how to check whether from basicblock A we can go to basicblock B within the same function?

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1

Dear all,
Is there a way to check whether we can go from A to B within a function when I’m writing a pass?
Thanks.

Best,
–Wenbin

2

Wenbin Zhang wrote:

Dear all,
Is there a way to check whether we can go from A to B within a
function when I'm writing a pass?

I assume you are asking whether control flow can pass from BasicBlock A
to BasicBlock B. In that case, yes: all you need to do is look at the
terminator instruction of BasicBlock A and see if it can branch to
BasicBlock B.

There's a new indirect branch instruction; I don't believe it is used
very often. You might have to handle those specially, but I'll let
others comment on that.

-- John T.

3

basicblock B within the same function?

Wenbin Zhang wrote:

Dear all,
Is there a way to check whether we can go from A to B within a
function when I'm writing a pass?

I assume you are asking whether control flow can pass from BasicBlock A
to BasicBlock B. In that case, yes: all you need to do is look at the
terminator instruction of BasicBlock A and see if it can branch to
BasicBlock B.

Yes, also switch and invoke instruction have the similar effects.
So does it mean that I need to recursively traverse all the possible destinations from A (following the terminator instructions) to see whether I can reach B?

Thanks.

There's a new indirect branch instruction; I don't believe it is used
very often. You might have to handle those specially, but I'll let
others comment on that.

-- John T.

Thanks.
Best,
--Wenbin

Best,
--Wenbin

4

Wenbin Zhang wrote:

From: "John Criswell" <criswell@illinois.edu>
To: "Wenbin Zhang" <zhangwen@cse.ohio-state.edu>
Cc: <llvmdev@cs.uiuc.edu>
Sent: Friday, August 27, 2010 5:09 PM
Subject: Re: [LLVMdev] how to check whether from basicblock A we can go to
basicblock B within the same function?

Wenbin Zhang wrote:
    

Dear all,
Is there a way to check whether we can go from A to B within a
function when I'm writing a pass?
      

I assume you are asking whether control flow can pass from BasicBlock A
to BasicBlock B. In that case, yes: all you need to do is look at the
terminator instruction of BasicBlock A and see if it can branch to
BasicBlock B.
    
Yes, also switch and invoke instruction have the similar effects.
  
Every basic block ends with a terminator instruction that indicates
which basic block or basic blocks it should branch to next. Switch,
invoke, and branch are all terminator instructions. There may be more
that I don't remember.

So does it mean that I need to recursively traverse all the possible
destinations from A (following the terminator instructions) to see whether I
can reach B?
  
Are you wanting to know if B is a successor of A in the control flow
graph, or are you wanting to know if there is a path from A to B in the
control flow graph? The first question only requires that you look at
the terminator instruction of basic block A. The second question
requires that you traverse the control-flow graph, starting at A, and
see if there is a path from A to B.

-- John T.

5

Every basic block ends with a terminator instruction that indicates
which basic block or basic blocks it should branch to next. Switch,
invoke, and branch are all terminator instructions. There may be more
that I don't remember.

Why not just use the predecessor and successor iterator?
http://llvm.org/docs/ProgrammersManual.html#iterate_preds

6

Are those iterators enought for this purpose?

7

I believe so.