[llvm] [LoopInterchange] Add metadata to control loop-interchange (PR #127474)

[Ryotaro Kasuga via llvm-commits]

================
@@ -569,6 +609,152 @@ struct LoopInterchange {
 
     return true;
   }
+
+  bool processEnabledLoop(SmallVectorImpl<Loop *> &LoopList,
+                          std::vector<std::vector<char>> &DependencyMatrix,
+                          const DenseMap<const Loop *, unsigned> &CostMap) {
+    bool Changed = false;
+
+    // Manage the index so that LoopList[Loop2Index[L]] == L for each loop L.
+    DenseMap<Loop *, unsigned> Loop2Index;
+    for (unsigned I = 0; I != LoopList.size(); I++)
+      Loop2Index[LoopList[I]] = I;
+
+    // Hold outer loops to be exchanged (i.e., loops that have
+    // "llvm.loop.interchange.enable" is true), in the current nest order.
+    SmallVector<Loop *, 4> Worklist;
----------------
kasuga-fj wrote:

As for the cases where the loops to be interchanged do not interfere with each other, I plan to handle them independently. That is, the following code 

```c
#pragma clang loop interchange(enable)
for (i=0; i<N; i++)
  for (j=0; j<N; j++)
    #pragma clang loop interchange(enable)
    for (k=0; k<N; k++)
      for (l=0; l<N; l++)
        ...
```

will be translated like as

```
!interchange_ij = !{!"interchange_ij", !interchange_enable}
!interchange_kl = !{!"interchange_kl", !interchange_enable}
!interchange_enable = !{!"llvm.loop.interchange.enable", i1 true}
```

not as follows.

```
!interchange_kl = !{!"interchange_kl", !interchange_enable, !followup_ij}
!interchange_enable = !{!"llvm.loop.interchange.enable", i1 true}
!followup_ij = !{"followup_next_next_outer", ...}
```

https://github.com/llvm/llvm-project/pull/127474