Re: [PATCH 2/9] Use unsigned as base type for some enums

[Simon Marchi <simon.marchi@ericsson.com>]

On 2018-08-27 04:21 PM, Tom Tromey wrote:
>>>>>> "Simon" == Simon Marchi <simon.marchi@ericsson.com> writes:
> 
> Simon> Can you give an example of how the error manifests itself (I'm not really
> Simon> familiar with -fsanitize=undefined).  Is the error reported at compile-time
> Simon> or run-time?  I'm not able to make a synthetic standalone example to reproduce
> Simon> the error.
> 
> You will get an error at runtime, and with the flags added by the last
> patch in the series, a crash.
> 
> The error looks somewhat like the error from the expression dumping
> patch:
> 
>   runtime error: load of value 2887952, which is not a valid value for type 'exp_opcode'
> 
> (I don't have an exact error handy, this was just taken from the other
> patch.)
> 
> Simon> In any case, that LGTM if that makes the compiler happy.  If the error reported
> Simon> by -fsanitize=undefined is at run-time, could we add a static assert in there
> Simon> to make sure the underlying types of types used with DEF_ENUM_FLAGS_TYPE are
> Simon> unsigned, to get a compilation error?
> 
> With the final patch, any UB will cause gdb to crash (in development
> mode), presumably leading to a test suite failure.  I think it isn't
> necessary to require unsigned as the underlying type -- any type will
> do.  However I don't know how to assert that.

Indeed, it's only necessary if the ~ operator is used.  OTOH, it doesn't really
make sense to use a signed type for flags, so we wouldn't lose anything by enforcing
unsigned types.  If I understand correctly, the errors come from code like this, when
making a bit mask to clear some bits:

  btinfo->flags &= ~(BTHR_MOVE | BTHR_STOP);

Doing a static assert like this:

diff --git a/gdb/common/enum-flags.h b/gdb/common/enum-flags.h
index 82568a5..c82970c 100644
--- a/gdb/common/enum-flags.h
+++ b/gdb/common/enum-flags.h
@@ -92,6 +92,7 @@ class enum_flags
 public:
   typedef E enum_type;
   typedef typename enum_underlying_type<enum_type>::type underlying_type;
+  gdb_static_assert (std::is_unsigned<underlying_type>::value);

 private:
   /* Private type used to support initializing flag types with zero:


would enforce it at compile time, which is preferable than finding it at
runtime.

Simon