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Re: [patch, avr] Take 2: Fix PR64331: insn output and insn length computation rely on REG_DEAD notes.


2015-02-27 1:45 GMT+03:00 Steven Bosscher <stevenb.gcc@gmail.com>:
> On Thu, Feb 26, 2015 at 8:35 PM, Georg-Johann Lay wrote:
>> Take #2 introduces a new, avr-specific rtl pass whose sole purpose is to
>> rectify notes.  The pass is scheduled right before cfg does down (right
>> before .*free_cfg) so that cfg and hence df machinery is available.
>>
>> Regression tests look fine and for the test case the patches produce correct
>> code and correct insn length.
>
> Sorry for party-pooping, but it seems to me that the real bug is that
> the target is lying to reload.
>
> IIUC the AVR port selects the insn alternative after register
> allocation (after reload). It bases its selection on REG_DEAD notes.
> In PR64331 an alternative is used that clobbers r20 that has a
> REG_DEAD note, but r20 is not actually dead because hardreg-cprop has
> propagated it forward without adjusting the note.
>
> The "normal" way of things is that the insn alternative is selected in
> reload (or in LRA) and that the clobbers are added as necessary. In
> PR64331, an alternative for insn r17 would be selected that has a
> CLOBBER for r20, prevent hardreg-cprop from propagating r20.
>
> Selecting insns based on REG-notes is dangerous business. Lying to
> reload and to post-RA passes is a mortal sin, the compiler will punish
> you. There is no guarantee that nothing will change between your new
> pass to recompute notes, and the final pass that emits the insns.
>
> It's not my port, for sure, but I would look for a real fix instead:
> Don't select insns to output based on unreliable information like
> REG-notes.

Steven rights.
We will have an endless fight with this problem.
Better to completely drop `reg_unused_after'. (I know that it used
around 40 times in port)

What do you think Georg  ?

Denis.


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