[Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type

[Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40752

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ebrahim at mohammadi dot ir

--- Comment #19 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2011-09-25 21:12:33 UTC ---
*** Bug 50519 has been marked as a duplicate of this bug. ***

[Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40752

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |DeusExSophismata at gmail
                   |                            |dot com

--- Comment #20 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-03-24 19:27:54 UTC ---
*** Bug 52703 has been marked as a duplicate of this bug. ***

[Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type

[DeusExSophismata at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40752

--- Comment #21 from David Stone <DeusExSophismata at gmail dot com> 2012-03-24 21:25:08 UTC ---
Why was this patch rejected, and is there a way to improve it so that obviously
safe cases (such as PR52703) are not warned about without having to specify a
'-Wno-' option?

Yes, according to the standard (C++03 5/9), calculations done on variables
smaller than int are first promoted to int, then the calculation is done, then
the value is converted back to the target size. However, C++03 1.8/3, the
"as-if rule", states that it the program can't tell the difference, you can do
whatever you want (see my answer to a similar question on Stack Overflow here:
http://stackoverflow.com/questions/5563000/implicit-type-conversion-rules-in-c-operators/8935697#8935697).

The C++ standard does not require a diagnostic for this, and the apparent
behavior is identical. Therefore, there can be no appeals to the C++ standard
on the behavior of the warning.

Because this is a purely option warning for which gcc defines the rules, we
should define it to be useful. If gcc can prove that all of the values are
greater than 0 (for instance, if all of the values are unsigned prior to
implicit promotion or are positive integral constant expressions), then there
is no possibility of having a negative value. Thanks to signed integer overflow
being undefined, there is no risk of creating a negative value that way,
either. Therefore, we should not warn. Having to manually say "Turn off stuff
that no one could ever possibly want to see" seems like a sure way to make this
warning useless.

[Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type

[wessjunk at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40752

Wes <wessjunk at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |wessjunk at gmail dot com

--- Comment #22 from Wes <wessjunk at gmail dot com> 2012-05-05 15:09:22 UTC ---
I agree with David.  My code has many instances of things like
a+=2
where a is a char and this makes -Wconversion useless to me.

It's too bad, since it would really be helpful as I am in the process of
changing some data types in the code.

If someone really thinks there should be a warning for this behavior, how about
adding a separate
-Wchar-arithmetic
warning which warns on all char arithmetic and see how much people use it.

[Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40752

--- Comment #23 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2012-05-05 15:30:06 UTC ---
(In reply to comment #22)
> If someone really thinks there should be a warning for this behavior, how about
> adding a separate
> -Wchar-arithmetic
> warning which warns on all char arithmetic and see how much people use it.

I think adding a new option Wconversion-after-promotion that covers all cases
where the conversion occurs to the same type that was originally promoted from
would be the most appropriate. I have a feeling that it should not be hard to
implement, but I am not sure how, and I have many other things I would like to
do first. So, please David, Wes, photon, give it a try. The code is in
c-family/c-common.c (conversion_warning).

[Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type

[dichlofos-mv at yandex dot ru]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40752

Mikhail Veltishchev <dichlofos-mv at yandex dot ru> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dichlofos-mv at yandex dot
                   |                            |ru

--- Comment #24 from Mikhail Veltishchev <dichlofos-mv at yandex dot ru> 2012-06-20 14:57:31 UTC ---
Well, please fix this. My cases are instructions like
unsigned short x = 100;
unsigned short y = 200;
// gives a warning
x += y;

[Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type

[chengniansun at gmail dot com]

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40752

--- Comment #25 from Chengnian Sun <chengniansun at gmail dot com> ---
Today I encountered a similar case as follows. The conversion warning by gcc is
not true as right-shifting an unsigned short decreases the value. 

BTW clang does not emit warnings for this code snippet. 


$: cat s1.c
unsigned short fn(unsigned short a, int right) {
  return a >> right;
}
$: gcc-trunk -c -Wconversion s1.c
s1.c: In function ‘fn’:
s1.c:2:3: warning: conversion to ‘short unsigned int’ from ‘int’ may alter its
value [-Wconversion]
   return a >> right;
   ^
$: clang-trunk -c -Wconversion s1.c
$:
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id`113

--- Comment #2 from lmat <dartmetrash at gmail dot com> ---
Created attachment 32090
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id2090&actioníit
Bug report including command line output.

My apologies! I was quite sure I had attached it, but I see quite plainly that
I had failed! It appears that my upload was too big. I've divided it up using

split --lines 35000 ...

which creates three files.

[Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type

[hariharan.gcc at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=40752

hariharan.gcc at gmail dot com changed:

           What    |Removed                     |Added
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                 CC|                            |hariharan.gcc at gmail dot com

--- Comment #27 from hariharan.gcc at gmail dot com ---
I encountered a similar problem with -Wconversion. This option is useless if it
would warn for cases like

short int i;
i += 5;

I concede Andrew's point about this being technically a place to warn, but in
practice, i would like for it to warn only when the target is a "smaller" than
any of the inputs.

[Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type

[photon at seznam dot cz]

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------- Comment #18 from photon at seznam dot cz  2010-06-12 16:46 -------
(In reply to comment #17)
> The patch was rejected but it may be accepted by using a new -Wno-* option to
> disable these warnings. Perhaps -Wno-conversion-after-promotion?
> 
> Suggestions are welcome.
> 


-Wconversion should be fixed to work as specified. No warning should be
generated for the following code:

char c = 2;
// warning: conversion to ‘char’ from ‘int’ may alter its value
c >>= 1;


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40752

[Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type

[manu at gcc dot gnu dot org]

------- Comment #17 from manu at gcc dot gnu dot org  2010-06-11 20:22 -------
The patch was rejected but it may be accepted by using a new -Wno-* option to
disable these warnings. Perhaps -Wno-conversion-after-promotion?

Suggestions are welcome.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40752

[Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type

[manu at gcc dot gnu dot org]

------- Comment #16 from manu at gcc dot gnu dot org  2009-09-05 14:05 -------
*** Bug 41274 has been marked as a duplicate of this bug. ***


-- 

manu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ashaduri at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40752

[Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type

[manu at gcc dot gnu dot org]

------- Comment #15 from manu at gcc dot gnu dot org  2009-07-23 15:35 -------
Patch: http://gcc.gnu.org/ml/gcc-patches/2009-07/msg01179.html


-- 

manu at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                URL|                            |http://gcc.gnu.org/ml/gcc-
                   |                            |patches/2009-
                   |                            |07/msg01179.html


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40752

[Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type

[photon at seznam dot cz]

-- 

photon at seznam dot cz changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|enhancement                 |normal
            Summary|-Wconversion: do not warn   |-Wconversion generates false
                   |for operands not larger than|warnings for operands not
                   |target type                 |larger than target type


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40752