[GVN] Improve processBlock for instruction erasure

This patch deletes the instructions right away by using
the approapriate iterators. Thus avoids collecting the instructions
in a vector and then doing the erasure.

Author: madhur13490

llvm/include/llvm/Transforms/Scalar/GVN.h

@@ -25,6 +25,8 @@
 #include "llvm/IR/ValueHandle.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/Compiler.h"
+#include "llvm/Transforms/Utils/AssumeBundleBuilder.h"
+#include "llvm/Transforms/Utils/Local.h"
 #include <cstdint>
 #include <optional>
 #include <utility>
@@ -137,9 +139,11 @@ class GVNPass : public PassInfoMixin<GVNPass> {
 
   /// This removes the specified instruction from
   /// our various maps and marks it for deletion.
-  void markInstructionForDeletion(Instruction *I) {
+  void doInstructionDeletion(Instruction *I) {
+    salvageKnowledge(I, AC);
+    salvageDebugInfo(*I);
     VN.erase(I);
-    InstrsToErase.push_back(I);
+    removeInstruction(I);
   }
 
   DominatorTree &getDominatorTree() const { return *DT; }
@@ -306,7 +310,6 @@ class GVNPass : public PassInfoMixin<GVNPass> {
   // propagate to any successors. Entries added mid-block are applied
   // to the remaining instructions in the block.
   SmallMapVector<Value *, Value *, 4> ReplaceOperandsWithMap;
-  SmallVector<Instruction *, 8> InstrsToErase;
 
   // Map the block to reversed postorder traversal number. It is used to
   // find back edge easily.

llvm/lib/Transforms/Scalar/GVN.cpp

@@ -730,8 +730,9 @@ void GVNPass::ValueTable::erase(Value *V) {
 /// verifyRemoved - Verify that the value is removed from all internal data
 /// structures.
 void GVNPass::ValueTable::verifyRemoved(const Value *V) const {
-  assert(!valueNumbering.contains(V) &&
-         "Inst still occurs in value numbering map!");
+  if (V != nullptr)
+    assert(!valueNumbering.contains(V) &&
+           "Inst still occurs in value numbering map!");
 }
 
 //===----------------------------------------------------------------------===//
@@ -1574,7 +1575,7 @@ void GVNPass::eliminatePartiallyRedundantLoad(
     I->setDebugLoc(Load->getDebugLoc());
   if (V->getType()->isPtrOrPtrVectorTy())
     MD->invalidateCachedPointerInfo(V);
-  markInstructionForDeletion(Load);
+  doInstructionDeletion(Load);
   ORE->emit([&]() {
     return OptimizationRemark(DEBUG_TYPE, "LoadPRE", Load)
            << "load eliminated by PRE";
@@ -1798,7 +1799,7 @@ bool GVNPass::PerformLoadPRE(LoadInst *Load, AvailValInBlkVect &ValuesPerBlock,
       // Erase instructions generated by the failed PHI translation before
       // trying to number them. PHI translation might insert instructions
       // in basic blocks other than the current one, and we delete them
-      // directly, as markInstructionForDeletion only allows removing from the
+      // directly, as doInstructionDeletion only allows removing from the
       // current basic block.
       NewInsts.pop_back_val()->eraseFromParent();
     }
@@ -1997,7 +1998,7 @@ bool GVNPass::processNonLocalLoad(LoadInst *Load) {
         I->setDebugLoc(Load->getDebugLoc());
     if (V->getType()->isPtrOrPtrVectorTy())
       MD->invalidateCachedPointerInfo(V);
-    markInstructionForDeletion(Load);
+    doInstructionDeletion(Load);
     ++NumGVNLoad;
     reportLoadElim(Load, V, ORE);
     return true;
@@ -2067,7 +2068,7 @@ bool GVNPass::processAssumeIntrinsic(AssumeInst *IntrinsicI) {
       }
     }
     if (isAssumeWithEmptyBundle(*IntrinsicI)) {
-      markInstructionForDeletion(IntrinsicI);
+      doInstructionDeletion(IntrinsicI);
       return true;
     }
     return false;
@@ -2179,7 +2180,7 @@ bool GVNPass::processLoad(LoadInst *L) {
     return false;
 
   if (L->use_empty()) {
-    markInstructionForDeletion(L);
+    doInstructionDeletion(L);
     return true;
   }
 
