[LLVMdev] some llvm/clang missed optimizations

John Regehr regehr at cs.utah.edu
Tue Jan 26 17:55:53 PST 2010

>> 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...


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