Volatile stores of undef values are deleted

[llvmbot]

Bugzilla Link	27309
Version	trunk
OS	All
Reporter	LLVM Bugzilla Contributor
CC	@sanjoy

Extended Description

All stores of undef values are deleted at the moment - including volatile stores:

define void @f(i32 *%p) {
  store volatile i32 undef, i32 *%p;
  ret void;
}

Compiles to:

f: retq

In theory, the pointer could point to a hardware device where the store has some side-effects like increasing a queue pointer. A driver that wants to write dummy data could reasonably store an undef and need it to be actually performed.

Bug found when browsing idly through the code, I don't actually have a real-world case for it.

[enferex]

Closed as fixed by 404071b