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

[Steven Bosscher <stevenb.gcc@gmail.com>]

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 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.

Ciao!
Steven