llvm dev - Aug 2016 - invariant.load metadata semantics

Hi Daniel,

Daniel Berlin wrote:
 > To be specific, does this imply "if we see a store, we can assume it
is
 > of the same value the load already has"?
 >
 > IE if i have:
 >
 > func()
 > {
 > load a, invariant.load !1
 > <use of a>
 > store a, 5
 > }
 >
 > Can i assume that a in <use of a> has the value 5?

Yes, that is what I had in mind.  Moreover, in

func()
{
   int k = load a, invariant.load !1
   print(k);
   store a, 5
}

k can be optimized to 5, in a form of "time traveling store
forwarding" :).  The store (at least if non-atomic and non-volatile)
is also trivially dead.

But I'd rather not allow stores at all.

-- Sanjoy

>
> Yes, that is what I had in mind.  Moreover, in
>
> func()
> {
>   int k = load a, invariant.load !1
>   print(k);
>   store a, 5
> }
>
> k can be optimized to 5, in a form of "time traveling store
> forwarding" :).  The store (at least if non-atomic and non-volatile)
> is also trivially dead.
>
>
Only if we know that print() returns, right?

Otherwise, we have the usual trouble with things like:
void print(int i) {
  if (k != 5) abort();
  ...
}
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Hi Michael,

Michael Kuperstein wrote:
 >
 >
 >     Yes, that is what I had in mind.  Moreover, in
 >
 >     func()
 >     {
 >        int k = load a, invariant.load !1
 >        print(k);
 >        store a, 5
 >     }
 >
 >     k can be optimized to 5, in a form of "time traveling store
 >     forwarding" :).  The store (at least if non-atomic and
non-volatile)
 >     is also trivially dead.
 >
 >
 > Only if we know that print() returns, right?

Yes.

-- Sanjoy