llvm pass replaceAllUsesWith type not match？ - c++

Question

use a encryptedString replace a GlobalVariable but type not match.
the GlobalVariable a const char * string.
code like that:
GlobalVariable* GV = *it;
//get clear text string
std::string clearstr = getGlobalStringValue(GV);
GlobalVariable::LinkageTypes lt = GV->getLinkage();
//encrypt current string
std::string encryptedString = stringEncryption(clearstr);
//create new global string with the encrypted string
std::ostringstream oss;
oss << ".encstr" << encryptedStringCounter << "_" << sys::Process::GetRandomNumber();
Constant *cryptedStr = ConstantDataArray::getString(M.getContext(), encryptedString, true);
GlobalVariable* gCryptedStr = new GlobalVariable(M, cryptedStr->getType(), true, GV->getLinkage(), cryptedStr, oss.str());
StringMapGlobalVars[oss.str()] = gCryptedStr;
//replace use of clear string with encrypted string
GV->replaceAllUsesWith(gCryptedStr);
but failed with:
Assertion failed: (New->getType() == getType() && "replaceAllUses of
value with new value of different type!"),

Answer 1

At first: I recommend replacing everything with the right type in LLVM IR that's why this assertion is there.
However:
You get this assertion because your strings does not match in length. A global string is represented as an array of characters (i.e. i8 values). So the type of your string is [len x i8] where len is the length of your string.
#.str = private unnamed_addr constant [12 x i8] c"hello world\00", align 1
What you can do is write your own replacement function like this:
template<typename T>
void ReplaceUnsafe(T *from, T *to) {
while (!from->use_empty()) {
auto &U = *from->use_begin();
U.set(to);
}
from->eraseFromParent();
}
However, this is (as the function name indicates) unsafe and here is why:
Consider the following C/C++ code:
int main() {
return "hello world"[9];
}
which will just return the int representation of l.
Compiled to IR it looks like this:
#.str = private unnamed_addr constant [12 x i8] c"hello world\00", align 1
; Function Attrs: nounwind
define i32 #main() #0 {
entry:
%retval = alloca i32, align 4
store i32 0, i32* %retval
%0 = load i8* getelementptr inbounds ([12 x i8]* #.str, i32 0, i64 9), align 1
%conv = sext i8 %0 to i32
ret i32 %conv
}
if the string is now replaced with somiting of unequal type (e.g., something of type [7 x i8]), then you may end up with a problem because your GEP instruction has the 9 as contant index. This will result in an out of bounds access. I don't know if the llvm verify pass catches this when it looks at GEP instructions (if you run it).

Answer 2

Constant *cryptedStr = ConstantDataArray::getString(M.getContext(), encryptedString, true);
change to
Constant *cryptedStr = ConstantDataArray::getString(M.getContext(), encryptedString, false);