* [Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type
[not found] <bug-40752-4@http.gcc.gnu.org/bugzilla/>
@ 2011-09-25 21:55 ` manu at gcc dot gnu.org
2012-03-24 20:21 ` manu at gcc dot gnu.org
` (6 subsequent siblings)
7 siblings, 0 replies; 13+ messages in thread
From: manu at gcc dot gnu.org @ 2011-09-25 21:55 UTC (permalink / raw)
To: gcc-bugs
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. ***
^ permalink raw reply [flat|nested] 13+ messages in thread* [Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type
[not found] <bug-40752-4@http.gcc.gnu.org/bugzilla/>
2011-09-25 21:55 ` [Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type manu at gcc dot gnu.org
@ 2012-03-24 20:21 ` manu at gcc dot gnu.org
2012-03-24 21:40 ` DeusExSophismata at gmail dot com
` (5 subsequent siblings)
7 siblings, 0 replies; 13+ messages in thread
From: manu at gcc dot gnu.org @ 2012-03-24 20:21 UTC (permalink / raw)
To: gcc-bugs
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. ***
^ permalink raw reply [flat|nested] 13+ messages in thread* [Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type
[not found] <bug-40752-4@http.gcc.gnu.org/bugzilla/>
2011-09-25 21:55 ` [Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type manu at gcc dot gnu.org
2012-03-24 20:21 ` manu at gcc dot gnu.org
@ 2012-03-24 21:40 ` DeusExSophismata at gmail dot com
2012-05-05 15:31 ` wessjunk at gmail dot com
` (4 subsequent siblings)
7 siblings, 0 replies; 13+ messages in thread
From: DeusExSophismata at gmail dot com @ 2012-03-24 21:40 UTC (permalink / raw)
To: gcc-bugs
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.
^ permalink raw reply [flat|nested] 13+ messages in thread* [Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type
[not found] <bug-40752-4@http.gcc.gnu.org/bugzilla/>
` (2 preceding siblings ...)
2012-03-24 21:40 ` DeusExSophismata at gmail dot com
@ 2012-05-05 15:31 ` wessjunk at gmail dot com
2012-05-05 16:05 ` manu at gcc dot gnu.org
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7 siblings, 0 replies; 13+ messages in thread
From: wessjunk at gmail dot com @ 2012-05-05 15:31 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40752
Wes <wessjunk at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
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.
^ permalink raw reply [flat|nested] 13+ messages in thread* [Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type
[not found] <bug-40752-4@http.gcc.gnu.org/bugzilla/>
` (3 preceding siblings ...)
2012-05-05 15:31 ` wessjunk at gmail dot com
@ 2012-05-05 16:05 ` manu at gcc dot gnu.org
2012-06-20 14:59 ` dichlofos-mv at yandex dot ru
` (2 subsequent siblings)
7 siblings, 0 replies; 13+ messages in thread
From: manu at gcc dot gnu.org @ 2012-05-05 16:05 UTC (permalink / raw)
To: gcc-bugs
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).
^ permalink raw reply [flat|nested] 13+ messages in thread* [Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type
[not found] <bug-40752-4@http.gcc.gnu.org/bugzilla/>
` (4 preceding siblings ...)
2012-05-05 16:05 ` manu at gcc dot gnu.org
@ 2012-06-20 14:59 ` dichlofos-mv at yandex dot ru
2014-02-10 1:14 ` chengniansun at gmail dot com
2014-09-16 21:07 ` hariharan.gcc at gmail dot com
7 siblings, 0 replies; 13+ messages in thread
From: dichlofos-mv at yandex dot ru @ 2012-06-20 14:59 UTC (permalink / raw)
To: gcc-bugs
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;
^ permalink raw reply [flat|nested] 13+ messages in thread* [Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type
[not found] <bug-40752-4@http.gcc.gnu.org/bugzilla/>
` (5 preceding siblings ...)
2012-06-20 14:59 ` dichlofos-mv at yandex dot ru
@ 2014-02-10 1:14 ` chengniansun at gmail dot com
2014-09-16 21:07 ` hariharan.gcc at gmail dot com
7 siblings, 0 replies; 13+ messages in thread
From: chengniansun at gmail dot com @ 2014-02-10 1:14 UTC (permalink / raw)
To: gcc-bugs
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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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--- 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.
^ permalink raw reply [flat|nested] 13+ messages in thread* [Bug c++/40752] -Wconversion generates false warnings for operands not larger than target type
[not found] <bug-40752-4@http.gcc.gnu.org/bugzilla/>
` (6 preceding siblings ...)
2014-02-10 1:14 ` chengniansun at gmail dot com
@ 2014-09-16 21:07 ` hariharan.gcc at gmail dot com
7 siblings, 0 replies; 13+ messages in thread
From: hariharan.gcc at gmail dot com @ 2014-09-16 21:07 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=40752
hariharan.gcc at gmail dot com changed:
What |Removed |Added
----------------------------------------------------------------------------
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.
^ permalink raw reply [flat|nested] 13+ messages in thread