[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 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.

Dan