[Bug debug/78685] -Og generates too many "<optimized out>"s

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78685

Eric Gallager <egallager at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |egallager at gcc dot gnu.org
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=54200

--- Comment #9 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to Tom de Vries from comment #8)
> Created attachment 44333 [details]
> proof of concept patch
> 
> I ran into the same problem with guality test-case pr54200.c, which fails
> for Og.
> 
> The relevant part of the test-case is:
> ...
> int __attribute__((noinline,noclone))
> foo (int z, int x, int b)
> {
>   if (x == 1)
>     {
>       bar ();
>       return z;
>     }
>   else
>     {
>       int a = (x + z) + b;
>       return a; /* { dg-final { gdb-test 20 "z" "3" } } */
>     }
> }
> ...
> 
> The problem is that the '(x + z) + b' calculation has a temporary register
> which  gets allocated the register that holds 'z', so when we get to the
> gdb-test line, z is no longer available.
> 
> Using this patch I managed to print the correct value of z at the gdb-test
> line.
> 
> The patch uses clobbers in gimple to mark the out-of-scope point, purely
> because that's similar to what was already done for local array variables,
> and I thought that was the fastest path to getting a proof of concept
> working. It's more accurate to model this as some sort of use in gimple, and
> doing so may prevent gimple optimizations which wreck debug info, but
> perhaps that's not necessary, I suppose that depends on which optimizations
> are enabled in Og.
> 
> Anyway, at expand we emit a use for the clobber which seems to do the trick.

Please submit the patch to the gcc-patches mailing list for review! Thanks.