llvm dev - Sep 2013 - [LLVMdev] Is it safe to insert instructions in batches into BBs?

Hi,

I'm getting a very strange behaviour while adding already created 
instructions in batches into basicblocks instead of creating and 
inserting them immediately.

Because I need to insert instructions in a certain specific order inside 
multiple different BBs I found it easy to use the IRBuilder to create 
instructions without inserting them into a BB, storing them somewhere 
(vector, map ... etc) and later on inserting all of them in one go into 
their positions in the BBs.

What I do is similar to this:

IRBuilder<> IB(Context);

Vector.push_back(IB.CreateXXX());
Vector.push_back(IB.CreateXXX());
Vector.push_back(IB.CreateXXX());
...
// Other stuff

for (SmallVectorImpl<Instruction*>::iterator I = Vector.begin(), E = 
Vector.end(); I != E; ++I) {

   SomeBB->getInstList().insert(SomeInstruction, *I);

}

If I add the instructions like this I get strange results. It's like if 
llvm gets corrupted or something similar, because in the backend strange 
things happen, like MCRegisterInfo returning wrong register aliasing 
information in a random fashion (sometimes happens and sometimes doesn't) .

If I create and add the instructions on the fly like this:

IB.SetInsertionPoint(SomeInstruction);

for () {
   IB.CreateXXX();
}

Everything is fine.

So , in the end, is it safe to add instructions like in the first method 
I mentioned or actually the only safe way is to use the second way? In 
case it should be safe, what could be the cause of the random breakages?

Cheers,
Marcello

PS = Some of the instructions I'm adding are using one the output value 
of the other.

Marcello Maggioni <marcello at codeplay.com> writes:
> I'm getting a very strange behaviour while adding already created
> instructions in batches into basicblocks instead of creating and
> inserting them immediately.
>
> Because I need to insert instructions in a certain specific order
> inside multiple different BBs I found it easy to use the IRBuilder to
> create instructions without inserting them into a BB, storing them
> somewhere (vector, map ... etc) and later on inserting all of them in
> one go into their positions in the BBs.
[snip]
> So , in the end, is it safe to add instructions like in the first
> method I mentioned or actually the only safe way is to use the second
> way? In case it should be safe, what could be the cause of the random
> breakages?
>
> Cheers,
> Marcello
>
> PS = Some of the instructions I'm adding are using one the output
> value of the other.
Does your module pass the verifier? (llvm::verifyModule)

> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at
cs.uiuc.edu]
> On Behalf Of Marcello Maggioni
> Subject: [LLVMdev] Is it safe to insert instructions in batches into BBs?
> Because I need to insert instructions in a certain specific order inside 
> multiple different BBs I found it easy to use the IRBuilder to create 
> instructions without inserting them into a BB, storing them somewhere 
> (vector, map ... etc) and later on inserting all of them in one go into 
> their positions in the BBs.
You cannot insert the same instruction object into more than one basic block. 
You might be able to clone instructions and add the clones to the other blocks.

 - Chuck

Il 18/09/2013 03:40, Óscar Fuentes ha scritto:
> Marcello Maggioni <marcello at codeplay.com> writes:
>
>> I'm getting a very strange behaviour while adding already created
>> instructions in batches into basicblocks instead of creating and
>> inserting them immediately.
>>
>> Because I need to insert instructions in a certain specific order
>> inside multiple different BBs I found it easy to use the IRBuilder to
>> create instructions without inserting them into a BB, storing them
>> somewhere (vector, map ... etc) and later on inserting all of them in
>> one go into their positions in the BBs.
> [snip]
>> So , in the end, is it safe to add instructions like in the first
>> method I mentioned or actually the only safe way is to use the second
>> way? In case it should be safe, what could be the cause of the random
>> breakages?
>>
>> Cheers,
>> Marcello
>>
>> PS = Some of the instructions I'm adding are using one the output
>> value of the other.
> Does your module pass the verifier? (llvm::verifyModule)
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
Yes, it does. I get no complaints by the module verifier, and the CFG at 
the IR level looks fine.

If I try to print the CFG at Machine level though in some occasions the 
graph is not shown (probably another one of the strange breakages I get 
together with the RegisterInfo one)

Marcello

Il 18/09/2013 04:04, Caldarale, Charles R ha scritto:
>> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at
cs.uiuc.edu]
>> On Behalf Of Marcello Maggioni
>> Subject: [LLVMdev] Is it safe to insert instructions in batches into
BBs?
>> Because I need to insert instructions in a certain specific order
inside
>> multiple different BBs I found it easy to use the IRBuilder to create
>> instructions without inserting them into a BB, storing them somewhere
>> (vector, map ... etc) and later on inserting all of them in one go into
>> their positions in the BBs.
> You cannot insert the same instruction object into more than one basic
block.  You might be able to clone instructions and add the clones to the other
blocks.
>
>   - Chuck
I'm not doing that, I only insert an instruction in a single basicblock. 
I'm using that method of insertion because it makes the code clearer to 
insert the instructions like that instead of inserting them immediately 
where they are created (the actual code is more complex than the example 
I shown , but is conceptually the same)

Marcello