[llvm-bugs] [Bug 26774] New: Doing certain kinds of IPO over comdat functions is unsound

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26774

            Bug ID: 26774
           Summary: Doing certain kinds of IPO over comdat functions is
                    unsound
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Interprocedural Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: sanjoy at playingwithpointers.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 15960
  --> https://llvm.org/bugs/attachment.cgi?id=15960&action=edit
Reproducer

This was discussed on llvm-dev:
http://lists.llvm.org/pipermail/llvm-dev/2016-February/095833.html

Description copy-pasted from the thread:

"""
Let's start with an example that shows that we have a problem (direct
copy/paste from the guard intrinsics thread). Say we have:

```
void foo() available_externally {
  %t0 = load atomic %ptr
  %t1 = load atomic %ptr
  if (%t0 != %t1) print("X");
}
void main() {
  foo();
  print("Y");
}
```

The possible behaviors of the above program are {print("X"),
print("Y")} or {print("Y")}.  But if we run opt then we have

```
void foo() available_externally readnone nounwind {
  ;; After CSE'ing the two loads and folding the condition
}
void main() {
  foo();
  print("Y");
}
```

and some generic reordering

```
void foo() available_externally readnone nounwind {
  ;; After CSE'ing the two loads and folding the condition
}
void main() {
  print("Y");
  foo();  // legal since we're moving a readnone nounwind function that
          // was guaranteed to execute (hence can't have UB)
}
```

If we do not inline @foo(), and instead re-link the call site in @main
to some non-optimized copy (or differently optimized copy) of @foo,
then it is possible for the program to have the behavior {print("Y");
print ("X")}, which was disallowed in the earlier program.

In other words, opt refined the semantics of @foo() (i.e. reduced the
set of behaviors it may have) in ways that would make later
optimizations invalid if we de-refine the implementation of @foo().

The above example is clearly fabricated, but such cases can come up
even if everything is optimized to the same level.  E.g. one of the
atomic loads in the unrefined implementation of @foo() could have been
hidden behind a function call, whose body existed in only one module.
That module would then be able to refine @foo() to `ret void` but
other modules won't.
"""

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20160229/1d1de535/attachment.html>