Help with bit-field semantics in C and C++

[Mark Mitchell]

Joe Buck wrote:

>[ issues connected with limited range of C++ enums ]
>On Tue, Aug 24, 2004 at 10:32:39AM -0600, Roger Sayle wrote:
>  
>
>>I'm happy with the explanations so far.  The next question is, if
>>the middle-end is supposed to treat loads, stores and comparisons
>>of these types identically to the underlying integer type for the
>>enumeration, is there a benefit for setting TYPE_PRECISION lower
>>than GET_MODE_BITSIZE for C++'s enumerated types?
>>    
>>
>
>I think that the middle end should, at least by default, try to generate
>the best code possible for conformant programs that use C++ enums.  In
>most cases, that probably means treating them as belonging to the
>underlying integer type; the exception would be where the limited range
>allows for, say, more efficient switch statements, or to determine that
>some code is unreachable.
>  
>
I agree.

PR 10796 was the motivation for the current implementation.  A secondary 
motivation was, as Joe says, to allow the middle-end to generate better 
code for switch/if/etc. situations where it could prove that, because of 
the limited range, certain values were impossible.

-- 
Mark Mitchell
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mark@codesourcery.com