[PATCH] D36113: [Loop Vectorize] Vectorize Loops with Backward Dependence

[Matthew Simpson via Phabricator via llvm-commits]

mssimpso added inline comments.


================
Comment at: lib/Transforms/Vectorize/LoopVectorizePred.cpp:187
+      if (MSSA->isLiveOnEntryDef(D))
+        Reordered |= checkDepAndReorder(St, Ld);
+      else if (Instruction *DefInst = dyn_cast<Instruction>(D)) {
----------------
dberlin wrote:
> DIVYA wrote:
> > hfinkel wrote:
> > > I'm somewhat concerned that a number of these cases are actually handling loop-invariant values, meaning that we're doing a suboptimal handling of something that LICM should handle more fully. The problem is that dealing with these after vectorization could be more difficult than before, and if we have a phase-ordering problem such that we're missing these cases, we might just end up with suboptimal code. Are you seeing these cases in practice?
> > > 
> > 
> > I think the loop invariant codes will be already hoisted outside the loop before this pass.The Defining access will be liveOnEntry for  the loads  and stores from pointer arguments .
> > For example in the below code,  the defining access for the load Instruction for a[i+1], is Live on Entry .However the load instruction is not loop invariant.
> > 
> > For the case
> > int  foo1(int n,int * restrict a, int * restrict b, int *restrict m){
> >   int i;
> >   for (i = 0; i < n; i++){
> >     a[i] = b[i];
> >     m[i] = a[i+1];
> >   }
> > }
> > 
> > 
> I'm surprised it can prove that it does not conflict with the store.
> The load instruction actually is loop invariant in the sense of whether a given loaded address can change during the loop, it just takes on different loop invariant values each iteration.
> 
> It is possible to hoist it out of this loop, it just requires making a different loop :)
> 
> In particular, this generates the same result:
> 
> ```
> int foo1(int n,int * restrict a, int * restrict b, int *restrict m){
> 
> int i;
> for (i = 0; i < n; i++){
>   m[i] = a[i+1];
> }
> for (i = 0; i < n; i++){
>   a[i] = b[i];
> }
> }
> ```
I wonder if our loop distribution pass (disabled by default) can handle this case?


https://reviews.llvm.org/D36113