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Re: RFC: implement DW_OP_call_frame_cfa


On Mon, Jun 08, 2009 at 04:23:20PM -0600, Tom Tromey wrote:
> GCC developers would like to change GCC to emit DW_OP_call_frame_cfa,
> as this would reduce the size of the generated debuginfo.
> 
> A prerequisite to this is that GDB understand this.  So, this patch
> implements this feature.  This is PR 10224.
> 
> I'm interested in feedback on this.  I am not sure whether the
> implementation of dwarf2_frame_cfa is ok.

It isn't, sorry.  It will crash with bad debug info (e.g. manually
stripped .debug_frame), because it runs the unwinder without passing
through the sniffer.  It also allocates an entire unwinding cache for
every local variable using this operation, which is very wasteful.

I think, as much as we've tried to avoid it, you're going to need a
back channel to find the existing cache iff the frame has a particular
unwinder type.

> No test case since at some point GCC will start generating this
> (perhaps optionally -- but I feel certain we'll do it by default in
> Fedora), and since it therefore seemed like a lot of work for little
> payoff.

IMO, not good enough; this is what gdb.dwarf2/ is for.  My compiler
doesn't generate this extension but I'd still like to not break it.
Maybe an x86-specific test?


-- 
Daniel Jacobowitz
CodeSourcery


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