[llvm-commits] [llvm] r72276 - in /llvm/trunk: include/llvm/CodeGen/Passes.h include/llvm/CodeGen/RuntimeLibcalls.h lib/CodeGen/CMakeLists.txt lib/CodeGen/DwarfEHPrepare.cpp lib/CodeGen/LLVMTargetMachine.cpp lib/CodeGen/SelectionDAG/FastISel.cpp

[Duncan Sands]

Hi Eli,

>> Note that it is currently rare for this pass to actually do
>> anything.  And in fact it normally shouldn't do anything at
>> all given the code coming out of llvm-gcc!  But it does fire
>> a few times in the testsuite.  As far as I can see this is
>> almost always due to the LoopStrengthReduce codegen pass
>> introducing pointless loop preheader blocks which are landing
>> pads and only contain a branch to another block.  This other
>> block contains an eh.exception call.  So probably by tweaking
>> LoopStrengthReduce a bit this can be avoided.
> 
> AFAIK, LoopStrengthReduce doesn't modify the CFG itself; it's probably
> an effect of LoopSimplify.

actually it does modify the CFG, by splitting edges for example.
However the preheader does seem to have been created by LoopSimplify.

Ciao,

Duncan.