[llvm-bugs] [Bug 31675] New: clang incorrectly assumes extern addresses cannot be identical

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=31675

            Bug ID: 31675
           Summary: clang incorrectly assumes extern addresses cannot be
                    identical
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: bruening at google.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Clang seems to assume that extern addresses cannot be identical.  If we try to
compare two assembly labels, clang does not bother to emit a comparison and
assumes they are different:

--------------------------------------------------
#include <stdio.h>
extern void * asm_label;
extern void * asm_label2;
int main() {
  __asm("asm_label: asm_label2:");
  if (&asm_label != &asm_label2)
    puts("bug");
  else
    puts("no bug");
  return 0;
}
--------------------------------------------------

# /usr/bin/clang clangbug.c -o clangbug && ./clangbug
bug
# /usr/bin/gcc clangbug.c -o clangbug && ./clangbug
no bug

Examining the generated code, there is no comparison at all and simply a call
to puts("bug"), even at -O0.

# /usr/bin/clang --version
clang version 3.5.0 (tags/RELEASE_350/final)
Target: x86_64-redhat-linux-gnu
Thread model: posix

More recent versions of clang have the same behavior, including a recent clang
built from sources at r290297.

Xref https://github.com/DynamoRIO/dynamorio/issues/2124 where a recent
clang (r290297 in fact) extends this problem to static const copies of extern
addresses.

Is there something in the C language spec that allows clang to make this
assumption?  This does not seem limited to assembly labels: presumably function
aliases could hit the same problem (certainly it happens if these are not
"extern void *" but rather declared as extern functions which are supplied in a
separately compiled file).

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