[llvm-dev] The undef story

Hal Finkel via llvm-dev llvm-dev at lists.llvm.org
Thu Jun 29 15:42:57 PDT 2017

On 06/29/2017 04:56 PM, Chandler Carruth via llvm-dev wrote:
> On Thu, Jun 29, 2017 at 2:52 PM Peter Lawrence via llvm-dev 
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>     I would start looking for a more complete example myself, but my
>     Belief is so strong that "optimizing undefined behavior" seems
>     like a self-contradiction to me, and I don’t know where to
>     Even start looking.

To be clear (and to reiterate what Sanjoy said), you're not looking for 
optimization *of* UB. You're looking for optimizing on the basis of the 
assumption that the UB is not dynamically reachable. In some contexts 
you can think of this as an assumption that undef/poison values don't 
contribute to any observable side effects of the program.


> If you can't take the time to provide evidence to back up your belief, 
> then...
>     So I’m eagerly awaiting Hal’s (or anyone else's) next email
>     That has a complete example.
> ... I don't think it is reasonable to expect others to take their time.
> I understand you have this belief and find it incompatible with the 
> LLVM community. I think several folks have tried to persuade you that 
> there is merit in the design of LLVM here, but I understand you remain 
> unconvinced.
> Perhaps you will start a compiler project based on your convictions 
> here and develop it and show why that is a better way to start a 
> compiler project. But without actually contributing to LLVM and 
> showing clearly (and incrementally) how to improve it based on your 
> belief with evidence to back it up, I don't think reiterating your 
> belief on the list is a productive thing to do.
> -Chandler
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Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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