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Re: RFC: implement DW_OP_call_frame_cfa
- From: Tom Tromey <tromey at redhat dot com>
- To: gdb-patches at sourceware dot org
- Date: Wed, 22 Jul 2009 15:57:58 -0600
- Subject: Re: RFC: implement DW_OP_call_frame_cfa
- References: <firstname.lastname@example.org> <20090609123521.GA3787@caradoc.them.org>
- Reply-to: Tom Tromey <tromey at redhat dot com>
>>>>> "Daniel" == Daniel Jacobowitz <email@example.com> writes:
Tom> A prerequisite to this is that GDB understand this. So, this patch
Tom> implements this feature. This is PR 10224.
Tom> I'm interested in feedback on this. I am not sure whether the
Tom> implementation of dwarf2_frame_cfa is ok.
Daniel> It isn't, sorry. It will crash with bad debug info (e.g. manually
Daniel> stripped .debug_frame), because it runs the unwinder without passing
Daniel> through the sniffer. It also allocates an entire unwinding cache for
Daniel> every local variable using this operation, which is very wasteful.
Daniel> I think, as much as we've tried to avoid it, you're going to need a
Daniel> back channel to find the existing cache iff the frame has a particular
Daniel> unwinder type.
I looked into this a little today.
My understanding is that we can only really implement
DW_OP_call_frame_cfa for the case where a frame is found by the
dwarf frame-sniffing code.
So, I ended up just adding a simple checking function:
frame_base_is (struct frame_info *fi, struct frame_base *base)
if (fi->base == NULL)
fi->base = frame_base_find_by_frame (fi);
return fi->base == base;
Then the code to implement DW_OP_call_frame_cfa checks this before
fetching the frame base.
dwarf2_frame_cfa (struct frame_info *this_frame)
if (! frame_base_is (this_frame, &dwarf2_frame_base))
error (_("can't compute CFA for this frame"));
return get_frame_base_address (this_frame);
This fixes the cache problem as a side effect.
Is this the sort of thing you were thinking of?
I've dug through the frame code a bit, but I'm still by no means an
expert. I'd appreciate any advice you might have.