[llvm] [Utils] Consolidate `LockstepReverseIterator` into own header (NFC) (PR #116657)

[David Blaikie via llvm-commits]

================
@@ -0,0 +1,155 @@
+//===- LockstepReverseIterator.h ------------------------------*- C++ -*---===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TRANSFORMS_UTILS_LOCKSTEPREVERSEITERATOR_H
+#define LLVM_TRANSFORMS_UTILS_LOCKSTEPREVERSEITERATOR_H
+
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/SetVector.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/Instruction.h"
+
+namespace llvm {
+
+struct NoActiveBlocksOption {};
+
+struct ActiveBlocksOption {
+protected:
+  SmallSetVector<BasicBlock *, 4> ActiveBlocks;
+
+public:
+  ActiveBlocksOption() = default;
+};
+
+/// Iterates through instructions in a set of blocks in reverse order from the
+/// first non-terminator. For example (assume all blocks have size n):
+///   LockstepReverseIterator I([B1, B2, B3]);
+///   *I-- = [B1[n], B2[n], B3[n]];
+///   *I-- = [B1[n-1], B2[n-1], B3[n-1]];
+///   *I-- = [B1[n-2], B2[n-2], B3[n-2]];
+///   ...
+///
+/// The iterator continues processing until all blocks have been exhausted if \p
+/// EarlyFailure is explicitly set to \c false. Use \c getActiveBlocks() to
+/// determine which blocks are still going and the order they appear in the list
+/// returned by operator*.
+template <bool EarlyFailure = true>
+class LockstepReverseIterator
+    : public std::conditional_t<EarlyFailure, NoActiveBlocksOption,
+                                ActiveBlocksOption> {
+private:
+  ArrayRef<BasicBlock *> Blocks;
+  SmallVector<Instruction *, 4> Insts;
+  bool Fail;
+
+public:
+  LockstepReverseIterator(ArrayRef<BasicBlock *> Blocks) : Blocks(Blocks) {
+    reset();
+  }
+
+  void reset() {
+    Fail = false;
+    if constexpr (!EarlyFailure) {
+      this->ActiveBlocks.clear();
+      for (BasicBlock *BB : Blocks)
+        this->ActiveBlocks.insert(BB);
+    }
+    Insts.clear();
+    for (BasicBlock *BB : Blocks) {
+      Instruction *Prev = BB->getTerminator()->getPrevNonDebugInstruction();
+      if (!Prev) {
+        // Block wasn't big enough - only contained a terminator.
+        if constexpr (EarlyFailure) {
+          Fail = true;
+          return;
+        } else {
+          this->ActiveBlocks.remove(BB);
+          continue;
+        }
+      }
+      Insts.push_back(Prev);
+    }
+    if (Insts.empty())
+      Fail = true;
+  }
+
+  bool isValid() const { return !Fail; }
+  ArrayRef<Instruction *> operator*() const { return Insts; }
+
+  // Note: This needs to return a SmallSetVector as the elements of
+  // ActiveBlocks will be later copied to Blocks using std::copy. The
+  // resultant order of elements in Blocks needs to be deterministic.
+  // Using SmallPtrSet instead causes non-deterministic order while
+  // copying. And we cannot simply sort Blocks as they need to match the
+  // corresponding Values.
+  SmallSetVector<BasicBlock *, 4> &getActiveBlocks() {
+    static_assert(!EarlyFailure, "Unknown method");
+    return this->ActiveBlocks;
+  }
+
----------------
dwblaikie wrote:

Could this be a member of the active blocks base, so it's just totally not defined on the inactive version? (then there'd be no need for the static assert, and maybe it'd be better for compiler diagnostics?)

https://github.com/llvm/llvm-project/pull/116657