[llvm] [VPlan] Model first memory runtime checks as VPlan recipes. (PR #221483)

[Ramkumar Ramachandra via llvm-commits]

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@@ -1809,8 +1799,33 @@ class GeneratedRTChecks {
   }
 
   /// Return true if any runtime checks have been added
-  bool hasChecks() const {
-    return getSCEVChecks().first || getMemRuntimeChecks().first;
+  bool hasChecks() const { return getSCEVChecks().first || HasMemChecks; }
+
+  /// Drop the pre-built memory check block in favour of VPlan recipes.
+  /// TODO: Remove once the checks can be costed in VPlan, before VF selection.
+  void dropMemRuntimeChecks() {
+    assert(MemCheckBlock && pred_empty(MemCheckBlock) &&
+           "cannot drop memory checks that are missing or already connected");
+    eraseMemCheckBlock();
+  }
+
+private:
+  /// Erase the memory check block, its instructions and their SCEV expansions.
+  void eraseMemCheckBlock() {
+    SCEVExpanderCleaner MemCheckCleaner(MemCheckExp);
+    auto &SE = *MemCheckExp.getSE();
+    // Memory runtime check generation creates compares that use expanded
+    // values. Remove them before running the SCEVExpanderCleaner.
+    for (auto &I : make_early_inc_range(reverse(*MemCheckBlock))) {
+      if (MemCheckExp.isInsertedInstruction(&I))
+        continue;
+      SE.forgetValue(&I);
+      I.eraseFromParent();
+    }
+    MemCheckCleaner.cleanup();
+    MemCheckBlock->eraseFromParent();
+    MemCheckBlock = nullptr;
+    MemRuntimeCheckCond = nullptr;
   }
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artagnon wrote:

I just had an idea to avoid this -- is it possible to re-use SCEVExpander::isHighCostExpansionHelper? Seems unnecessary to generate IR to just cost something, right?

https://github.com/llvm/llvm-project/pull/221483