diff --git a/llvm/include/llvm/Analysis/TargetLibraryInfo.h b/llvm/include/llvm/Analysis/TargetLibraryInfo.h
--- a/llvm/include/llvm/Analysis/TargetLibraryInfo.h
+++ b/llvm/include/llvm/Analysis/TargetLibraryInfo.h
@@ -192,6 +192,9 @@
   /// This queries the 'wchar_size' metadata.
   unsigned getWCharSize(const Module &M) const;
 
+  /// Returns the size of the size_t type in bits or 0 if the size is unknown.
+  unsigned getSizeTSize(const Module &M, unsigned AddressSpace) const;
+
   /// Get size of a C-level int or unsigned int, in bits.
   unsigned getIntSize() const {
     return SizeOfInt;
@@ -403,6 +406,11 @@
     return Impl->getWCharSize(M);
   }
 
+  /// \copydoc TargetLibraryInfoImpl::getSizeTSize()
+  unsigned getSizeTSize(const Module &M, unsigned AddressSpace) const {
+    return Impl->getSizeTSize(M, AddressSpace);
+  }
+
   /// \copydoc TargetLibraryInfoImpl::getIntSize()
   unsigned getIntSize() const {
     return Impl->getIntSize();
diff --git a/llvm/lib/Analysis/TargetLibraryInfo.cpp b/llvm/lib/Analysis/TargetLibraryInfo.cpp
--- a/llvm/lib/Analysis/TargetLibraryInfo.cpp
+++ b/llvm/lib/Analysis/TargetLibraryInfo.cpp
@@ -785,11 +785,7 @@
 bool TargetLibraryInfoImpl::isValidProtoForLibFunc(const FunctionType &FTy,
                                                    LibFunc F,
                                                    const Module &M) const {
-  // FIXME: There is really no guarantee that sizeof(size_t) is equal to
-  // sizeof(int*) for every target. So the assumption used here to derive the
-  // SizeTBits based on the size of an integer pointer in address space zero
-  // isn't always valid.
-  unsigned SizeTBits = M.getDataLayout().getPointerSizeInBits(/*AddrSpace=*/0);
+  unsigned SizeTBits = getSizeTSize(M, /*AddressSpace=*/0);
   unsigned NumParams = FTy.getNumParams();
 
   switch (F) {
@@ -1796,6 +1792,19 @@
   return 0;
 }
 
+unsigned TargetLibraryInfoImpl::getSizeTSize(const Module &M,
+                                             unsigned AddressSpace) const {
+  // There is really no guarantee that sizeof(size_t) is equal to sizeof(int*)
+  // for every target. If that isn't true then it should be possible to derive
+  // the SizeTTy from the target tripe here instead and do an early return.
+
+  // Here we assume that sizeof(size_t) is equal to sizeof(int*) in address
+  // space zero. This should work for most targets.
+  // TODO: Should this use getIndexSizeInBits instead?
+  const DataLayout &DL = M.getDataLayout();
+  return DL.getPointerSizeInBits(AddressSpace);
+}
+
 TargetLibraryInfoWrapperPass::TargetLibraryInfoWrapperPass()
     : ImmutablePass(ID), TLA(TargetLibraryInfoImpl()) {
   initializeTargetLibraryInfoWrapperPassPass(*PassRegistry::getPassRegistry());
diff --git a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
--- a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -3292,11 +3292,8 @@
   else
     return nullptr;
 
-  // FIXME: There is really no guarantee that sizeof(size_t) is equal to
-  // sizeof(int*) for every target. So the assumption used here to derive the
-  // SizeTBits based on the size of an integer pointer in address space zero
-  // isn't always valid.
-  Type *SizeTTy = DL.getIntPtrType(CI->getContext(), /*AddressSpace=*/0);
+  unsigned SizeTBits = TLI->getSizeTSize(*CI->getModule(), /*AddressSpace=*/0);
+  Type *SizeTTy = IntegerType::get(CI->getContext(), SizeTBits);
   Value *LenV = ConstantInt::get(SizeTTy, Len);
   Value *Ret = emitMemCpyChk(Dst, Src, LenV, ObjSize, B, DL, TLI);
   // If the function was an __stpcpy_chk, and we were able to fold it into