r194288 - If a linkonce_odr dtor/ctor is identical to another one, just rauw.

[Rafael Espíndola]

On 11 November 2013 11:18, David Blaikie <dblaikie at gmail.com> wrote:
> This seems to have broken the GDB bot:
> http://lab.llvm.org:8011/builders/clang-x86_64-ubuntu-gdb-75/builds/10344 (
> http://lab.llvm.org:8011/builders/clang-x86_64-ubuntu-gdb-75/builds/10344/steps/gdb-75-check/logs/gdb.cp__destrprint.exp
> )
>
> I haven't looked too closely at the failure yet, but let me know if you need
> a hand understanding it or deciding if it's a true/false positive.
>
> I suspect, while the optimization you've implemented is legitimate, it will
> reduce -O0 debuggability somewhat (as COMDAT/function deduplication tends
> to) - if it does so too severely we might want to enable it only in -O1 and
> above. (times like these it might be nice to have -Og)

The issue seems to be us replacing a derived destructor with a base
one. We were not using aliases since we could not guarantee we would
get the same comdat in every file, but with rauw that is legal. We
would have exactly the same issue with older clangs if the classes had
out of line destructors.

With the new clang we get:

#0  Base::~Base (this=0x7fffffffde50) at
/home/espindola/llvm/cte/gdb/testsuite/gdb.cp/destrprint.cc:14
#1  0x00000000004007cf in main () at
/home/espindola/llvm/cte/gdb/testsuite/gdb.cp/destrprint.cc:35

By disabling the optimization we get

#0  Base::~Base (this=0x7fffffffde50) at
/home/espindola/llvm/cte/gdb/testsuite/gdb.cp/destrprint.cc:14
#1  0x0000000000400835 in Derived::~Derived (this=0x7fffffffde50) at
/home/espindola/llvm/cte/gdb/testsuite/gdb.cp/destrprint.cc:28
#2  0x0000000000400815 in Derived::~Derived (this=0x7fffffffde50) at
/home/espindola/llvm/cte/gdb/testsuite/gdb.cp/destrprint.cc:27
#3  0x00000000004007cf in main () at
/home/espindola/llvm/cte/gdb/testsuite/gdb.cp/destrprint.cc:3

I don't know enough dwarf to say if we could represent this in the
debug info. If not, should we just disable this opimization (derived
to base alias/rauw) at -O0?

Cheers,
Rafael