I am building a lifter that translates armv7m assembly instructions into llvm IR. A sample of my C++ code is:
IRBuilder<> builder(TheContext); //line 0
llvm::ConstantInt* I_0 = llvm::ConstantInt::get(TheContext, llvm::APInt(/nbits/32, 0, true)); //line 1
V = builder.CreateAlloca(Type::getInt32Ty(TheContext), nullptr, "V"); //line 2
Value* s3 = builder.CreateStore(I_0, V, /isVolatile=/false); //line 3
Value* loaded_V = builder.CreateLoad(V); //line 4
Value* result_V = builder.CreateICmpEQ(I_0, loaded_V); //line 5
result_V->print(errs()); //line 6
The result I get is:
%22 = icmp eq i32 0, %18
The result I want is:
So my question is, even though I stored an Immediate value of 0 at the memory location pointed to by the ptr V in line 3. When I load this value from memory using memory load from ptr V in line 4 and then compare it to an immediate value of 0 in the icmpEq in line 5, why dont I get a true result in line 6 but instead it seems that the loaded_V doesnt evaluate to an immediate value of 0, its just represented by the variable %18.
1- How can I get the value stored inside the llvm variable %18 ?.
2- Is there a way to get the data stored in the memory address pointed to by the ptr V directly without having to load.
As you can see in another sample of my code where I am trying to translate the assembly instruction
add DST, SRC1, SRC2 to an equivalent llvm IR:
llvm::Value* SRC1 = builder.CreateAlloca(Type::getInt32Ty(TheContext), nullptr); llvm::Value* SRC2 = builder.CreateAlloca(Type::getInt32Ty(TheContext), nullptr); llvm::Value* s = builder.CreateStore(register1_val, SRC1, /*isVolatile=*/false); llvm::Value* s2 = builder.CreateStore(register2_val, SRC2, /*isVolatile=*/false); llvm::Value* LHS = builder.CreateLoad(SRC1, register1); llvm::Value* RHS = builder.CreateLoad(SRC2, register2); llvm::Value* add_ll = builder.CreateAdd(LHS, RHS, output_register); llvm::Value* DST = builder.CreateAlloca(Type::getInt32Ty(TheContext), nullptr); llvm::Value* s3 = builder.CreateStore(add_ll, DST, /*isVolatile=*/false);
I represent an operand as a ptr to a memory address by using an alloca instruction and then loading from that memory address when using the operand in an llvm instruction creation.
3- Is there perhaps a better way of representing operands (virtual registers?!, instead of ptrs to memory).
Source: Windows Questions C++