[PATCH] D21665: [LoopSimplify] Update LCSSA after separating nested loops.

[Sanjoy Das via llvm-commits]

sanjoy added inline comments.

================
Comment at: lib/Transforms/Utils/LoopSimplify.cpp:370
@@ +369,3 @@
+            NewPN->addIncoming(V, PredBB);
+          PN->setIncomingValue(i, NewPN);
+          break;
----------------
sanjoy wrote:
> What if you have
> 
> ```
> loop:
>   %iv = phi [ 0, %entry ] [ %iv, (%loop, %left, %right)],  [ %iv.inc, %outer ]
>   %iv.inc = %iv + 1
>   switch(condition), %left, %right, %loop
> 
> left:
>   br condition, %outer, %loop
> 
> right:
>   br condition, %outer, %loop
> 
> outer:
>   br %loop
> ```
> 
> then the separated loop structure will be
> 
> ```
> outer_header:
>   %iv = [ 0, %entry ] , [ %iv.inc, %outer ]
>   br loop
> 
> loop:
>   %iv.inc = %iv + 1
>   switch(condition), %left, %right, %loop
> 
> left:
>   br condition, %outer, %loop
> 
> right:
>   br condition, %outer, %loop
> 
> outer:
>   br %loop
> ```
> 
> Now %iv.inc is live out of the inner loop and none of the inner loop
> exits (%left, %right) dominate %outer.
Nit: the latter code snippet should be

```
outer_header:
  %iv = [ 0, %entry ] , [ %iv.inc, %outer ]
  br loop

loop:
  %iv.inc = %iv + 1
  switch(condition), %left, %right, %loop

left:
  br condition, %outer, %loop

right:
  br condition, %outer, %loop

outer:
  br %outer_header
```



http://reviews.llvm.org/D21665