[SCEVExp] Add helper to clean up dead instructions after expansion. (#157308)

Add new helper to erase dead instructions inserted during SCEV expansion
but not being used due to InstSimplifyFolder simplifications.

Together with https://github.com/llvm/llvm-project/pull/157307 this also
allows removing some specialized folds, e.g.
https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp#L2205

PR: https://github.com/llvm/llvm-project/pull/157308

1 files changed, 16 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
index 7c12dac..ef9173b 100644
--- a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
@@ -26,6 +26,7 @@
 #include "llvm/IR/PatternMatch.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Transforms/Utils/LoopUtils.h"
 
 #if LLVM_ENABLE_ABI_BREAKING_CHECKS
@@ -175,6 +176,21 @@ SCEVExpander::findInsertPointAfter(Instruction *I,
   return IP;
 }
 
+void SCEVExpander::eraseDeadInstructions(Value *Root) {
+  SmallVector<Value *> WorkList;
+  append_range(WorkList, getAllInsertedInstructions());
+  while (!WorkList.empty()) {
+    Instruction *I = dyn_cast<Instruction>(WorkList.pop_back_val());
+    if (!I || I == Root || !isInsertedInstruction(I) ||
+        !isInstructionTriviallyDead(I))
+      continue;
+    append_range(WorkList, I->operands());
+    InsertedValues.erase(I);
+    InsertedPostIncValues.erase(I);
+    I->eraseFromParent();
+  }
+}
+
 BasicBlock::iterator
 SCEVExpander::GetOptimalInsertionPointForCastOf(Value *V) const {
   // Cast the argument at the beginning of the entry block, after