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* [Bug c/95213] New: GCC -Werror=conversion error when assigning to a bitfield (when mixing constants and variables)
@ 2020-05-19 14:59 in-gcc at baka dot org
2020-05-19 15:01 ` [Bug c/95213] " in-gcc at baka dot org
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: in-gcc at baka dot org @ 2020-05-19 14:59 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95213
Bug ID: 95213
Summary: GCC -Werror=conversion error when assigning to a
bitfield (when mixing constants and variables)
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: in-gcc at baka dot org
Target Milestone: ---
When trying to assign to a bitfield with -Werror=conversion, having a constant
mix with a variable causes a "conversion from 'unsigned int' to 'unsigned
int:24' may change value", where as alternate formulations with two constants
or two variables work fine. This error seems to be present on every version of
gcc ever (the ones I tried since 4.9.1, at least one in each major version, all
did the same thing).
Watch it fail on godbolt: https://gcc.godbolt.org/z/vavr-e
struct foo { unsigned int a:8; unsigned int b:24; };
void bar(struct foo num, unsigned int x) {
num.b = (5U << 1) | (1 & 1);
unsigned int z = 5U;
num.b = ((z << 1) | (x & 1)) & 0xffffffU;
num.b = ((5U << 1) | (x & 1)) & 0xffffffU;
num.a = (unsigned char)x;
}
In the above example, the last assignment of num.b fails to compile where the
other two succeed. Note in the last example, casting the bit (x&1) to _Bool
will fix the problem, but is obviously not portable to larger expressions (e.g.
x&7).
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/95213] GCC -Werror=conversion error when assigning to a bitfield (when mixing constants and variables)
2020-05-19 14:59 [Bug c/95213] New: GCC -Werror=conversion error when assigning to a bitfield (when mixing constants and variables) in-gcc at baka dot org
@ 2020-05-19 15:01 ` in-gcc at baka dot org
2020-05-20 21:56 ` egallager at gcc dot gnu.org
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: in-gcc at baka dot org @ 2020-05-19 15:01 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95213
--- Comment #1 from Seth Robertson <in-gcc at baka dot org> ---
FYI, discussion on
https://stackoverflow.com/questions/61877799/son-of-gcc-conversion-warning-when-assigning-to-a-bitfield
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/95213] GCC -Werror=conversion error when assigning to a bitfield (when mixing constants and variables)
2020-05-19 14:59 [Bug c/95213] New: GCC -Werror=conversion error when assigning to a bitfield (when mixing constants and variables) in-gcc at baka dot org
2020-05-19 15:01 ` [Bug c/95213] " in-gcc at baka dot org
@ 2020-05-20 21:56 ` egallager at gcc dot gnu.org
2020-05-20 23:16 ` in-gcc at baka dot org
2022-03-02 2:39 ` egallager at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: egallager at gcc dot gnu.org @ 2020-05-20 21:56 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95213
Eric Gallager <egallager at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |DUPLICATE
Status|UNCONFIRMED |RESOLVED
CC| |egallager at gcc dot gnu.org
--- Comment #2 from Eric Gallager <egallager at gcc dot gnu.org> ---
dup of bug 39170
*** This bug has been marked as a duplicate of bug 39170 ***
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/95213] GCC -Werror=conversion error when assigning to a bitfield (when mixing constants and variables)
2020-05-19 14:59 [Bug c/95213] New: GCC -Werror=conversion error when assigning to a bitfield (when mixing constants and variables) in-gcc at baka dot org
2020-05-19 15:01 ` [Bug c/95213] " in-gcc at baka dot org
2020-05-20 21:56 ` egallager at gcc dot gnu.org
@ 2020-05-20 23:16 ` in-gcc at baka dot org
2022-03-02 2:39 ` egallager at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: in-gcc at baka dot org @ 2020-05-20 23:16 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95213
Seth Robertson <in-gcc at baka dot org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|RESOLVED |UNCONFIRMED
Resolution|DUPLICATE |---
--- Comment #3 from Seth Robertson <in-gcc at baka dot org> ---
I disagree that this the same bug as 39170. I know how to disable these
warning entirely. I know how to disable them just for the instance it is
wrong. However, in this bug, under some circumstances gcc is incorrectly
claiming that there is a conversion error where essentially isomorphic code is
thought just fine.
Thank you for your consideration.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/95213] GCC -Werror=conversion error when assigning to a bitfield (when mixing constants and variables)
2020-05-19 14:59 [Bug c/95213] New: GCC -Werror=conversion error when assigning to a bitfield (when mixing constants and variables) in-gcc at baka dot org
` (2 preceding siblings ...)
2020-05-20 23:16 ` in-gcc at baka dot org
@ 2022-03-02 2:39 ` egallager at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: egallager at gcc dot gnu.org @ 2022-03-02 2:39 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95213
Eric Gallager <egallager at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
See Also| |https://gcc.gnu.org/bugzill
| |a/show_bug.cgi?id=39170
--- Comment #4 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to Seth Robertson from comment #3)
> I disagree that this the same bug as 39170. I know how to disable these
> warning entirely. I know how to disable them just for the instance it is
> wrong. However, in this bug, under some circumstances gcc is incorrectly
> claiming that there is a conversion error where essentially isomorphic code
> is thought just fine.
>
> Thank you for your consideration.
ok, well I still think it's at least *related*, even if it isn't exactly the
same...
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2022-03-02 2:39 UTC | newest]
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2020-05-19 14:59 [Bug c/95213] New: GCC -Werror=conversion error when assigning to a bitfield (when mixing constants and variables) in-gcc at baka dot org
2020-05-19 15:01 ` [Bug c/95213] " in-gcc at baka dot org
2020-05-20 21:56 ` egallager at gcc dot gnu.org
2020-05-20 23:16 ` in-gcc at baka dot org
2022-03-02 2:39 ` egallager at gcc dot gnu.org
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