[Bug c++/43680] G++ is too aggressive in optimizing away bounds checking with enums

[rguenth at gcc dot gnu dot org]

------- Comment #5 from rguenth at gcc dot gnu dot org  2010-04-08 08:39 -------
The middle-end trusts the C++ frontends TYPE_MIN/MAX_VALUE which is according
to the C++ standard the range for an (unsigned) integer type with a precision
just as wide as to represent all values in the enum.

I did want to remove that optimization at some point but people complained
that it was highly useful (the C++ FE can still set TYPE_PRECISION
accordingly if it wants to, but TYPE_MIN/MAX_VALUE cannot really be trusted
in the middle-end as conversions between integer types of same precision
but different TYPE_MIN/MAX_VALUE are 1) useless and 2) do not produce code
to truncate the values).

It's VRP btw, that does this optimization.  If I have your support I'll rip
out this optimization from VRP ... (I didn't yet manage to create a wrong-code
bug due to the useless conversion thing, but I believe that is possible).

I also believe that the TYPE_MIN/MAX_VALUE on enums might be needed for
proper debug information.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu dot
                   |                            |org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43680