llvm dev - Feb 2014 - [LLVMdev] Custom allocators for MachineBasicBlocks and STL containers

Hello,

I was looking at how the memory for MachineBasicBlocks is managed and
something seems strange.

On one hand, a MachineFunction uses a recycling allocator to create and
destroy MBBs without costly heap allocation (the same is done for
MachineInstrs, MachineOperands, etc.).

On the other hand, each MBB also has a number of STL vectors within it. For
example, for Predecessors, Successors, Weights, LiveIns. Maintaining these
vectors involves heap allocation and they don't have custom allocators,
AFAICS.

Is there some magic I'm missing here, or does this make little sense?

Eli
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On Feb 14, 2014, at 1:52 PM, Eli Bendersky <eliben at google.com> wrote:
> Hello,
> 
> I was looking at how the memory for MachineBasicBlocks is managed and
something seems strange.
> 
> On one hand, a MachineFunction uses a recycling allocator to create and
destroy MBBs without costly heap allocation (the same is done for MachineInstrs,
MachineOperands, etc.).
> 
> On the other hand, each MBB also has a number of STL vectors within it. For
example, for Predecessors, Successors, Weights, LiveIns. Maintaining these
vectors involves heap allocation and they don't have custom allocators,
AFAICS.
> 
> Is there some magic I'm missing here, or does this make little sense?
Jakob would know for sure. My guess is it takes time to move all the data
structure over to the custom allocators so we just tackled the ones that provide
the most bang for the buck.

Evan
> 
> Eli
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

On Feb 14, 2014, at 3:34 PM, Evan Cheng <evan.cheng at apple.com> wrote:
> 
> On Feb 14, 2014, at 1:52 PM, Eli Bendersky <eliben at google.com>
wrote:
> 
>> Hello,
>> 
>> I was looking at how the memory for MachineBasicBlocks is managed and
something seems strange.
>> 
>> On one hand, a MachineFunction uses a recycling allocator to create and
destroy MBBs without costly heap allocation (the same is done for MachineInstrs,
MachineOperands, etc.).
>> 
>> On the other hand, each MBB also has a number of STL vectors within it.
For example, for Predecessors, Successors, Weights, LiveIns. Maintaining these
vectors involves heap allocation and they don't have custom allocators,
AFAICS.
>> 
>> Is there some magic I'm missing here, or does this make little
sense?
> 
> Jakob would know for sure. My guess is it takes time to move all the data
structure over to the custom allocators so we just tackled the ones that provide
the most bang for the buck.
What Evan said.

The MachineInstr class is completely heap independent. It would be nice to do
the same for MBB, but the gains are small.

Thanks,
/jakob