On 19/03/14 15:55, Paulo Matos wrote:
>
>
>> -----Original Message-----
>> From: David Brown [mailto:da...@westcontrol.com]
>> Sent: 19 March 2014 14:44
>> To: Paulo Matos; gcc@gcc.gnu.org
>> Subject: Re: returning short-enum and truncate doesn't trigger
>> conversion warning
>>
>> On 19/03/14 15:33, Paulo Matos wrote:
>>> Hi all,
>>>
>>> This is either a C standard subtlety or a bug in GCC.
>>
>> This is perfectly normal behaviour for C, and not a bug. It is also a
>> topic for the gcc help list, rather than the development list.
>>
>
> Apologies, but I submitted this to gcc devel because I do think it is a bug.
Usually the discovery of bugs gets discussed on the help list - fixes
for the bugs are an issue for the developers list. And it is certainly
a "help" issue until we know you have found a bug.
>
>>>
>>> enum xpto
>>> {
>>> A = 0,
>>> B = 1,
>>> X = 512
>>> };
>>>
>>> extern void print (unsigned int);
>>>
>>> unsigned char bar (enum xpto a)
>>> {
>>> print (sizeof (unsigned char));
>>> print (sizeof (enum xpto));
>>> return a;
>>> }
>>
>> The sizeof operator returns an integer of type size_t (typically the
>> same as unsigned int or unsigned long, depending on the platform
>> details). But the compiler can see that the particular values in
>> question - 1 and 2 - can be converted to unsigned int without loss of
>> precision of changing the value. Therefore no warning is giving.
>>
>
> 512 cannot be converted to an unsigned char without loss of precision.
> The prints are actually not relevant for the question. They were just used
> for me to confirm the sizes of the types.
Not only are the prints not relevant, but they hid the real issue - I
missed the point of your question!
Conversions involving enumerations are not listed in the documentation
for -Wconversion, but it would be nice to get a warning here. Filing it
as an enhancement request seems reasonable.
> enum xpto
> {
> A = 0,
> B = 1,
> X = 512
> };
> extern void print (unsigned int);
>
> unsigned char bar (enum xpto a)
> {
> return a;
> }
>
> Variable a could be 0, 1 or 512. In case of the latter there is loss of
> precision but still no warning.
>
>>>
>>> $ ~/work/tmp/GCC/builds/gcc-trunk/gcc/cc1 -O2 test.c -Wall
>>> -Wconversion --short-enums -quiet
>>> test.c: In function 'foo':
>>> test.c:3:3: warning: conversion to 'short unsigned int' from
>> 'unsigned int' may alter its value [-Wconversion]
>>> return a;
>>> ^
>>>
>>> I was expecting a warning for bar as well since sizeof unsigned char
>> is 1 and sizeof enum xpto is 2, therefore the value is truncated but
>> no warning is issued.
>>>
>>> Shall I open a PR?
>>>
>>> Paulo Matos
>>>
>>
>>
>
