Re: [PATCH, C++] Make Canonical ICE instead of just warn when mismatching

["Doug Gregor" <doug.gregor@gmail.com>]

On 1/4/07, Brooks Moses <brooks.moses@codesourcery.com> wrote:
> My impression from your argument is that this case is not like that;
> that here we know that a failure to pass this particular internal check
> will never result in invalid code.  And thus a warning is appropriate
> here, because the user will still get valid usable code if they ignore
> the warning.
>
> Is this a correct impression?  I haven't seen any mention of this
> distinction, but it seems to me that it's the only way that the argument
> for a warning instead of a hard error would make sense.

Yes, your impression is correct.

The canonical types implementation is a compile-time optimization that
makes comparing types for equality (e.g., is "int*" the same as
"my_int_t*"?) faster. Previously, we would have to look at the
structure of the types to determine if they are equivalent; with
canonical types, we only do a single pointer comparison. However, the
code that looks at the structure of types is still needed for some
comparisons, so it's still available for us to use when checking.

As it stands now, there is a parameter "verify-canonical-types". When
it is zero (the default for --disable-checking builds), we trust the
canonical type system to be correct and get our 5-10% speedup on
template-heavy code. When it is one (the default for --enable-checking
builds), we compare the results of the older, slower structure-based
comparison against the results of the canonical type comparison. If
they differ, we complain. Then, we can recover from the failure of the
canonical-types system by reporting the result that the
structure-based comparison determined.

  Cheers,
  Doug