[LLVMdev] Branching to Entry block

[Chris Lattner]

On Thu, 17 Feb 2005, Tanu Sharma wrote:
> I created a new block and inserted it into the present basic block list 
> , but I get this error:
>
> opt: BasicBlock.cpp:83: virtual llvm::BasicBlock::~BasicBlock(): Assertion `getParent() == 0 && "BasicBlock still linked into the program!"' failed.
>
> The program completes its task and i get this in the end. I am unable to 
> resolve this.

The problem there is that you're trying to delete some basic block that is 
still embedded into a function.  There are several ways to accomplish 
this:

BasicBlock *BB = ...

// Approach number 1, the easy way:
BB->eraseFromParent();

// Approach number 2, the more explicit way:
BB->getParent()->getInstList().erase(BB);     // Remove from list & delete

// Approach number 3, remove from the list, then delete it explicitly:
BB->getParent()->getInstList().remove(BB);    // Just remove from list
delete BB;                                    // Delete block


I strongly suggest using approach #1, but I listed all of them so you can 
see the relation between remove and erase.

-Chris


> John Criswell <criswell at cs.uiuc.edu> wrote:
> Tanu Sharma wrote:
>> Hello,
>>
>> In an attempt to randomise the basic blocks in a function, is it
>> possible that I can randomise the entry block as well? And maybe insert
>> some instructions in the pass to call entry block while running the
>> program ?
>>
>> Is it feasible?
>>
>> What does entry block consist of ?
>
> The entry block, by definition, is the first basic block (BB) to be
> executed by a function. As I understand it, it has the additional
> restriction that it cannot be dominated by other basic blocks (i.e. no
> other BBs in the function can branch to it).
>
> If your entry BB performs useful computations and you want to move it,
> you could create a new entry BB that does nothing but branch to the old
> entry BB. The old entry BB (now just a regular BB) can then be moved.
>
> I'm assuming thus far that you're randomizing the basic block order at
> the LLVM level (i.e. take LLVM function, randomize order of BBs, and
> then codegen each BB in order). Another approach, as I see it, would be
> to change the code generator so that it codegens the BBs in a random
> order, instead of codegen'ing them in order.
>
> The first approach, I think, is a lot easier.
>
> Regards,
>
> -- John T.
>
>>
>> Thanks
>>
>> Tanu
>>
>>
>>
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-Chris

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