[llvm] [LoopPeel] Add new option to peeling loops to convert PHI into IV (PR #121104)

[Nikita Popov via llvm-commits]

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@@ -155,45 +159,170 @@ namespace {
 // corresponding calls to g are determined and the code for computing
 // x, y, and a can be removed.
 //
+// Similarly, there are cases where peeling makes Phi nodes loop-inductions
+// (i.e., the value is increased or decreased by a fixed amount on every
+// iteration). For example, consider the following function.
+//
+//   #define N 100
+//   void f(int a[], int b[]) {
+//     int im = N - 1;
+//     for (int i = 0; i < N; i++) {
+//       a[i] = b[i] + b[im];
+//       im = i;
+//     }
+//   }
+//
+// The IR of the loop will look something like the following.
+//
+//   %i = phi i32 [ 0, %entry ], [ %i.next, %for.body ]
+//   %im = phi i32 [ 99, %entry ], [ %i, %for.body ]
+//   ...
+//   %i.next = add nuw nsw i32 %i, 1
+//   ...
+//
+// In this case, %im becomes a loop-induction variable by peeling 1 iteration,
+// because %i is a loop-induction one. The peeling count can be determined by
+// the same algorithm with loop-invariant case. Such peeling is profitable for
+// loop-vectorization.
+//
 // The PhiAnalyzer class calculates how many times a loop should be
 // peeled based on the above analysis of the phi nodes in the loop while
 // respecting the maximum specified.
 class PhiAnalyzer {
 public:
-  PhiAnalyzer(const Loop &L, unsigned MaxIterations);
+  PhiAnalyzer(const Loop &L, unsigned MaxIterations, bool PeelForIV);
 
   // Calculate the sufficient minimum number of iterations of the loop to peel
   // such that phi instructions become determined (subject to allowable limits)
   std::optional<unsigned> calculateIterationsToPeel();
 
 protected:
-  using PeelCounter = std::optional<unsigned>;
+  enum class PeelCounterType {
+    Invariant,
+    Induction,
+  };
+
+  using PeelCounterValue = std::pair<unsigned, PeelCounterType>;
+  using PeelCounter = std::optional<PeelCounterValue>;
   const PeelCounter Unknown = std::nullopt;
 
   // Add 1 respecting Unknown and return Unknown if result over MaxIterations
   PeelCounter addOne(PeelCounter PC) const {
     if (PC == Unknown)
       return Unknown;
-    return (*PC + 1 <= MaxIterations) ? PeelCounter{*PC + 1} : Unknown;
+    auto [Val, Ty] = *PC;
+    return (Val + 1 <= MaxIterations) ? PeelCounter({Val + 1, Ty}) : Unknown;
   }
 
-  // Calculate the number of iterations after which the given value
-  // becomes an invariant.
+  // Return a value representing zero for the given counter type.
+  PeelCounter makeZero(PeelCounterType Ty) const {
+    return PeelCounter({0, Ty});
+  }
+
+  // Calculate the number of iterations after which the given value becomes an
+  // invariant or an induction.
   PeelCounter calculate(const Value &);
 
+  // Auxiliary function to calculate the number of iterations for a comparison
+  // instruction or a binary operator.
+  PeelCounter mergeTwoCounter(const Instruction &CmpOrBinaryOp,
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nikic wrote:

nit: mergeTwoCounters?

https://github.com/llvm/llvm-project/pull/121104