Whats going on with the conversion warning?

[Andy H]

Thanks for explanation and help

But this leave me with the conclusion that one of the following must be 
wrong:

signed char xi;

 xi =  (int) (unsigned short int) sc;    /* testcase NO WARNING - think 
this is bug*/
 xi =  (long) (unsigned short int) sc;    /* warning: conversion to 
'signed char' from 'short unsigned int' may alter its value - correct*/

Following your logic, (long).... appears to be wrong. Yet for i686 the 
first case  (int)..... generates PASS with expected warning and that 
would seem similar.

So  I'm still completely stuck knowing how I can patch testcase 
correctly for AVR or  post a bug.

best regards
Andy


Manuel López-Ibáñez wrote:
> 2008/5/20 Andy H <hutchinsonandy@aim.com>:
>   
>> I came across this odd issue with testsuite test Wconversion-5.c and AVR
>> target.
>> I should get warning going from a unsigned value that is wider than signed
>> result.
>>
>>     
>
> Yes. You should also get a warning from a unsigned value converted to
> a same-width signed type.
>
>   
>> void foo(void)
>> {
>>  signed char sc;
>>  signed char xi;
>>
>>  xi =  (int) (unsigned short int) sc;    /* testcase NO WARNING - think this
>> is bug*/
>>     
>
> I may be wrong but I think (unsigned short int) sc is zero-extended,
> then (int)(unsigned short int) sc is again zero extended. This means
> that the complete conversion results in an integer value that when
> converted to signed char gives back the original signed char. So the
> assignment is actually equivalent to xi = sc. Ergo, no conversion
> warning.
>
>   
>>  xi =  (unsigned short int) sc;   /* NO WARNING - think this is bug*/
>>     
>
> The same applies here. Zero-extending to a wider type and then
> conversion to the original type does not change the value. So now
> warning. (That is, Wconversion can see whether the casts actually
> affect the result or not.)
>
> So I think this is not a bug. There are bugs in Wconversion, nonetheless.
>
> http://gcc.gnu.org/PR35635
> http://gcc.gnu.org/PR35701
> http://gcc.gnu.org/PR34389
> http://gcc.gnu.org/PR35852
>
> Cheers,
>
> Manuel.
>