[llvm] [DA] Check monotonicity for subscripts (PR #154527)

[Ryotaro Kasuga via llvm-commits]

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@@ -3308,6 +3308,322 @@ void DependenceInfo::updateDirection(Dependence::DVEntry &Level,
     llvm_unreachable("constraint has unexpected kind");
 }
 
+namespace {
+
+enum class MonotonicityType {
+  MaySignedWrap, ///< The expression contains arithmetic operations that may
+                 ///< cause signed wrap.
+  Constant,      ///< The expression is constant. If a SCEV is classified as
+                 ///< Constant, it also implies that it doesn't contain any
+                 ///< arithmetic operations that may cause signed wrap.
+  Monotonic, ///< The expression is monotonically increasing or decreasing. This
+             ///< is exclusive of Constant. That is, we say an SCEV is Monotonic
+             ///< iff it contains at least one AddRec where its step reccurence
+             ///< value is non-zero.
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kasuga-fj wrote:

I'm not entirely sure whether the term "monotonic" is appropriate here, but I believe `A[i + j]` in the example is "valid" in the context of DA. I think what I'm trying to conceptualize is something like how multilinear relates to linear -- perhaps something like 'multimonotonic' for monotonic?

So, I intended these properties to be considered with respect to the outermost loop, rather than any specific loop within the loop nest. In any case, I'm now feeling strongly that I should add some tests to illustrate the expected behavior.

https://github.com/llvm/llvm-project/pull/154527