necessarily be per-function -- you might require a whole image to be
built for the same vector length (if any).

Cheers
---Dave