[LLVMdev] some llvm/clang missed optimizations

John Regehr
Wed, 27 Jan 2010 01:55:53 -0000

>> Repetitive code with lots of bitwise operations is compiled by LLVM
into
>> much larger code than the other compilers:
>>
>>
http://embed.cs.utah.edu/embarrassing/jan_10/harvest/source/ED/ED37DAF5.shtml
>>
http://embed.cs.utah.edu/embarrassing/jan_10/harvest/source/1F/1F4003C7.shtml
>>
>> Note that this is straight-line code, so LLVM's output will run 4-5
>> times longer than everyone else's.
>>
>> I'll be interested to learn the source of this one.
>
> This looks like a one-off case; instcombine destroys the symmetry of
> the code that the test harness duplicated by reducing the masking
> constants.  Probably too complicated for too little gain to be worth
> pursuing.

There are a bunch of these actually, I can try to make a list...

John