Add support for simplifying a load from a computed value to a load from a global when it

is provable that they're equivalent.  This fixes PR4855.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112994 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/LazyValueInfo.cpp

@@ -452,14 +452,15 @@ LVILatticeVal LVIQuery::getBlockValue(BasicBlock *BB) {
     
     // If this is a pointer, and there's a load from that pointer in this BB,
     // then we know that the pointer can't be NULL.
+    bool NotNull = false;
     if (Val->getType()->isPointerTy()) {
-      const PointerType *PTy = cast<PointerType>(Val->getType());
       for (BasicBlock::iterator BI = BB->begin(), BE = BB->end();BI != BE;++BI){
         LoadInst *L = dyn_cast<LoadInst>(BI);
         if (L && L->getPointerAddressSpace() == 0 &&
             L->getPointerOperand()->getUnderlyingObject() ==
               Val->getUnderlyingObject()) {
-          return LVILatticeVal::getNot(ConstantPointerNull::get(PTy));
+          NotNull = true;
+          break;
         }
       }
     }
@@ -475,11 +476,19 @@ LVILatticeVal LVIQuery::getBlockValue(BasicBlock *BB) {
       if (Result.isOverdefined()) {
         DEBUG(dbgs() << " compute BB '" << BB->getName()
                      << "' - overdefined because of pred.\n");
+        // If we previously determined that this is a pointer that can't be null
+        // then return that rather than giving up entirely.
+        if (NotNull) {
+          const PointerType *PTy = cast<PointerType>(Val->getType());
+          Result = LVILatticeVal::getNot(ConstantPointerNull::get(PTy));
+        }
+        
         return Result;
       }
       ++NumPreds;
     }
     
+    
     // If this is the entry block, we must be asking about an argument.  The
     // value is overdefined.
     if (NumPreds == 0 && BB == &BB->getParent()->front()) {

lib/Transforms/Scalar/CorrelatedValuePropagation.cpp

@@ -21,8 +21,9 @@
 #include "llvm/ADT/Statistic.h"
 using namespace llvm;
 
-STATISTIC(NumPhis,    "Number of phis propagated");
-STATISTIC(NumSelects, "Number of selects propagated");
+STATISTIC(NumPhis,      "Number of phis propagated");
+STATISTIC(NumSelects,   "Number of selects propagated");
+STATISTIC(NumMemAccess, "Number of memory access targets propagated");
 
 namespace {
   class CorrelatedValuePropagation : public FunctionPass {
@@ -30,6 +31,7 @@ namespace {
     
     bool processSelect(SelectInst *SI);
     bool processPHI(PHINode *P);
+    bool processMemAccess(Instruction *I);
     
   public:
     static char ID;
@@ -54,6 +56,7 @@ Pass *llvm::createCorrelatedValuePropagationPass() {
 
 bool CorrelatedValuePropagation::processSelect(SelectInst *S) {
   if (S->getType()->isVectorTy()) return false;
+  if (isa<Constant>(S->getOperand(0))) return false;
   
   Constant *C = LVI->getConstant(S->getOperand(0), S->getParent());
   if (!C) return false;
@@ -97,6 +100,23 @@ bool CorrelatedValuePropagation::processPHI(PHINode *P) {
   return Changed;
 }
 
+bool CorrelatedValuePropagation::processMemAccess(Instruction *I) {
+  Value *Pointer = 0;
+  if (LoadInst *L = dyn_cast<LoadInst>(I))
+    Pointer = L->getPointerOperand();
+  else
+    Pointer = cast<StoreInst>(I)->getPointerOperand();
+  
+  if (isa<Constant>(Pointer)) return false;
+  
+  Constant *C = LVI->getConstant(Pointer, I->getParent());
+  if (!C) return false;
+  
+  ++NumMemAccess;
+  I->replaceUsesOfWith(Pointer, C);
+  return true;
+}
+
 bool CorrelatedValuePropagation::runOnFunction(Function &F) {
   LVI = &getAnalysis<LazyValueInfo>();
   
@@ -106,10 +126,18 @@ bool CorrelatedValuePropagation::runOnFunction(Function &F) {
     bool BBChanged = false;
     for (BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE; ) {
       Instruction *II = BI++;
-      if (SelectInst *SI = dyn_cast<SelectInst>(II))
-        BBChanged |= processSelect(SI);
-      else if (PHINode *P = dyn_cast<PHINode>(II))
-        BBChanged |= processPHI(P);
+      switch (II->getOpcode()) {
+      case Instruction::Select:
+        BBChanged |= processSelect(cast<SelectInst>(II));
+        break;
+      case Instruction::PHI:
+        BBChanged |= processPHI(cast<PHINode>(II));
+        break;
+      case Instruction::Load:
+      case Instruction::Store:
+        BBChanged |= processMemAccess(II);
+        break;
+      }
     }
     
     // Propagating correlated values might leave cruft around.

test/Transforms/CorrelatedValuePropagation/basic.ll

@@ -39,3 +39,21 @@ bb3:            ; preds = %bb1
 ; CHECK: ret i1 %res
         ret i1 %res
 }
+
+; PR4855
+@gv = internal constant i8 7
+; CHECK: @test3
+define i8 @test3(i8* %a) nounwind {
+entry:
+        %cond = icmp eq i8* %a, @gv
+        br i1 %cond, label %bb2, label %bb
+
+bb:             ; preds = %entry
+        ret i8 0
+
+bb2:            ; preds = %entry
+; CHECK-NOT: load i8* %a
+        %should_be_const = load i8* %a
+; CHECK: ret i8 7
+        ret i8 %should_be_const
+}