@@ -2209,13 +2210,13 @@ bool GVNPass::processLoad(LoadInst *L) {
   // MaterializeAdjustedValue is responsible for combining metadata.
   ICF->removeUsersOf(L);
   L->replaceAllUsesWith(AvailableValue);
-  markInstructionForDeletion(L);
   if (MSSAU)
     MSSAU->removeMemoryAccess(L);
   ++NumGVNLoad;
   reportLoadElim(L, AvailableValue, ORE);
   // Tell MDA to reexamine the reused pointer since we might have more
   // information after forwarding it.
+  doInstructionDeletion(L);
   if (MD && AvailableValue->getType()->isPtrOrPtrVectorTy())
     MD->invalidateCachedPointerInfo(AvailableValue);
   return true;
@@ -2605,7 +2606,7 @@ bool GVNPass::processInstruction(Instruction *I) {
       Changed = true;
     }
     if (isInstructionTriviallyDead(I, TLI)) {
-      markInstructionForDeletion(I);
+      doInstructionDeletion(I);
       Changed = true;
     }
     if (Changed) {
@@ -2723,7 +2724,7 @@ bool GVNPass::processInstruction(Instruction *I) {
   patchAndReplaceAllUsesWith(I, Repl);
   if (MD && Repl->getType()->isPtrOrPtrVectorTy())
     MD->invalidateCachedPointerInfo(Repl);
-  markInstructionForDeletion(I);
+  doInstructionDeletion(I);
   return true;
 }
 
@@ -2799,10 +2800,6 @@ bool GVNPass::runImpl(Function &F, AssumptionCache &RunAC, DominatorTree &RunDT,
 }
 
 bool GVNPass::processBlock(BasicBlock *BB) {
-  // FIXME: Kill off InstrsToErase by doing erasing eagerly in a helper function
-  // (and incrementing BI before processing an instruction).
-  assert(InstrsToErase.empty() &&
-         "We expect InstrsToErase to be empty across iterations");
   if (DeadBlocks.count(BB))
     return false;
 
@@ -2820,41 +2817,12 @@ bool GVNPass::processBlock(BasicBlock *BB) {
     VN.erase(PN);
     removeInstruction(PN);
   }
-
-  for (BasicBlock::iterator BI = BB->begin(), BE = BB->end();
-       BI != BE;) {
+  for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
+    Instruction *Inst = &*BI++;
     if (!ReplaceOperandsWithMap.empty())
-      ChangedFunction |= replaceOperandsForInBlockEquality(&*BI);
-    ChangedFunction |= processInstruction(&*BI);
-
-    if (InstrsToErase.empty()) {
-      ++BI;
-      continue;
-    }
-
-    // If we need some instructions deleted, do it now.
-    NumGVNInstr += InstrsToErase.size();
-
-    // Avoid iterator invalidation.
-    bool AtStart = BI == BB->begin();
-    if (!AtStart)
-      --BI;
-
-    for (auto *I : InstrsToErase) {
-      assert(I->getParent() == BB && "Removing instruction from wrong block?");
-      LLVM_DEBUG(dbgs() << "GVN removed: " << *I << '\n');
-      salvageKnowledge(I, AC);
-      salvageDebugInfo(*I);
-      removeInstruction(I);
-    }
-    InstrsToErase.clear();
-
-    if (AtStart)
-      BI = BB->begin();
-    else
-      ++BI;
+      ChangedFunction |= replaceOperandsForInBlockEquality(Inst);
+    ChangedFunction |= processInstruction(Inst);
   }
-
   return ChangedFunction;
 }