[PR debug/46931] don't crash propagating removed DEFs into debug stmts

[Alexandre Oliva]

On Dec 26, 2010, Richard Guenther <richard.guenther@gmail.com> wrote:

> Yeah.  But wasn't there a correctness problem with propagating as well?

> the the debug info will refer to the wrong value of i.  Remember that
> we can have overlapping life ranges for SSA names but not for the
> decls for which we emit debug info.

The value expressions in debug stmts refer to the SSA names, so there's
no problem, at least in this regard.  The overlapping ranges, if
expanded to different pseudos, it will get the correct RTL expressions;
if no longer available at a point, it should not get any RTL expression
(although it would in theory be possible to do better, looking for some
equivalence).

Now, I don't think the latter has been actually verified, especially
after the change that made us go straight from SSA to RTL, so if you
have evidence that we're doing it wrong, I'd love to see it.

>> Regstrapping your proposed patch now.

> It's ok to commit if it works (with your testcase).

Here's what I installed.

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Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
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