[llvm] My dce (PR #120468)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: None (RashmiSalankar)

<details>
<summary>Changes</summary>



---
Full diff: https://github.com/llvm/llvm-project/pull/120468.diff


5 Files Affected:

- (added) llvm/include/llvm/Transforms/Utils/MyDeadCodeEliminationPass.h (+27) 
- (modified) llvm/lib/Passes/PassBuilder.cpp (+1) 
- (modified) llvm/lib/Passes/PassRegistry.def (+1) 
- (modified) llvm/lib/Transforms/Utils/CMakeLists.txt (+5) 
- (added) llvm/lib/Transforms/Utils/MyDeadCodeEliminationPass.cpp (+59) 


``````````diff
diff --git a/llvm/include/llvm/Transforms/Utils/MyDeadCodeEliminationPass.h b/llvm/include/llvm/Transforms/Utils/MyDeadCodeEliminationPass.h
new file mode 100644
index 00000000000000..24187cb4436b76
--- /dev/null
+++ b/llvm/include/llvm/Transforms/Utils/MyDeadCodeEliminationPass.h
@@ -0,0 +1,27 @@
+#ifndef LLVM_TRANSFORMS_UTILS_MYDEADCODEELIMINATIONPASS_H
+#define LLVM_TRANSFORMS_UTILS_MYDEADCODEELIMINATIONPASS_H
+
+#include "llvm/IR/PassManager.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/Instruction.h"
+#include <unordered_set>
+
+namespace llvm {
+
+class MyDeadCodeEliminationPass : public PassInfoMixin<MyDeadCodeEliminationPass> {
+public:
+  // Main run method for the pass
+  PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
+
+private:
+  // Helper function for iterative analysis
+  void analyzeInstructionsIteratively(Function &F);
+
+  // Helper function to check if an instruction is dead
+  bool isInstructionDead(Instruction *Inst, const std::unordered_set<Instruction *> &potentialDeadInstructions);
+};
+
+} // namespace llvm
+
+#endif // LLVM_TRANSFORMS_UTILS_MYDEADCODEELIMINATIONPASS_H
+
diff --git a/llvm/lib/Passes/PassBuilder.cpp b/llvm/lib/Passes/PassBuilder.cpp
index 77dea7d06d0900..80809e3ea1b486 100644
--- a/llvm/lib/Passes/PassBuilder.cpp
+++ b/llvm/lib/Passes/PassBuilder.cpp
@@ -302,6 +302,7 @@
 #include "llvm/Transforms/Scalar/StructurizeCFG.h"
 #include "llvm/Transforms/Scalar/TailRecursionElimination.h"
 #include "llvm/Transforms/Scalar/WarnMissedTransforms.h"
+#include "llvm/Transforms/Utils/MyDeadCodeEliminationPass.h"
 #include "llvm/Transforms/Utils/AddDiscriminators.h"
 #include "llvm/Transforms/Utils/AssumeBundleBuilder.h"
 #include "llvm/Transforms/Utils/BreakCriticalEdges.h"
diff --git a/llvm/lib/Passes/PassRegistry.def b/llvm/lib/Passes/PassRegistry.def
index 772ec5fd10e633..c797955ee969e9 100644
--- a/llvm/lib/Passes/PassRegistry.def
+++ b/llvm/lib/Passes/PassRegistry.def
@@ -328,6 +328,7 @@ FUNCTION_ALIAS_ANALYSIS("tbaa", TypeBasedAA())
 #ifndef FUNCTION_PASS
 #define FUNCTION_PASS(NAME, CREATE_PASS)
 #endif
+FUNCTION_PASS("my-dce", MyDeadCodeEliminationPass())
 FUNCTION_PASS("aa-eval", AAEvaluator())
 FUNCTION_PASS("adce", ADCEPass())
 FUNCTION_PASS("add-discriminators", AddDiscriminatorsPass())
diff --git a/llvm/lib/Transforms/Utils/CMakeLists.txt b/llvm/lib/Transforms/Utils/CMakeLists.txt
index 65bd3080662c4d..221cb31e20dd9a 100644
--- a/llvm/lib/Transforms/Utils/CMakeLists.txt
+++ b/llvm/lib/Transforms/Utils/CMakeLists.txt
@@ -1,4 +1,5 @@
 add_llvm_component_library(LLVMTransformUtils
+  MyDeadCodeEliminationPass.cpp	
   AddDiscriminators.cpp
   AMDGPUEmitPrintf.cpp
   ASanStackFrameLayout.cpp
@@ -93,6 +94,10 @@ add_llvm_component_library(LLVMTransformUtils
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/Transforms
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/Transforms/Utils
 
+  target_include_directories(LLVMMyDeadCodeElimination PRIVATE
+    ${LLVM_MAIN_INCLUDE_DIR}
+  )
+
   DEPENDS
   intrinsics_gen
 
diff --git a/llvm/lib/Transforms/Utils/MyDeadCodeEliminationPass.cpp b/llvm/lib/Transforms/Utils/MyDeadCodeEliminationPass.cpp
new file mode 100644
index 00000000000000..f906341acd3a5f
--- /dev/null
+++ b/llvm/lib/Transforms/Utils/MyDeadCodeEliminationPass.cpp
@@ -0,0 +1,59 @@
+#include "llvm/Transforms/Utils/MyDeadCodeEliminationPass.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/Instruction.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/Support/raw_ostream.h"
+#include <unordered_set>
+
+using namespace llvm;
+
+PreservedAnalyses MyDeadCodeEliminationPass::run(Function &F, FunctionAnalysisManager &AM) {
+  errs() << "I'm here in my Pass" << "\n";
+
+  // Call the helper function to iteratively analyze instructions
+  analyzeInstructionsIteratively(F);
+
+  return PreservedAnalyses::all(); // Preserve analyses since the code isn't modified.
+}
+
+void MyDeadCodeEliminationPass::analyzeInstructionsIteratively(Function &F) {
+  std::unordered_set<Instruction *> potentialDeadInstructions; // To track potential dead instructions
+  bool foundNewDead; // Flag to track if we find new dead instructions in an iteration
+
+  do {
+    foundNewDead = false; // Reset the flag at the start of each iteration
+
+    for (BasicBlock &BB : F) {
+      for (Instruction &I : BB) {
+        // Skip if already identified as dead
+        if (potentialDeadInstructions.count(&I)) {
+          continue;
+        }
+
+        // Check if the instruction is "dead" (not used anywhere or used only by dead instructions)
+        if (isInstructionDead(&I, potentialDeadInstructions)) {
+          errs() << "Potential dead instruction: " << I << "\n";
+          potentialDeadInstructions.insert(&I);
+          foundNewDead = true; // Mark that we found a new dead instruction
+        }
+      }
+    }
+  } while (foundNewDead); // Continue until no new dead instructions are found
+}
+
+bool MyDeadCodeEliminationPass::isInstructionDead(Instruction *Inst, const std::unordered_set<Instruction *> &potentialDeadInstructions) {
+  // Check if the instruction's result is not used, or all users are in the potentialDeadInstructions set
+  if (Inst->use_empty()) {
+    return true; // No users, definitely dead
+  }
+
+  for (const Use &U : Inst->uses()) {
+    auto *User = dyn_cast<Instruction>(U.getUser());
+    if (!User || potentialDeadInstructions.find(User) == potentialDeadInstructions.end()) {
+      return false; // Found a user that is not "dead"
+    }
+  }
+
+  return true; // All users are "dead"
+}
+

``````````

</details>


https://github.com/llvm/llvm-project/pull/120468