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Re: regrename creates invalid insn
On 03/26/2012 07:37 PM, Eric Botcazou wrote:
>> Does 4.7 still have the failure at all? I've checked with the 4.6
>> branch, and regrename gets confused because there's a REG_DEAD note for
>> the register, and another REG_UNUSED for the same reg. As far as I
>> remember, it used to be the case that there should not be a REG_DEAD
>> note for a register that gets set in the insn, but maybe df changed the
>> rules? Or maybe it was a df bug in 4.6?
> My understanding is that the REG_UNUSED note causes the chain opened for a dest
> register operand to be immediately closed but, when you have multiple such
> dest register operands, one would need to have the chain live "during the
> instruction" or right after, so that you have a conflict with the other dest
> register operands for the instruction. This looks awkward though.
You're right on the behaviour of REG_UNUSED, but it only takes effect
after all destinations have been opened. So they still conflict with
each other, usually.
Here, I think the problem is that we have an in-out operand whose chain
is closed prematurely due to a bogus REG_DEAD note which shouldn't be
there for a register set in the instruction.