[llvm] [LoopInterchange] Don't consider loops with BTC=0 (PR #167113)

[Ryotaro Kasuga via llvm-commits]

================
@@ -0,0 +1,73 @@
+; RUN: opt < %s -passes=loop-interchange -verify-dom-info -verify-loop-info \
+; RUN:       -pass-remarks-output=%t -pass-remarks='loop-interchange' -S
+; RUN: cat %t |  FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+ at D = common global [100 x [100 x [100 x i32]]] zeroinitializer
+
+; Test for interchange in
+;
+;  for(int i=0;i<1;i++)
+;    for(int j=0;j<100;j++)
+;      for(int k=0;k<100;k++)
+;        D[i][k][j] = D[i][k][j]+t;
+;
+
+; CHECK:        --- !Analysis
+; CHECK-NEXT:   Pass:            loop-interchange
+; CHECK-NEXT:   Name:            Dependence
+; CHECK-NEXT:   Function:        interchange_i_and_j
+; CHECK-NEXT:   Args:
+; CHECK-NEXT:     - String:          Computed dependence info, invoking the transform.
+; CHECK-NEXT:   ...
+; CHECK-NEXT:   --- !Passed
+; CHECK-NEXT:   Pass:            loop-interchange
+; CHECK-NEXT:   Name:            Interchanged
+; CHECK-NEXT:   Function:        interchange_i_and_j
+; CHECK-NEXT:   Args:
+; CHECK-NEXT:     - String:          Loop interchanged with enclosing loop.
+; CHECK-NEXT:   ...
+; CHECK-NEXT:   --- !Missed
+; CHECK-NEXT:   Pass:            loop-interchange
+; CHECK-NEXT:   Name:            InterchangeNotProfitable
+; CHECK-NEXT:   Function:        interchange_i_and_j
+; CHECK-NEXT:   Args:
+; CHECK-NEXT:     - String:          Insufficient information to calculate the cost of loop for interchange.
+; CHECK-NEXT:   ...
+
+define void @interchange_i_and_j(i32 %t){
+entry:
+  br label %outer.header
+
+outer.header:
+  %i = phi i64 [ 0, %entry ], [ %inc16, %for.inc15 ]
+  br label %inner1.header
+
+inner1.header:
+  %j = phi i64 [ 0, %outer.header ], [ %inc13, %for.inc12 ]
+  br label %inner2.body
+
+inner2.body:
+  %k = phi i64 [ 0, %inner1.header ], [ %inc, %inner2.body ]
+  %arrayidx8 = getelementptr inbounds [100 x [100 x [100 x i32]]], ptr @D, i64 0, i64 %i, i64 %k, i64 %j
----------------
kasuga-fj wrote:

```suggestion
  %arrayidx8 = getelementptr inbounds [100 x [100 x i32]], ptr @D, i64 %i, i64 %k, i64 %j
```

I remember hearing somewhere that this is the canonical form.

https://github.com/llvm/llvm-project/pull/167113