[PATCH/AARCH64] Emit brk #0 for __builtin_trap

[Andrew Pinski]

On Tue, Nov 19, 2013 at 1:22 AM, Marcus Shawcroft
<marcus.shawcroft@arm.com> wrote:
> On 13 November 2013 00:04, Andrew Pinski
>
> <andrew.pinski@caviumnetworks.com> wrote:
>>
>> Hi all,
>>   This patch implements the trap pattern for the AARCH64 back-end.  I
>> used the "brk #0" instruction as that is the breakpoint instruction
>> that GDB uses.  I looked at what other targets did when the
>> instruction set did not have a trap instruction and found that using
>> the breakpoint instruction was a common theme between them if there
>> was not explicit defined undefined instruction to use.
>
>
>
> Hi Andrew, We can exploit the immediate field in the brk instruction to
> distinguish the origin of various traps.  There was some discussion on
> this topic within ARM a (long) while back, at that time we discussed a
> scheme along the following lines:
>
>                 POSIX siginfo
> BRK #imm16      si_signo si_code        Purpose
> ----------      -------- -------        -------
>
> 0000-0fff  S/w breakpoint, reserved for debuggers
>  0000-3ff       SIGTRAP TRAP_BRKPT-       EL0 breakpoint (e.g. gdb)
>  0400-7ff       SIGILL ILL_ILLTRP*        EL1 breakpoint (e.g. kgdb)
>  0800-bff       SIGILL ILL_ILLTRP*        EL2 breakpoint
>  0c00-fff       SIGILL ILL_ILLTRP*        EL3 breakpoint
>
> 1000-1fff                               C/C++ runtime errors
>  1000           SIGABRT n/a               libc abort()
>  1001           SIGFPE  FPE_INTDIV        integer divide by zero
>  1002           SIGFPE  FPE_INTOVF        integer overflow
>  1003           SIGFPE  FPE_FLTDIV        floating-point divide by zero
>  1004           SIGFPE  FPE_FLTOVF        floating-point overflow
>  1005           SIGFPE  FPE_FLTUND        floating-point underflow
>  1006           SIGFPE  FPE_FLTRES        floating-point inexact result
>  1007           SIGFPE  FPE_FLTINV        floating-point invalid op
>  1008           SIGFPE  FPE_FLTSUB        subscript out of range
>  1009-1fff      SIGILL  ILL_ILLTRP        unused but reserved
>
> 2000-ffff       SIGILL  ILL_ILLOPC+   Guaranteed unused, resvd for apps
>
> - This is the signal generated now for all values of BRK
>   immediate. GDB currently uses "BRK #0"
>
> * The EL1/EL2/EL3 breakpoints would deliver a SIGILL if they are
>   executed by EL0 code, and caught by the EL1 kernel. A s/w debugger
>   operating at a higher EL which placed such breakpoints would
>   presumably catch them and handle them, without the EL1 kernel ever
>   seeing them.
>
> + Immediate values 0x2000-ffff generate the same signal as any other
>   UNDEFINED instruction encoding, but with guaranteed behaviour for
>   JITs etc. The original imm16 value could be made available in the
>   si_trapno field of the signal context.
>
> Following this scheme I suggest __builtin_trap() generate brk #1000
>
> Thoughts?


This sounds like a good scheme.  I will update my patch to use #1000
based on this.

Thanks,
Andrew Pinski

>
> Cheers
> /Marcus
>