[Bug tree-optimization/61493] New: Bug exposed by speculative devirtualizing

[Bug tree-optimization/61493] New: Bug exposed by speculative devirtualizing

[wmi at google dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61493

            Bug ID: 61493
           Summary: Bug exposed by speculative devirtualizing
           Product: gcc
           Version: 4.10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wmi at google dot com

1.cxx is attached.

$~/workarea/gcc-r211604/build/install/bin/g++ -v
Using built-in specs.
COLLECT_GCC=/usr/local/google/home/wmi/workarea/gcc-r211604/build/install/bin/g++
COLLECT_LTO_WRAPPER=/usr/local/google/home/wmi/workarea/gcc-r211604/build/install/libexec/gcc/x86_64-unknown-linux-gnu/4.10.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../src/configure --enable-languages=c,c++ --disable-bootstrap
--prefix=/usr/local/google/home/wmi/workarea/gcc-r211604/build/install
Thread model: posix
gcc version 4.10.0 20140613 (experimental) (GCC)

$~/workarea/gcc-r211604/build/install/bin/g++ -O2 1.cxx
$./a.out
not equal

$~/workarea/gcc-r211604/build/install/bin/g++ -O2 1.cxx
$./a.out
equal

$~/workarea/gcc-r211604/build/install/bin/g++ -O2
-fno-devirtualize-speculatively 1.cxx
$./a.out
equal

Google ref b/15521306

[Bug tree-optimization/61493] Bug exposed by speculative devirtualizing

[wmi at google dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61493

--- Comment #1 from wmi at google dot com ---
Created attachment 32931
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=32931&action=edit
testcase

[Bug tree-optimization/61493] [4.10 Regression] Bug exposed by speculative devirtualizing

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61493

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
                 CC|                            |hubicka at gcc dot gnu.org
   Target Milestone|---                         |4.10.0
            Summary|Bug exposed by speculative  |[4.10 Regression] Bug
                   |devirtualizing              |exposed by speculative
                   |                            |devirtualizing

[Bug tree-optimization/61493] [4.10 Regression] Bug exposed by speculative devirtualizing

[ppluzhnikov at google dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61493

Paul Pluzhnikov <ppluzhnikov at google dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppluzhnikov at google dot com

--- Comment #2 from Paul Pluzhnikov <ppluzhnikov at google dot com> ---
Also broken in 4.9

[Bug tree-optimization/61493] [4.10 Regression] Bug exposed by speculative devirtualizing

[wmi at google dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61493

--- Comment #3 from wmi at google dot com ---
Fix a typo in the first post.

$~/workarea/gcc-r211604/build/install/bin/g++ -O2 1.cxx
$./a.out
not equal

$~/workarea/gcc-r211604/build/install/bin/g++ -O0 1.cxx
$./a.out
equal

$~/workarea/gcc-r211604/build/install/bin/g++ -O2
-fno-devirtualize-speculatively 1.cxx
$./a.out
equal

[Bug tree-optimization/61493] [4.10 Regression] Bug exposed by speculative devirtualizing

[wmi at google dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61493

wmi at google dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #4 from wmi at google dot com ---
This is a source problem. So close the bug.

void foo(FST *fst) {
  const PAIR &final_pair = fst->Final().getpair();
  if (final_pair == global_pair)
    __builtin_printf("equal\n");
  else
    __builtin_printf("not equal\n");

  return;
}

The life time of the temporary object generated by fst->Final() will not be
extended after the statement generating it, according to the following rule :

a temporary bound to a return value of a function in a return statement is not
extended: it is destroyed immediately at the end of the return expression. Such
function always returns a dangling reference.
(http://en.cppreference.com/w/cpp/language/reference_initialization)

So it is meaningless to access final_pair afterwards.