[llvm-commits] [llvm] r86871 - in /llvm/trunk: lib/CodeGen/BranchFolding.cpp lib/CodeGen/MachineBasicBlock.cpp test/CodeGen/Thumb2/thumb2-cbnz.ll test/CodeGen/Thumb2/thumb2-ifcvt3.ll test/CodeGen/X86/loop-blocks.ll test/CodeGen/X86/tail-opts.ll

[Dan Gohman]

On Nov 12, 2009, at 1:54 PM, Chris Lattner wrote:

> 
> On Nov 12, 2009, at 10:39 AM, Dan Gohman wrote:
> 
>> 
>> On Nov 11, 2009, at 11:42 PM, Duncan Sands wrote:
>> 
>>> Hi Dan,
>>> 
>>>> Add support for tail duplication to BranchFolding, and extend
>>>> tail merging support to handle more cases.
>>>> - Recognize several cases where tail merging is beneficial even when
>>>> the tail size is smaller than the generic threshold.
>>>> - Make use of MachineInstrDesc::isBarrier to help detect
>>>> non-fallthrough blocks.
>>>> - Check for and avoid disrupting fall-through edges in more cases.
>>> 
>>> can this result in a label being sucked off the end of a block, and
>>> moved into another?  Exception handling labels come in pairs, and it
>>> would be wrong to move one of them without the other (even all the
>>> instructions in between).
>> 
>> My understanding is that each EH_LABEL has a unique ID, and this
>> automatically prevents two of them from being merged.
>> 
>> I just made a change to mark all label instructions as non-duplicable,
>> to prevent tail-duplication and other code-duplicating optimizations
>> from getting into trouble.
> 
> Does this cause debug labels to change optimization/codegen behavior?

Hrm yes. I'll look into this.

Dan