[PATCH] D153932: [AliasAnalysis] Mark fences as not Mod'ing unescaped local values

[Eli Friedman via Phabricator via llvm-commits]

efriedma added a comment.

The relevant scenario would be a bit more complicated, I think, since we're returning that the fence is "ModRefInfo::Ref".  Basically, that means non-atomic loads can move across the fence, but stores and atomic ops can't move across the fence.  But consider transforming something like the following:

  atomic<int*> y(nullptr);
  
  // Thread 1:
  x = 17;
  z = x;
  atomic_thread_fence(memory_order::release);  // Address of x not yet taken.
  y.store(&x, memory_order::relaxed);
  assert(z == 17);
  
  // Thread 2:
  int* tmp = y.load(memory_order::acquire);
  if (tmp != nullptr) *tmp += 100;

To something like:

  atomic<int*> y(nullptr);
  
  // Thread 1:
  x = 17;
  atomic_thread_fence(memory_order::release);  // Address of x not yet taken.
  z = x;
  y.store(&x, memory_order::relaxed);
  assert(z == 17);
  
  // Thread 2:
  int* tmp = y.load(memory_order::acquire);
  if (tmp != nullptr) *tmp += 100;

The original is legal C++, but the modified version has undefined behavior, I think.

And actually, I guess this reasoning also applies to hoisting/sinking across arbitrary calls, since any function can contain a fence.  So BasicAAResult::getModRefInfo for CallInst also needs to be fixed.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D153932/new/

https://reviews.llvm.org/D153932