Allow GVN to eliminate read-only function calls when it can detect that they are redundant.

llvm-svn: 44323

2 files changed, 23 insertions, 5 deletions

diff --git a/llvm/lib/Analysis/MemoryDependenceAnalysis.cpp b/llvm/lib/Analysis/MemoryDependenceAnalysis.cpp
index 5375d52c33c..68366f6d912 100644
--- a/llvm/lib/Analysis/MemoryDependenceAnalysis.cpp
+++ b/llvm/lib/Analysis/MemoryDependenceAnalysis.cpp
@@ -79,9 +79,6 @@ Instruction* MemoryDependenceAnalysis::getCallSiteDependency(CallSite C,
     if (StoreInst* S = dyn_cast<StoreInst>(QI)) {
       pointer = S->getPointerOperand();
       pointerSize = TD.getTypeStoreSize(S->getOperand(0)->getType());
-    } else if (LoadInst* L = dyn_cast<LoadInst>(QI)) {
-      pointer = L->getPointerOperand();
-      pointerSize = TD.getTypeStoreSize(L->getType());
     } else if (AllocationInst* AI = dyn_cast<AllocationInst>(QI)) {
       pointer = AI;
       if (ConstantInt* C = dyn_cast<ConstantInt>(AI->getArraySize()))
@@ -98,7 +95,11 @@ Instruction* MemoryDependenceAnalysis::getCallSiteDependency(CallSite C,
       // FreeInsts erase the entire structure
       pointerSize = ~0UL;
     } else if (CallSite::get(QI).getInstruction() != 0) {
-      if (AA.getModRefInfo(C, CallSite::get(QI)) != AliasAnalysis::NoModRef) {
+      AliasAnalysis::ModRefBehavior result =
+                   AA.getModRefBehavior(cast<CallInst>(QI)->getCalledFunction(),
+                                        CallSite::get(QI));
+      if (result != AliasAnalysis::DoesNotAccessMemory &&
+          result != AliasAnalysis::OnlyReadsMemory) {
         if (!start && !block) {
           depGraphLocal.insert(std::make_pair(C.getInstruction(),
                                               std::make_pair(QI, true)));
diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index 7cbd617a2eb..11fa336c7bb 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -476,7 +476,8 @@ uint32_t ValueTable::lookup_or_add(Value* V) {
   
   if (CallInst* C = dyn_cast<CallInst>(V)) {
     if (C->getCalledFunction() &&
-        AA->doesNotAccessMemory(C->getCalledFunction())) {
+        (AA->doesNotAccessMemory(C->getCalledFunction()) ||
+         AA->onlyReadsMemory(C->getCalledFunction()))) {
       Expression e = create_expression(C);
     
       DenseMap<Expression, uint32_t>::iterator EI = expressionNumbering.find(e);
@@ -1038,6 +1039,22 @@ bool GVN::processInstruction(Instruction* I,
   } else if (currAvail.test(num)) {
     Value* repl = find_leader(currAvail, num);
     
+    if (CallInst* CI = dyn_cast<CallInst>(I)) {
+      AliasAnalysis& AA = getAnalysis<AliasAnalysis>();
+      if (CI->getCalledFunction() &&
+          !AA.doesNotAccessMemory(CI->getCalledFunction())) {
+        MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
+        if (MD.getDependency(CI) != MD.getDependency(cast<CallInst>(repl))) {
+          // There must be an intervening may-alias store, so nothing from
+          // this point on will be able to be replaced with the preceding call
+          currAvail.erase(repl);
+          currAvail.insert(I);
+          
+          return false;
+        }
+      }
+    }
+    
     VN.erase(I);
     I->replaceAllUsesWith(repl);
     toErase.push_back(I);