Re: gcc 4.3.0, -Wconversion: assignment-by operators for shorter types

[Andriy Gapon <avg at icyb dot net dot ua>]

on 27/05/2008 13:50 Manuel López-Ibáñez said the following:
> Please, open a bug report: http://gcc.gnu.org/bugzilla/

Manuel,

I looked through open GCC bugs and it seems that the following bugs are 
very similar to what I report:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34389
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32643

Should I still open a new one or maybe it's better to follow up on one 
of those two.

And just in case, here's a better description of what I am reporting 
(demonstration by comparison):
**********************
int main()
{
        int int_x;
        int int_y;
        short short_x;
        short short_y;

        short_x += short_y;
        short_x = short_x + short_y;

        int_x += int_y;
        int_x = int_x + int_y;

        return 0;
}
**********************

$ g++ -Wconversion test-conv.cc -o test-conv
test-conv.cc: In function ‘int main()’:
test-conv.cc:8: warning: conversion to ‘short int’ from ‘int’ may alter 
its value
test-conv.cc:9: warning: conversion to ‘short int’ from ‘int’ may alter 
its value

$ g++ -dumpversion
4.3.0


> 2008/5/26 Andriy Gapon <avg@icyb.net.ua>:
> > If I write something like the following:
> > uint8_t x;
> > uint8_t y;
> > x ^= y;
> >
> > and compile with gcc 4.3.0 with  -Wconversion option, then I get the
> > following warning:
> > warning: conversion to 'unsigned char' from 'int' may alter its value
> > [-Wconversion]
> >
> > While technically this warning is correct (because x and y are
> > implicitly advanced to "int"before actual xor operation), it seems that
> > it doesn't make much practical sense.
> >
> > I think that this applies to all "assignment-by" operators working on
> > types shorter than int.
> >
> > OTOH, proper implementation should be smart enough to warn about the
> > following:
> > uint8_t x;
> > uint16_t y;
> > x ^= y;
> >
> >
> > --
> > Andriy Gapon


--
Andriy Gapon