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

[Daniel Berlin via llvm-dev]

Sure, and you definitely can't transform an unconditional store to a
conditional one unless the uses are guarded by the same condition, or you
can prove it already had that value (in which case, the memset would also
be dead) :)

ie
memset(a, 0)
use a

can't be transformed to
if (c)
  memset(a, 0)
use a

So again, if polly is making a lifetime.start conditional when the use is
not conditional, that's a bug in polly, regardless of the bug in stack
coloring's ability to handle it.



On Thu, Mar 30, 2017 at 4:21 PM, Reid Kleckner <rnk at google.com> wrote:

> It's useful to think about lifetime.start/end as being equivalent to a
> memset of undef. They effectively clobber what was there and reinitialize
> it with "nothing". If you transform lifetime.start in a way that would be
> incorrect if it were storing undef, then that transform is incorrect.
>
> On Thu, Mar 30, 2017 at 4:16 PM, Daniel Berlin via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Interesting.
>> They are marked this way specifically to prevent things from moving them
>> this way, and it blocks other optimization. So if we are ignoring it to
>> move them, it seems either "they shouldn't be moved", or "we shouldn't
>>  mark them" :P
>>
>>
>> Thinking harder about this.
>> if you transformed
>>
>> lifetime.start(%p)
>> use %p
>> lifetime.end(%p)
>> into
>>
>> if (c)
>>   lifetime.start(%p)
>> use %p
>> lifetime.end(%p)
>>
>> That is *definitely* a bug in polly.
>> Stack coloring should be able to handle it if that is the original IR but
>> that is definitely not a legal transform of the lifetime.start.
>>
>>
>>
>> On Thu, Mar 30, 2017 at 4:09 PM, Michael Kruse <llvmdev at meinersbur.de>
>> wrote:
>>
>>> 2017-03-31 0:54 GMT+02:00 Daniel Berlin <dberlin at dberlin.org>:
>>> > I'm curious, btw, what made it think the above is legal.
>>> > The intrinsics are marked as touching memory, having side effects, etc.
>>>
>>> The precedes my involvement with Polly.
>>>
>>> Polly has a list of intrinsics that can be safely ignored, e.g.
>>> llvm.dbg.value/llvm.gbg.declare. The lifetime markers are also in this
>>> list. Ignored intrinsic are ... well ... ignored when generating the
>>> optimized code.
>>>
>>> Michael
>>>
>>
>>
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>>
>>
>
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