[llvm] [CodeGen] Move rollback capabilities outside of the rematerializer (PR #184341)

[Quentin Colombet via llvm-commits]

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@@ -361,27 +355,26 @@ class Rematerializer {
                                  MachineBasicBlock::iterator InsertPos,
                                  DependencyReuseInfo &DRI);
 
-  /// Rolls back all rematerializations of original register \p RootIdx,
-  /// transfering all their users back to it and permanently deleting them from
-  /// the MIR. The root register is revived if it was fully rematerialized (this
-  /// requires that rollback support was set at that time). Transitive
-  /// dependencies of the root register that were fully rematerialized are
-  /// re-vived at their original positions; this requires that rollback support
-  /// was set when they were rematerialized.
-  void rollbackRematsOf(RegisterIdx RootIdx);
-
-  /// Rolls back register \p RematIdx (which must be a rematerialization)
-  /// transfering all its users back to its origin. The latter is revived if it
-  /// was fully rematerialized (this requires that rollback support was set at
-  /// that time).
-  void rollback(RegisterIdx RematIdx);
-
-  /// Revives original register \p RootIdx at its original position in the MIR
-  /// if it was fully rematerialized with rollback support set. Transitive
-  /// dependencies of the root register that were fully rematerialized are
-  /// revived at their original positions; this requires that rollback support
-  /// was set when they were themselves rematerialized.
-  void reviveRegIfDead(RegisterIdx RootIdx);
+  /// Rematerializes register \p RegIdx at \p InsertPos in \p UseRegion, adding
+  /// the new rematerializable register to the backing vector \ref Regs and
+  /// returning its index inside the vector. Sets the new register's
+  /// rematerializable dependencies to \p Dependencies (these are assumed to
+  /// already exist in the MIR) and its unrematerializable dependencies to the
+  /// same as \p RegIdx. The new register initially has no user. Since the
+  /// method appends to \ref Regs, references to elements within it should be
+  /// considered invalidated across calls to this method unless the vector can
+  /// be guaranteed to have enough space for an extra element.
+  RegisterIdx rematerializeReg(RegisterIdx RegIdx, unsigned UseRegion,
+                               MachineBasicBlock::iterator InsertPos,
+                               SmallVectorImpl<Reg::Dependency> &&Dependencies);
+
+  /// Re-creates a previously deleted register \p RegIdx at \p InsertPos in \p
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qcolombet wrote:

Same question on the InsertPos? Before or after?

https://github.com/llvm/llvm-project/pull/184341