[llvm-dev] [lldb-dev] Continuing from dbgtrap on different targets

Pavel Labath via llvm-dev llvm-dev at lists.llvm.org
Wed Mar 4 23:29:42 PST 2020

On 04/03/2020 21:45, Jim Ingham via llvm-dev wrote:
> As you have seen, different machine architectures do different things after hitting a trap.  On x86_64, the trap instruction is executed, and then you stop, so the PC is left after the stop.  On arm64, when execution halts the pc is still pointing at the trap instruction.
> I don't think lldb should be in the business of telling systems how they should report stops, especially since that is certainly something we can handle in lldb.
> For traps that lldb recognizes as ones it is using for breakpoints, it already has to handle this difference for you.  But for traps we know nothing about we don't do anything special. 
> I think it would be entirely reasonable that whenever lldb encounters a trap instruction that isn't one of ours it should always move the PC after the trap before returning control to the user.  I can't see why you would want to keep hitting the trap over and over.  I've received several bugs (on the Apple bug reporter side) for this feature.  This might be something we teach lldb-server & debugserver to do, rather than lldb but that's an implementation detail...
> For now, on architectures where the trap doesn't execute, you just need to move the pc past the trap by hand (with the "thread jump" command) before continuing.  That has always been safe on arm64 so far as I can tell.
> Jim

Yes, this is something that has bugged me too.

While I think it would be nice if the OSes hid these architecture quirks
(hell, I think it would be nice if the CPU manufacturers made this
consistent so that the OS doesn't need to hide it), I think that
changing that at this point is very unlikely, and so working around it
in lldb is probably the best we can do.

I am not sure what is the official position on continuing from a debug
trap, but I think that without that ability, the concept would be pretty
useless. A quick example <https://godbolt.org/z/-8voBz> shows that clang
produces the "expected" output even at -O3. In fact, on aarch64,
__builtin_debugtrap() and __builtin_trap() produce the same instruction,
and the only difference between them is that the latter also triggers
DCE of everything coming after it.


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