[llvm] [LoopVectorize] Teach LoopVectorizationLegality about more early exits (PR #107004)

[Paul Walker via llvm-commits]

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@@ -1070,6 +1070,50 @@ bool LoopVectorizationLegality::canVectorizeMemory() {
     return false;
   }
 
+  // For loops with uncountable early exiting blocks that are not the latch
+  // it's necessary to perform extra checks, since the vectoriser is currently
+  // only capable of handling simple search loops.
+  if (IsEarlyExitLoop) {
+    // We don't support calls or any memory accesses that write to memory.
+    if (LAI->getNumStores()) {
+      reportVectorizationFailure(
+          "Writes to memory unsupported in early exit loops",
+          "Cannot vectorize early exit loop with writes to memory",
+          "WritesInEarlyExitLoop", ORE, TheLoop);
+      return false;
+    }
+
+    if (LAI->getNumCalls()) {
+      reportVectorizationFailure(
+          "Calls unsupported in early exit loops",
+          "Cannot vectorize early exit loop with function calls",
+          "CallsInEarlyExitLoop", ORE, TheLoop);
+      return false;
+    }
+
+    // The vectoriser cannot handle loads that occur after the early exit block.
+    BasicBlock *LatchBB = TheLoop->getLoopLatch();
+    for (Instruction &I : *LatchBB) {
----------------
paulwalker-arm wrote:

~This seems like a naive question but is the latch block the only one that needs to be check? Are there no other blocks between the existing an latch blocks?~

I answered my own question but leaving comment here because it is referenced below.

https://github.com/llvm/llvm-project/pull/107004