[llvm-dev] Well-formed @llvm.lifetime.start and @llvm.lifetime.end intrinsics

[Daniel Berlin via llvm-dev]

(in particular, those things keep LICM from moving them)

On Thu, Mar 30, 2017 at 3:54 PM, Daniel Berlin <dberlin at dberlin.org> wrote:

>
>
> On Thu, Mar 30, 2017 at 3:45 PM, Michael Kruse <llvmdev at meinersbur.de>
> wrote:
>
>> 2017-03-31 0:19 GMT+02:00 Daniel Berlin <dberlin at dberlin.org>:
>> >> I modified Polly's code generator to produce
>> >>
>> >>     if (c) {
>> >>       llvm.lifetime.start(&var)
>> >>     } else {
>> >>       llvm.lifetime.start(&var)
>> >>     }
>> >>     [...]
>> >>     llvm.lifetime.end(&var)
>> >>
>> >> and it does not miscompile anymore. However, Polly is currently is not
>> >> able to compute the lifetime in the general case.
>> >
>> >
>> > Of course, nothing is.
>> > But they are also metadata, so you could just drop them if you wanted.
>>
>> This is actually exactly what Polly did, but only in the optimized
>> side of the loop versioning. The original code version is not touched,
>> which keeps the llvm.lifetime.start in it.
>>
>> So of the bug is on Polly's side, I would "fix" it by deleting all
>> lifetime markers also in the original code.
>>
>
> I'm curious, btw, what made it think the above is legal.
> The intrinsics are marked as touching memory, having side effects, etc.
>
> Do you guys specially ignore them or something?
>
>
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