llvm dev - Nov 2011 - [LLVMdev] Problem using a label to a MachineBasicBlock

Hi all,

I think that I came somewhat closer to a solution for splitting a
MachineBasicBlock for a PSEUDO_CALL_R instruction and having a label to the new
MBB:

For following piece of code:
---
typedef int callme_t(int a, int b);
callme_t* c01;

int foo(int a, int b)
{
  return c01(a,b); // MachineBasicBlock will be split at call instruction
}
---

I have initially following correspondence:
  BB1 -> MBB10    (aka, BasicBlock 1 corresponds to MachineBasicBlock 10)

After splitting MBB10 at the call instruction, we get :
  BB1 -> MBB10, MBB11

Because I need the address of MBB11, I found that I need to ensure 
that 'MBB11->hasAddressTaken()' returns true. Otherwise it can be
optimized
away.

This can be done by:
  MBB11->setHasAddressTaken();

But this is not sufficient: an assertion failure in 
  llvm::MMIAddrLabelMap::getAddrLabelSymbolToEmit(..)
is triggered:
  'BB->hasAddressTaken()' should also be set.

The only way I found to do this (although it does not look clean to me)
is the following:
 
(void)llvm::BlockAddress::get(const_cast<BasicBlock*>(MBB11->getBasicBlock()))
In this way, BB1 will have its reference count incremented and the assertion
will
not trigger.


So, after splitting the MachineBasicBlock and migrating the successors etc., I
have to do:
---
 NEW_MBB->setHasAddressTaken();
 (void)llvm::BlockAddress::get(const_cast<BasicBlock*>(NEW_MBB->getBasicBlock()))
---

With this, my basic example works as expected.

Now, I want to try this on a more complex example:
---
typedef int callme_t(int a, int b);
callme_t* c01;
callme_t* c02;

int foo(int a, int b)
{
  int result = c01(a,b); // MachineBasicBlock will be split at call instruction
  result += c02(a,b);  // MachineBasicBlock will also be split at this call
instruction
  return result;
}
---

This corresponds with:
 BB1 -> MBB10, MBB11, MBB12

where MBB11 and MBB12 have both their address taken, and the reference count for
BB1
is increased twice. With this, I am triggering a new assertion failure:

In lib/MC/MCAsmStreamer.cpp:328 of MCAsmStreamer::EmitLabel(llvm::MCSymbol*)
----
void MCAsmStreamer::EmitLabel(MCSymbol *Symbol) {
  assert(Symbol->isUndefined() && "Cannot define a symbol
twice!");
  MCStreamer::EmitLabel(Symbol);

  OS << *Symbol << MAI.getLabelSuffix();
  EmitEOL();
}
---

When I disable the assertion, I observe the following:
The different labels for the machine basic blocks are emitted, but the two new
basic blocks that were created, also have a common extra label '_tmp0'

---
	.text
	.globl _foo
_foo:
	....
	load_immediate  R15, __BB0_1
	branch_to_reg   R1
_tmp0:
_BB0_1:
     ...
     load_immediate   R15, __BB0_2:
     branch_to_reg    R2
_tmp0:                     // *** duplicate unnecessary label, _tmp0 is not used
_BB0_2
     ...
---

Any idea how this can be resolved in a clean way ?

Thanks,

Jeroen Dobbelaere

Using 'NEW_BB->setIsLandingPad(true);' seems to resolve everything.

Greetings,

Jeroen Dobbelaere
[...]

Can you please post the code to split a MachineBasicBlock?

I am trying to split a MachineBasicBlock at a specific instruction in the
MBB, let us say, into MBB1 and MBB2. This instruction should go into MBB2.
Also MBB1 should have an unconditional branch to MBB2 as the terminator.
(quite similar to splitBasicBlock in BasicBlock.cpp)

Meanwhile, I am trying to come up with a variant of SplitCriticalEdge to do
this but if someone can provide the code to split a MBB, it will be of great
help.


Jeroen Dobbelaere-2 wrote:
> 
> Using 'NEW_BB->setIsLandingPad(true);' seems to resolve
everything.
> 
> Greetings,
> 
> Jeroen Dobbelaere
> [...]
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> 
> 
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