[llvm-dev] Question concerning llvm::BlockAddress class

[Pei Wang via llvm-dev]

I personally have used blockaddress as virtual addresses of basic blocks without problems, as long as you store them as globals and retrieve the values from memory. Below is an example.

int foo(int n) {
    const void *targets[] = { &&BB1, &&BB2, &&BB3 };
    goto *targets[n % 3];
    BB1:
        return 1;
    BB2:
        return 2;
    BB3:
        return 3;
}

In the compiled bit code you will find that the "targets" array is defined as follows.

@foo.targets = private unnamed_addr constant [3 x i8*] [i8* blockaddress(@foo, %11), i8* blockaddress(@foo, %12), i8* blockaddress(@foo, %13)], align 16

You can read the addresses through the global variable @foo.targets.

Pei

On 4/16/18, 4:36 PM, "serge guelton via llvm-dev" <llvm-dev at lists.llvm.org> wrote:

    On Mon, Apr 16, 2018 at 04:14:03PM -0400, Brenda So via llvm-dev wrote:
    > Hi all,
    > 
    > I have a question concerning block address class in LLVM. I am currently
    > working on a project where I need to obtain and manipulate basic block virtual
    > addresses. I was searching the web and found the llvm::BlockAddress class (
    > http://llvm.org/doxygen/classllvm_1_1BlockAddress.html). With this class I was
    > able to obtain a printout like this:
    > 
    > i8* blockaddress(@func_name, %bb_label)
    > 
    > How do I obtain the virtual memory address from the Block address class? Can I
    > even do so? 
    > 
    > Moreover, I thought that one can only obtain the virtual memory address after
    > linking the object files together. So how would the BlockAddress class help me
    > when it's working on the IR level? If BlockAddress class is not the way to go,
    > is there another api function that I can use to obtain the addresses?
    
    Hi Brenda,
    
    you can bitcast a BlockAdress to an intptr and then manipulate it as you
    want. Unfortunately, there' is no guarentee that you can do anything
    appart jumping to this adress from the same function, as described in
    the langref http://llvm.org/docs/LangRef.html#addresses-of-basic-blocks
    
        This value only has defined behavior when used as an operand to the
        ‘indirectbr‘ instruction, or for comparisons against null. Pointer
        equality tests between labels addresses results in undefined
        behavior — though, again, comparison against null is ok, and no
        label is equal to the null pointer. This may be passed around as an
        opaque pointer sized value as long as the bits are not inspected.
    
    A glimpse of hope though:
    
        Finally, some targets may provide defined semantics when using the
        value as the operand to an inline assembly, but that is target
        specific.
    
    Hope it helps,
    
    Serge