[LLVMdev] Building a stable bitcode format for PNaCl - based on LLVM IR

[Eli Bendersky]

On Tue, Jun 18, 2013 at 3:08 PM, Sean Silva <silvas at purdue.edu> wrote:

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>
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> On Tue, Jun 18, 2013 at 2:06 PM, Eli Bendersky <eliben at google.com> wrote:
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>>
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>> On Tue, Jun 18, 2013 at 12:03 PM, Sean Silva <silvas at purdue.edu> wrote:
>>
>>> Instead of a blacklist, why not a whitelist? Given the size of LangRef,
>>> you're bound to leave something out of your blacklist that needs to be
>>> there (also, future additions to LLVM IR will need to be added to the
>>> blacklist; are you sure you can catch *all* of them?). A whitelist seems
>>> much less prone to breakage or unexpected behavior.
>>>
>>>
>> Hi Sean,
>>
>> Which blacklist are you referring to? In all places where we specifically
>> allow or disallow certain constructs (such as specific instructions,
>> intrinsics, linkage modes and so on) we use a whitelisting strategy.
>>
>>
> What I'm saying is that the approach to defining the format seems to be
> basically "the format is LLVM IR, except ...". The "except ..." is
> effectively a blacklist. You are starting with LLVM IR and then removing
> (i.e. blacklisting certain aspects)
>

I just think it's a more useful discussion format for people knowledgeable
about LLVM. Dumping a huge LangRef-like reference manual on people is less
discussion-friendly :-) As I've mentioned, in reality (= code), the
approach is whitelisting so we shouldn't miss things that get added in
future LLVMs.

Eli
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