llvm dev - Mar 2014 - [LLVMdev] How to extract the starting address of each basic block with llvm?

Dear all,

I use the clang frontend to produce the control flow graph of a C program
through
this command:
clang -Xclang -analyze -Xclang -analyzer-checker=debug.DumpCFG test.c

The produced information contains all the basic blocks identified by labels,
e.g. B1, B2, etc. along with their predecessors and successors.

Is there a way to get the starting address of each of these basic block?
I would like to map the basic block labels produced by CFG  with the actual
corresponding basic block starting addresses.

Could I use llvm to do this? From what I understand this issued is
unrelevant with
IR. Maybe I could get something from the machine code (MC). Perhaps
I have to write a pass for llvm backend. Maybe I could get this information
from
BranchFolder and IfConverter machine function passes:
http://llvm.org/docs/WritingAnLLVMBackend.html#branch-folding-and-if-conversion
but I am not sure.

I am new to llvm so I need some help,

Thank you,

-- 
Thanasis Petsas
Distributed Computing Systems (DCS)
Institute of Computer Science (ICS/FORTH)
Heraklion, Crete
Greece (GR)

http://www.thanasispetsas.com/
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On Fri, Mar 28, 2014 at 1:15 PM, Petsas Athanasios <petsas at
ics.forth.gr> wrote:
> Dear all,
>
> I use the clang frontend to produce the control flow graph of a C program
> through
> this command:
> clang -Xclang -analyze -Xclang -analyzer-checker=debug.DumpCFG test.c
>
> The produced information contains all the basic blocks identified by
labels,
> e.g. B1, B2, etc. along with their predecessors and successors.
>
> Is there a way to get the starting address of each of these basic block?
> I would like to map the basic block labels produced by CFG  with the actual
> corresponding basic block starting addresses.
>
> Could I use llvm to do this? From what I understand this issued is
> unrelevant with
> IR. Maybe I could get something from the machine code (MC). Perhaps
> I have to write a pass for llvm backend. Maybe I could get this information
> from
> BranchFolder and IfConverter machine function passes:
>
http://llvm.org/docs/WritingAnLLVMBackend.html#branch-folding-and-if-conversion
> but I am not sure.
>
> I am new to llvm so I need some help,
>
> Thank you,
>
Check out blockaddress(@function, %block) as documented in:
http://llvm.org/docs/LangRef.html#addresses-of-basic-blocks

The documentation says it only has defined behavior when used with
indirectbr, but I have been able to store the block address in a
global variable that survives lowering and is the correct address.

      GlobalVariable *GV = new GlobalVariable(M, bbPtr, /*isConstant=*/true,
                                   GlobalValue::ExternalLinkage,
                                   BlockAddress::get(bb), name);

Beware, the IR BB may be optimized away when lowering to assembly. You
can detect this case when blockaddress() returns 1.

Cheers,
Jevin

Thank you very much Jevin for the suggestion!

But how I can use blockaddress function? Do I have to write a pass in order
to use it?
As I told you I am new in llvm. Could you provide me a guide or an example?

Thank you very much,
Thanasis


On Fri, Mar 28, 2014 at 8:27 PM, Jevin Sweval <jevinsweval at gmail.com>
wrote:
> On Fri, Mar 28, 2014 at 1:15 PM, Petsas Athanasios <petsas at
ics.forth.gr>
> wrote:
> > Dear all,
> >
> > I use the clang frontend to produce the control flow graph of a C
program
> > through
> > this command:
> > clang -Xclang -analyze -Xclang -analyzer-checker=debug.DumpCFG test.c
> >
> > The produced information contains all the basic blocks identified by
> labels,
> > e.g. B1, B2, etc. along with their predecessors and successors.
> >
> > Is there a way to get the starting address of each of these basic
block?
> > I would like to map the basic block labels produced by CFG  with the
> actual
> > corresponding basic block starting addresses.
> >
> > Could I use llvm to do this? From what I understand this issued is
> > unrelevant with
> > IR. Maybe I could get something from the machine code (MC). Perhaps
> > I have to write a pass for llvm backend. Maybe I could get this
> information
> > from
> > BranchFolder and IfConverter machine function passes:
> >
>
http://llvm.org/docs/WritingAnLLVMBackend.html#branch-folding-and-if-conversion
> > but I am not sure.
> >
> > I am new to llvm so I need some help,
> >
> > Thank you,
> >
>
> Check out blockaddress(@function, %block) as documented in:
> http://llvm.org/docs/LangRef.html#addresses-of-basic-blocks
>
> The documentation says it only has defined behavior when used with
> indirectbr, but I have been able to store the block address in a
> global variable that survives lowering and is the correct address.
>
>       GlobalVariable *GV = new GlobalVariable(M, bbPtr,
> /*isConstant=*/true,
>                                    GlobalValue::ExternalLinkage,
>                                    BlockAddress::get(bb), name);
>
> Beware, the IR BB may be optimized away when lowering to assembly. You
> can detect this case when blockaddress() returns 1.
>
> Cheers,
> Jevin
>


-- 
Thanasis Petsas
Distributed Computing Systems (DCS)
Institute of Computer Science (ICS/FORTH)
Heraklion, Crete
Greece (GR)

http://www.thanasispetsas.com/
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