question on gdbarch_skip_prologue()

[Kevin Buettner]

On Mar 6, 10:28pm, David Mosberger wrote:

> I'm exploring the idea of changing the ia64 backend of gdb to use
> unwind information instead of code-reading.  While looking into this,
> I started to wonder how gdbarch_skip_prologue() should be implemented.
> Some backends simply return the PC that was passed into the routine,
> i.e., they always assume a zero-size prologue.  I'm tempted to do the
> same because I worry that with optimized code, the very notion of a
> prologue becomes quite fuzzy.  For example, a prologue might contain a
> branch and, if so, there may not even be a single PC that corresponds
> to the end of the prologue.
> 
> Are there any downsides to gdbarch_skip_prologue() always returning
> the original PC?  Since the unwind info is accurate no matter what the
> PC is, there are no problems with tracking the contents of preserved
> (callee-saved) registers, but I'm wondering whether I'm missing
> anything else.

GDB currently expects that the skip_prologue() function will return a
PC that's after the last prologue instruction that saved an argument
to its "home" location (if any) in memory (or whereever the debug info
says that a parameter's location is).  The difficulty with this, of
course, is that with optimized code, it can be very difficult to
discern where this is.

Kevin