[llvm] [IVDescriptors] Implement MonotonicDescriptor (PR #214490)

[Luke Lau via llvm-commits]

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@@ -1696,3 +1696,95 @@ bool InductionDescriptor::isInductionPHI(
                           /*InductionBinOp=*/nullptr, /*Casts=*/nullptr, Preds);
   return true;
 }
+
+// Recognize a monotonic PHI variable by matching the following pattern:
+// loop_header:
+//   %monotonic_phi = phi [ %start, %preheader ], [ %latch_phi, %latch ]
+//   br i1 %do_step, label %step_bb, label %latch
+//
+// step_bb:
+//   %step = add/gep %monotonic_phi, %step_val
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lukel97 wrote:

Is it safe to consider a phi that can wrap as monotonic? Looking at the test cases in #214491 the steps all have the `nsw` flag, I'm guessing it's not safe to consider a pointer as a compressing pointer without it? 

If so, do we need to check for that in this PR and return false for `isMonotonicPHI`, or do we handle it in #214491 by checking `SCEVNoWrapFlags`?

https://github.com/llvm/llvm-project/pull/214490