[llvm] [DA] Add initial support for monotonicity check (PR #162280)

[Ryotaro Kasuga via llvm-commits]

================
@@ -3479,10 +3732,19 @@ bool DependenceInfo::tryDelinearize(Instruction *Src, Instruction *Dst,
   // resize Pair to contain as many pairs of subscripts as the delinearization
   // has found, and then initialize the pairs following the delinearization.
   Pair.resize(Size);
+  SCEVMonotonicityChecker MonChecker(SE);
+  const Loop *OutermostLoop = SrcLoop ? SrcLoop->getOutermostLoop() : nullptr;
   for (int I = 0; I < Size; ++I) {
     Pair[I].Src = SrcSubscripts[I];
     Pair[I].Dst = DstSubscripts[I];
     unifySubscriptType(&Pair[I]);
+
+    if (EnableMonotonicityCheck) {
+      if (MonChecker.checkMonotonicity(Pair[I].Src, OutermostLoop).isUnknown())
+        return false;
+      if (MonChecker.checkMonotonicity(Pair[I].Dst, OutermostLoop).isUnknown())
+        return false;
+    }
----------------
kasuga-fj wrote:

Consider the following case ([godbolt](https://godbolt.org/z/Mro9bMrKW)):

```llvm
; char A[][32];
; for (i = 0; i < 1ll << 62; i++)
;   for (j = 0; j < 32; j++)
;     if (i < (1ll << 57))
;       A[i][j] = 0;
define void @outer_loop_may_wrap(ptr %a) {
entry:
  br label %loop.i.header

loop.i.header:
  %i = phi i64 [ 0, %entry ], [ %i.inc, %loop.i.latch ]
  br label %loop.j.header

loop.j.header:
  %j = phi i64 [ 0, %loop.i.header ], [ %j.inc, %loop.j.latch ]
  %cond = icmp slt i64 %i, 144115188075855872  ; 2^57
  br i1 %cond, label %if.then, label %loop.j.latch

if.then:
  %gep = getelementptr inbounds [32 x i8], ptr %a, i64 %i, i64 %j
  store i8 0, ptr %gep
  br label %loop.j.latch

loop.j.latch:
  %j.inc = add nuw nsw i64 %j, 1
  %ec.j = icmp eq i64 %j.inc, 32
  br i1 %ec.j, label %loop.i.latch, label %loop.j.header

loop.i.latch:
  %i.inc = add nuw nsw i64 %i, 1
  %ec.i = icmp eq i64 %i.inc, 4611686018427387904  ; 2^62
  br i1 %ec.i, label %exit, label %loop.i.header


exit:
  ret void
}
```

The subscripts `{0,+,1}<nuw><nsw><%loop.i.header>` and `{0,+,1}<nuw><nsw><%loop.j.header>` are monotonic, but the original offset `{{0,+,32}<%loop.i.header>,+,1}<nw><%loop.j.header>` is not.

https://github.com/llvm/llvm-project/pull/162280