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

[Michael Kruse via llvm-dev]

2017-03-31 3:38 GMT+02:00 Daniel Berlin <dberlin at dberlin.org>:
> 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.

InstCombine transforms memset(a,1) to a single StoreInst, but keeps
the memset if the size argument is zero. A StoreInst can be optimized
away, e.g. by DeadStoreElimination. I wonder, are the semantics of
memset(a, 0) different than "do nothing"?

Where is the chain of transformations

memset(&a, '0', sizeof a);
if (c)
  a = 1;

=>

if (c) {
  memset(&a, '0', sizeof a);
  a = 1;
} else
  memset(&a, '0', sizeof a);

=>

if (c) {
  memset(&a, '0', sizeof a);
  a = 1;
} else
  memset(&a, '0', sizeof a);

=>

if (c) {
  a = 0;
  a = 1;
} else
  memset(&a, '0', sizeof a);

=>

if (c)
  a = 1;
else
  memset(&a, '0', sizeof a);

=>
if (c)
  a = 0;
if (!c)
  memset(&a, '0', sizeof a);


not legal?

Michael