llvm dev - Oct 2010 - [LLVMdev] Splitting basic blocks while preserving machine code

On Wed, Oct 20, 2010 at 7:19 PM, Jim Grosbach <grosbach at apple.com>
wrote:
>
> On Oct 20, 2010, at 5:16 PM, Trevor Harmon wrote:
>> I'm writing a pass that does some static analysis on C code. A
>> critical requirement of the pass is that it must preserve not only the
>> correctness of the code but also its timing -- It cannot add or remove
>> even one CPU cycle from the resulting machine code. To put it another
>> way, the binary executable that is produced when the pass is present
>> must be identical to what is produced when the pass is not present.
>>
>> That said, I have run into a situation where it would help to split
>> certain basic blocks, probably by calling BasicBlock::splitBasicBlock.
>> But this method adds an unconditional branch instruction. Would this
>> new instruction alter the machine code that is later generated, or
>> would the additional branch somehow be optimized away in subsequent
>> passes?
>
> It probably would be optimized away, yes, but I would recommend against
counting on it with requirements as strict as yours.
>
> -Jim
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You could always undo the split right before your pass finishes.

On Oct 20, 2010, at 5:26 PM, Kenneth Uildriks wrote:
> You could always undo the split right before your pass finishes.
How would I undo a split? Perhaps by merging them back using  
llvm::MergeBlockIntoPredecessor?

Trevor

On Wed, Oct 20, 2010 at 7:35 PM, Trevor Harmon <trevor.w.harmon at
nasa.gov> wrote:
> On Oct 20, 2010, at 5:26 PM, Kenneth Uildriks wrote:
>
>> You could always undo the split right before your pass finishes.
>
> How would I undo a split? Perhaps by merging them back using
> llvm::MergeBlockIntoPredecessor?
>
> Trevor
Exactly.  It's for merging two blocks when there is an unconditional
branch between them.