(for general information
  about gdbserver, read coregrind/m_gdbserver/README_DEVELOPERS
  about gdbserver tests, read gdbserver_tests/README_DEVELOPERS).

ARM thumb and gdbserver
-----------------------

When executing thumb code, the ARM cpu program counter value is
not equal to the address of the instruction being executed: the fact
that the instruction being executed is a thumb instruction is
encoded in the bit0 of the program counter.
For more information about this, see VEX/pub/libvex_guest_arm.h.

This additional bit set in the ARM guest_IP caused some difficulties
at several places in the gdbserver code.


* breakpoint handling:
  gdbsrv code detects that a breakpoint is reached
  by comparing the current guest_IP with the breakpoint addresses given
  by gdb. As gdb is giving breakpoints without the thumb bit set,
  the gdbsrv code has to ensure breakpoint comparisons are done without
  the thumb bit.
  => the breakpoints are stored in the gdbserver breakpoints hash table
  without the thumb bit.
  => when the guest_IP is compared with these breakpoints, the thumb bit
  must be masked.

* Thumb bit in IstMark:
  When instrumenting a SB for gdbserver, a PUT of the current instruction
  address is done to the IP, to ensure that the gdbserver helper call
  sees the correct IP. This PUT must be done with the thumb bit set,
  as this value will be given back to gdb. gdb uses the thumb bit a.o.
  to guess the next program counter after this instruction.
  Such a guess is needed for gdb commands such as next/step/...
  If the thumb bit is not properly set, these commands are not working.
  As gdbserver instrumentation code uses the IstMark to find the IP
  of the instruction being instrumented, the thumb bit must be accessible
  or found. Multiple approaches were tried (see below) before the
  current one (which is to have a field Delta in IstMark which encodes
  the thumb bit).

* Thumb bit in extents
  When a breakpoint is placed at an instruction, the translation for this
  instruction must be discarded to have the SB re-instrumented for gdbserver.
  At least at this moment (r2155/r11786), the extents for ARM/thumb blocks
  contains addresses with the thumb bit set. It is not clear that this is
  a good thing: extents should preferably reference the real address range of
  instructions, not an address range which can be "off by one" due to the
  fact that the instruction is a thumb instruction.
 
  Due to this "off by one", gdbserver is discarding a range of two bytes
  to ensure it is not sensitive to the presence (or not) of the thumb
  bit in the range being discarded.

* Handling of monitor vg.translate
  When the translation is requested, the thumb bit must be properly set
  in the address, as this bit is used by the Valgrind translator
  to decide to decode in ARM or in thumb mode.
  The monitor command guesses the thumb bit based on the debug information.

* gdb 7.0 is buggy with thumb code
  The logic of gdb to guess the next PC in thumb code is not working
  properly in version gdb 7.0. Several gdbserver tests are disabled
  on ARM if gdb version is < 7.1.

* The (historical) list of difficulties encountered is the following:
  - initially, IstMark contained the thumb bit in the addr.
    This caused the breakpoints to not work (comparison between
    gdb address and gdbserver failed).
    (see breakpoint handling above).
  - A change was done to mask the thumb bit in the IstMark
    This made breakpoints work, but then gdb next/step/... was then failing
    (see Thumb bit in IstMark above).
  - So, it was needed to have the thumb bit when instrumenting an
    IstMark. 3 solutions were looked at:
     * use the debug info : this solution was discarded as often debug
       info does not allow a 100% correct solution. debug info is acceptable
       for vg.translate (which is only for internal valgrind debugging),
       but is better not used for 'end user functionality'.
     * Derive the thumb bit for a SB from the extent address.
       This was discarded as this implies that an SB cannot mix thumb
       and ARM code. This implied to disable chasing at transition between
       ARM/thumb code, which potentially decreases performance.
       Also, it would oblige to keep the thumb bit in the extents, which
       seems not nice.
     * the final solution implemented was to add a Delta fied in IstMark.
       This Delta field gives the delta to add to the IstMark addr field
       to obtain the guest_IP : for thumb code, this field is 1,
       and is 0 for ARM code and for all other architectures.
