Fix twolf -funroll-loops -O3 miscompilation (a semi-latent web.c bug)

[Paolo Bonzini]

Il 14/10/2012 22:59, Steven Bosscher ha scritto:
> On Sun, Oct 14, 2012 at 9:02 AM, Paolo Bonzini wrote:
>> Can we just simulate liveness for web, and drop REG_EQUAL/REG_EQUIV
>> notes that refer to a dead pseudo?
> 
> I don't think we want to do that. A REG_EQUAL/REG_EQUIV note can use a
> pseudo that isn't live and still be valid. Consider a simple example
> like this:
> 
> a = b + 3
> // b dies here
> c = a {REG_EQUAL b+3}
> 
> The REG_EQUAL note is valid and may help optimization. Removing it
> just because b is dead at that point would be unnecessarily
> pessimistic.

I disagree that it is valid.  At least it is risky to consider it valid,
because a pass that simulates liveness might end up doing something
wrong because of that note.  If simulation is done backwards, it doesn't
even require any interaction with REG_DEAD notes.

> I also don't want to compute DF_LR taking EQ_USES into account as real
> uses for liveness, because that involves recomputing and enlarging the
> DF_LR sets (all of them, both globally and locally) before LR&RD and
> after LR&RD. That's why I implemented the quick-and-dirty liveness
> computation for the notes: It's non-intrusive on DF_LR and it's cheap.

Yes, I agree on that part of the implementation. :)

Paolo