llvm dev - Mar 2017 - Well-formed @llvm.lifetime.start and @llvm.lifetime.end intrinsics

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

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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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
>>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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2017-03-31 1:16 GMT+02:00 Daniel Berlin <dberlin at
dberlin.org>:
> 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.
If it is legal for a frontend to generate it, why is it not legal for
a transformation?

Michael