[llvm] r193853 - LoopVectorizer: If dependency checks fail try runtime checks

[Arnold Schwaighofer]

On 11/01/13, Hal Finkel   wrote:
> ----- Original Message -----
> > 
> > On Oct 31, 2013, at 10:36 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> > 
> > > Arnold,
> > > 
> > > FWIW, this looks related to a question that I asked earlier today:
> > > http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-October/067083.html
> > > 
> > 
> > The array bounds check is only added when the dependency check fails.
> > Once we extend SCEV for your example, in whatever way we decide to
> > do that, we won’t emit these checks anymore.
> > 
> > > If a guard already exists (as in the example from my post), and the
> > > vectorizer also adds one, does anything after the vectorizer clean
> > > that up?
> > > 
> > 
> > The runtime checks we add are different from the guard you added in
> > your example. Not as clever. The loop vectorizer’s runtime checks
> > just test no-overlapping'ness of the accesses in the loop. We don’t
> > take dependency information into account for building those checks.
> 
> Do the runtime checks account for the store-to-load forwarding issue?

Yes (indirectly), because we check that the range of memory that two accesses touch don't overlap.


for (i =0; i <  n; )
 a[f(i)] = ...
 ... = a[g(i)]


then we test


  &a[f(0)] ... &a[f(n)]   does not overlap &a[g(0)] ... &a[g(n)]


In  your case, if 'k < 2047' this test would fail.