[InstCombine] Calloc-ed strings optimizations

Summary:
Example cases:
strlen(calloc(...)) -> 0

Reviewers: efriedma, bkramer

Reviewed By: bkramer

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47059

llvm-svn: 332990

2 files changed, 32 insertions, 3 deletions

diff --git a/llvm/lib/Analysis/MemoryBuiltins.cpp b/llvm/lib/Analysis/MemoryBuiltins.cpp
index deacdb9e324..988eacbf3c9 100644
--- a/llvm/lib/Analysis/MemoryBuiltins.cpp
+++ b/llvm/lib/Analysis/MemoryBuiltins.cpp
@@ -589,7 +589,7 @@ SizeOffsetType ObjectSizeOffsetVisitor::visitCallSite(CallSite CS) {
 
   // Handle strdup-like functions separately.
   if (FnData->AllocTy == StrDupLike) {
-    APInt Size(IntTyBits, GetStringLength(CS.getArgument(0)));
+    APInt Size(IntTyBits, GetStringLength(CS.getArgument(0), TLI));
     if (!Size)
       return unknown();
 
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 4442df8cfd5..95e7a9bfd74 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -3372,10 +3372,39 @@ static uint64_t GetStringLengthH(const Value *V,
   return NullIndex + 1;
 }
 
+static bool isStringFromCalloc(const Value *Str, const TargetLibraryInfo *TLI) {
+  const CallInst *Calloc = dyn_cast<CallInst>(Str);
+  if (!Calloc)
+    return false;
+
+  const Function *InnerCallee = Calloc->getCalledFunction();
+  if (!InnerCallee)
+    return false;
+
+  LibFunc Func;
+  if (!TLI->getLibFunc(*InnerCallee, Func) || !TLI->has(Func) ||
+      Func != LibFunc_calloc)
+    return false;
+
+  const ConstantInt *N = dyn_cast<ConstantInt>(Calloc->getOperand(0));
+  const ConstantInt *Size = dyn_cast<ConstantInt>(Calloc->getOperand(1));
+
+  if (!N || !Size)
+    return false;
+
+  if (N->isNullValue() || Size->isNullValue())
+    return false;
+
+  return true;
+}
+
 /// If we can compute the length of the string pointed to by
 /// the specified pointer, return 'len+1'.  If we can't, return 0.
-uint64_t llvm::GetStringLength(const Value *V, unsigned CharSize) {
-  if (!V->getType()->isPointerTy()) return 0;
+uint64_t llvm::GetStringLength(const Value *V, const TargetLibraryInfo *TLI, unsigned CharSize) {
+  if (!V->getType()->isPointerTy())
+    return 0;
+  if (isStringFromCalloc(V, TLI))
+    return 1;
 
   SmallPtrSet<const PHINode*, 32> PHIs;
   uint64_t Len = GetStringLengthH(V, PHIs, CharSize);