Silence warnings with new GCC

[Mario Torre]

2015-11-26 20:37 GMT+01:00 Lindenmaier, Goetz <goetz.lindenmaier at sap.com>:
> Hi Andrew,
>
> I know about this problem ... I guess a change of mine causes these warnings.
> While I found a row of good fixes, these with ShouldNotReachHere are annoying.
>
> What you propose has been discussed in April: 8065585: Change ShouldNotReachHere() to never return.
> I didn't follow the discussion all to the end, but it wasn't done for some reason.
> Also, I think, one can overrule ShoudlNotReachHere() with -XX:SuppressError=...

FYI, I believe the full thread is this:

http://mail.openjdk.java.net/pipermail/hotspot-dev/2015-April/017990.html

Cheers,
Mario

> Best regards,
>   Goetz.
>
> -----Original Message-----
> From: hotspot-dev [mailto:hotspot-dev-bounces at openjdk.java.net] On Behalf Of Andrew Haley
> Sent: Thursday, November 26, 2015 6:24 PM
> To: build-dev <build-dev at openjdk.java.net>; hotspot-dev Source Developers <hotspot-dev at openjdk.java.net>
> Subject: Silence warnings with new GCC
>
> I've been getting a lot of warnings such as
>
> warning: 'size' may be used uninitialized in this function [-Wmaybe-uninitialized]
>
> which error out with -Werror.  Almost all of them are bogus.  They are
> typically of the form
>
>     unsigned size;
>
>     if (i->get(26, 26)) { // float
>       switch(i->get(31, 30)) {
>       case 0b10:
>         size = 2; break;
>       case 0b01:
>         size = 1; break;
>       case 0b00:
>         size = 0; break;
>       default:
>         ShouldNotReachHere();
>       }
>     } else {
>       size = i->get(31, 31);
>     }
>
> The problem here is that GCC does not know that ShouldNotReachHere()
> should be treated as an unreachable statement.
>
> The patch here fixes it.  I'd rather do this than add pointless assignments
> all over the place.  Thoughts?  Opinions?
>
> Thanks,
>
> Andrew.
>
>
>
> diff --git a/src/share/vm/utilities/debug.hpp b/src/share/vm/utilities/debug.hpp
> --- a/src/share/vm/utilities/debug.hpp
> +++ b/src/share/vm/utilities/debug.hpp
> @@ -172,16 +172,24 @@
>    BREAKPOINT;                                                                     \
>  } while (0)
>
> +#ifdef __GNUC__
> +#  define UNREACHABLE __builtin_unreachable()
> +#else
> +#  define UNREACHABLE do { } while (0)
> +#endif
> +
>  #define ShouldNotReachHere()                                                      \
>  do {                                                                              \
>    report_should_not_reach_here(__FILE__, __LINE__);                               \
>    BREAKPOINT;                                                                     \
> +  UNREACHABLE;                                                                    \
>  } while (0)
>
>  #define Unimplemented()                                                           \
>  do {                                                                              \
>    report_unimplemented(__FILE__, __LINE__);                                       \
>    BREAKPOINT;                                                                     \
> +  UNREACHABLE;                                                                    \
>  } while (0)
>
>  #define Untested(msg)



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