[cfe-dev] A patch for integer promotions.

[Enea Zaffanella]

Eli Friedman wrote:
> On Mon, Aug 17, 2009 at 11:32 PM, Enea Zaffanella<zaffanella at cs.unipr.it> wrote:
>> Here is attached the cumulative patch.
>> Is it possible they get applied before code is frozen?
> 
> Patch looks great; applied in r79412.  Sorry about the delay!
> 
> -Eli
> 
> 

Hi.

Thank you for applying the patch. We seem to have identified another 
promotion problem, this time related to the promotion of bit-fields.

Apparently, clang accepts further integral bit-field types besides those 
that are mandated by the C99 standard (_Bool, signed int, unsigned int). 
However, the code meant to check for bit-field promotions will only 
insert the promotions for the types mandated by the standard, so that, 
e.g., a bit-field having `unsigned short' type will not be subject to 
bit-field promotion rules; it will instead be subject to usual promotion 
rules.

Example: suppose the following code is parsed on a target where short 
and int have the same size (e.g., pic16).

struct S {
   unsigned short us : 12;
} s;

int foo(void) {
   return s.us + s.us;
}

Here clang inserts the usual (i.e., non-bit-field) promotion, obtaining 
code equivalent to

int foo(void) {
   return ((int) (((unsigned int) (s.us)) + ((unsigned int) (s.us))));
}

In contrast, if the bit-field promotions would be applied, we would 
obtain code equivalent to:

int foo(void) {
   return (int) (s.us) + (int) (s.us);
}

Now, even though we are talking about an implementation-defined 
behavior, I guess that clang is striving for maximum compatibility with 
GCC and, according to our observation, GCC applies bit-field promotions 
to *all* bit-fields having a declared bit-size smaller than that of an 
int, no matter what their declared type is.

This behavior sounds reasonable, so please find here attached the 
corresponding patch.

Besides implementing gcc bit-field extensions behavior, the patch also 
moves the corresponding function into ASTContext, so as to make it 
easily available to clients:

   QualType ASTContext::isPromotableBitField(Expr *E);

Let us know if there is anything unclear.

Cheers,
Enea Zaffanella.
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