Re: [Patch] pr65779 - [5/6 Regression] undefined local symbol on powerpc

[Jakub Jelinek <jakub at redhat dot com>]

On Mon, Apr 20, 2015 at 06:12:26PM +0930, Alan Modra wrote:
> I had it that way in my first patch, then decided to try deleting..
> 
> I can certainly change it back even if only to do it the standard way
> for safety's sake, but I'm curious as to why they can't be deleted in
> this special case.  My thinking was that we're on a chain of blocks
> starting at the entry where there is a single outgoing live edge for
> the register being used.  So there shouldn't be any need for a debug
> insn to mark info about the variable as invalid.

The debug insns can be for arbitrary variables, there is no "the variable",
and there could be other debug insns for the same variable on that path,
say saying that decl lives in some other register, or can be computed using
an expression involving other registers, or memory etc.  Say you could have
(set regX (whatever))
...
(debug_insn var5 (some expression not referring to regX))
...
(debug_insn var5 (some expression referring to regX))
...
(debug_insn var5 (other expression not referring to regX))
...
(use regX)

where ... contains unrelated insns (not referring to regX) and edges live
for regX.  If shrink wrapping attempts to move the first set somewhere into
the last ..., if you delete debug insns referring to regX, you extend the
lifetime of the previous debug_insn, which is wrong, the registers
referenced in the first debug_insn might not contain the right values
afterwards.  And if you move the debug insn later, you might shorten the
lifetime of the third debug_insn, while regX is supposed to contain the same
value, perhaps some other register referenced there might have been changed
already.

	Jakub