llvm dev - Apr 2018 - Question concerning llvm::BlockAddress class

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?

Thanks!
Brenda
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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

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