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

[Daniel Berlin via llvm-dev]

On Fri, Mar 31, 2017 at 4:05 AM, Michael Kruse <llvmdev at meinersbur.de>
wrote:

> 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?
>

Errr.
Because, as mentioned, they are not meant to float.
They are meant to stay executing under precisely the same conditions the
frontend gave them.

Also, the set of things for which it is legal for the frontend to do, but
not you, is infinite.

For example, as mentioned, the frontend may generate:

if (c)
  memset (a, 0)
use(a)

But that does not make it legal for you to transform

memset(a, 0)
use(a)

into
if (c)
  memset(a, 0)
use(a)

unless you can prove a already had the value 0, or that condition c always
holds.


> Michael
>
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