Re: 0003-Improve-VBEout-computation.patch

[Jeff Law <law at redhat dot com>]

On 06/16/10 11:03, Maxim Kuvyrkov wrote:
> On 6/16/10 8:57 PM, Paolo Bonzini wrote:
> > +  that are calculated along every path from BB.
> > + E.g., it will not try to optimize the following case:
> > +
> > + 2
> > + | \
> > + 3* |
> > + | /
> > + 4
> > + / \
> > + 5* 6
> > +
> > + ;; "*" marks basic blocks that calculate same expression
> > + ;; Ideally, all calculation would be moved to block 2.
> >
> > No, this pessimizes the path 2->4->6.
>
> Code hoisting is used only when optimizing for size, otherwise PRE is 
> used.  Maybe I'm missing something, but /speed/ regression of the path 
> 2->4->6 is acceptable as long as overall code size goes down.
Well, within certain limits, a speed regression would be acceptable.  
But what is more important is the correctness issue.

Hoisting the expression into block #2 would introduce an evaluation of 
the expression on a path which did not have an evaluation in the 
original code (2->4->6) -- which could potentially cause a conforming 
program to begin to fail.  This can occur for memory references or other 
instructions that might potentially trap/fault on invalid input.

Jeff