Re: [RFA][rtl-optimization/52714] Do not allow combine to create invalid RTL

[Jeff Law <law at redhat dot com>]

On 02/11/14 02:06, Eric Botcazou wrote:
> > I pondered changing the condition for swapping the insn order, but it
> > didn't seem worth it.  I doubt we see many 3->2 combinations where I3 is
> > a JUMP_INSN, the result turns into two simple sets and the insn swapping
> > code you wrote decides it needs to swap the insns.
>
> I didn't actually write it, just enhanced it recently, that's why I suggested
> to do the same here.  It's one line of code and we have an example of valid
> simplification at hand so I think we ought to do it.
>
> > It seems to me that as long as we're re-using the existing insns to
> > contain the simple sets that we have to ensure that they're INSNs, not
> > CALL_INSNs or JUMP_INSNs.
>
> I disagree, nullifying JUMP_INSNs by changing them to (set (pc) (pc)) is a
> standard method in the combiner.
Right, but most of the time we're doing a 2->1 or 3->1 that turns the 
conditional jump into a nop -- at which point all we do is modify I3 
(which is the conditional jump) into a nop-jump.  That common case 
shouldn't be affected by my change.

It's only when we have a 3->2 or 4->2 combination where one of the 
earlier insns has a side effect we need to keep and we're able to 
simplify the conditional into a NOP that would be affected by my change. 
 And I suspect that is much more rare.

Regardless, I'll see what I can do.

THanks,
Jeff