[LLVMdev] Getting basic block address offset from its parent function

[John Criswell]

On 2/27/15 6:30 PM, Ziqiang Patrick Huang wrote:
> Hi, all
>
> Is there a way of getting the basic block offset from its parent 
> function ?

At the LLVM IR level, no.  At the code generator layer 
(MachineFunctionPass layer or the MC layer), probably yes.

>
> What I'm trying to do is to get an execution count of each basic 
> blocks, so I need to know the starting address of each basic blocks. 
> Obviously we can't get the absolute address before linking the 
> program, but the offset relative to parent function should be 
> available so I can take it and get the function start address from 
> objdump then figure out each basic block's absolute address.
>
> Or is there another way of doing this ...

On way to do it would be to instrument the program so that each basic 
block increments a counter every time it is executed.  This would be 
trivial to do.  To optimize it, you could analyze the CFG so that 
control equivalent basic blocks use a single counter (e.g., the 
single-entry block and the single-exit block are executed the same 
number of times, so they only need 1 counter).

Another option might be to use the pcmarker intrinsic.  Apparently it's 
used for matching up LLVM IR to machine instructions for use in 
processor simulators, though I have never used it myself.

The Giri project has support for recording the execution of every basic 
block, but it might be more heavy-weight than you need.  I also don't 
recall off-hand from where to download it; search the llvmdev archives 
for emails from Swarup Sahoo to get that information.

Hope this helps,

John Criswell

-- 
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell

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