[PATCH] Unbreak forward simulation of liveness (!)

[Paolo Bonzini <bonzini at gnu dot org>]

Hi all,

this patch is the main reason why I've not yet submitted the patch to 
restore memory usage of fwprop.  The patch had a regression and I found 
time to analyze it only now.

It seems that forward simulation of liveness is totally broken, in that 
it never adds a register to the set!  So, we get (way) too many dead 
registers.  In the case of fwprop this would call a missed optimization, 
with one single exception: this is PR39543, where the missed propagation 
causes reload failures.

The problem is that while scanning forwards, a def will _generate_ 
liveness!  The reason is that during the solution of the problem, the 
def had killed liveness towards the top of the basic block; now we have 
to assume that liveness was generated and use REG_UNUSED notes to clean 
up extra bits.

df_simulate_initialize_forwards is broken in the same way.  I have no 
time right now to fix it, and it is safe for fwprop, so I would 
appreciate if Kenny took a look at it.  The simplest way seems to be to 
restore DF_LR_TOP/DF_LIVE_TOP, and then use that in fwprop.  Possibly 
you could make that a changeable flag.

Finally, there's also df_simulate_finalize_forwards, which I think we 
can delete (it's just computing DF_LR_OUT in an expensive way).

The patch has been regtested together with the fwprop patch; a full 
bootstrap is in progress.  Ok when it finishes?

Paolo