[llvm] [LV] Initial support for stores in early exit loops (PR #137774)

[David Sherwood via llvm-commits]

================
@@ -654,6 +661,16 @@ class LoopVectorizationLegality {
   /// Keep track of the loop edge to an uncountable exit, comprising a pair
   /// of (Exiting, Exit) blocks, if there is exactly one early exit.
   std::optional<std::pair<BasicBlock *, BasicBlock *>> UncountableEdge;
+
+  /// Indicates that we will need to copy the early exit condition into
+  /// the vector preheader, as we will need to mask some operations in
+  /// the loop (e.g. stores).
+  bool RequiresEarlyExitConditionCopy = false;
+
+  /// The load used to determine an uncountable early-exit condition. This is
+  /// only used to allow further analysis in canVectorizeMemory if we found
+  /// what looks like a valid early exit loop with store beforehand.
+  std::optional<LoadInst *> EarlyExitLoad;
----------------
david-arm wrote:

What happens in the case of two loads?, i.e.

%ld1 = load i8, ...
%ld2 = load i8, ...
%cmp = icmp eq i8 %ld1, %ld2
br i1 %cmp, label %early.exit, %loop.inc


https://github.com/llvm/llvm-project/pull/137774