Re: [Valgrind-developers] allowing skins to instrument conditional jumps

[Jeremy F. <je...@go...>]

On Wed, 2002-10-02 at 04:23, Nicholas Nethercote wrote:
    Best way I can think of doing it, which only requires skin changes rather
    than core changes, is this:  using the `extended_UCode' need, add a new
    UInstr PRE_JCC, which gets inserted by SK_(instrument) before conditional
    JMPs, evaluates the condition, and calls a C function (or whatever) if
    it's true.  This would duplicate the condition evaluation but that
    shouldn't matter since they're trivial (just checking an EFLAGS bit I
    think).
    
    It's a bit nasty that something as simple as this requires a new UInstr...

Well, I've actually come up with a simpler approach.  Since what I want
is to get the (from, to) pair for a BB graph edge, I'm simply updating a
global (bb_from) with %EIP before each jump, and then create/update the
edge at the entry to each BB (bb_from, %EIP).

At present I'm using a single global, which means that I'll be creating
spurious edges when there's context switches between threads.  The
obvious place to store the information is in the baseBlock, and have it
copied to/from the thread state on context switch.  I didn't see a
mechanism for allocating variable space in the baseBlock, nor a way of
conveniently addressing baseBlock offsets directly.  Should I add it? 
Or some other way of storing per-thread information?

	J