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* [Bug c++/108342] New: std::complex: ignoring packed attribute because of unpacked non-POD field
@ 2023-01-09 12:57 ruilvo at ua dot pt
2023-01-09 16:08 ` [Bug c++/108342] " pinskia at gcc dot gnu.org
` (11 more replies)
0 siblings, 12 replies; 13+ messages in thread
From: ruilvo at ua dot pt @ 2023-01-09 12:57 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108342
Bug ID: 108342
Summary: std::complex: ignoring packed attribute because of
unpacked non-POD field
Product: gcc
Version: 12.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ruilvo at ua dot pt
Target Milestone: ---
Created attachment 54217
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54217&action=edit
Code to reproduce the bug report
Summary:
With the annexed code, reproduced on the following CE link:
https://godbolt.org/z/1q9a5Mq7r
I get the following warning:
```
<source>:29:16: warning: ignoring packed attribute because of unpacked non-POD
field 'fcomplex_t <unnamed struct>::bb_iq_samples [512]'
29 | fcomplex_t bb_iq_samples[BB_FRAME_IQ_SAMPLES_COUNT];
| ^~~~~~~~~~~~~
```
Clang doesn't complain facing the same code.
Details:
I was trying to write a C++ library, which is going to target an ARM Linux
system, but leaving C-compatible "bindings" and structs for using with CFFI on
other languages.
The library deals with a serial stream that I don't control, and that I wanted
to de-serialize, thus the weird structure.
I discovered the problem (warning) when I changed from a C source file and
`_Complex float` to C++ and `std::complex<float>`.
The compiler I first discovered the problem on was arm-linux-gnueabihf-g++
(GCC) 12.2.1 20221203 [releases/gcc-12 revision
c03cb4b762aceeba95da918b042583af0d9f6030] from Linaro.
The bug is reproducible on x86 and mainline GCC (see Compiler Explorer URL)
Expected behaviour:
For this code to compile without warnings and producing the desired effect
(packing). The C++ standard even carves out a guarantee than `_Complex
[float|double]` is memory-layout-compatible with
`std::complex<[float|double]>`.
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug c++/108342] std::complex: ignoring packed attribute because of unpacked non-POD field
2023-01-09 12:57 [Bug c++/108342] New: std::complex: ignoring packed attribute because of unpacked non-POD field ruilvo at ua dot pt
@ 2023-01-09 16:08 ` pinskia at gcc dot gnu.org
2023-01-09 16:13 ` [Bug libstdc++/108342] " pinskia at gcc dot gnu.org
` (10 subsequent siblings)
11 siblings, 0 replies; 13+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-01-09 16:08 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108342
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
>The C++ standard even carves out a guarantee than `_Complex [float|double]` is memory-layout-compatible with `std::complex<[float|double]>`.
I know about _Atomic and std::atomic but not std::complex and _Complex. Because
std::complex was part of C++98 which was done before C99's _Complex ...
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug libstdc++/108342] std::complex: ignoring packed attribute because of unpacked non-POD field
2023-01-09 12:57 [Bug c++/108342] New: std::complex: ignoring packed attribute because of unpacked non-POD field ruilvo at ua dot pt
2023-01-09 16:08 ` [Bug c++/108342] " pinskia at gcc dot gnu.org
@ 2023-01-09 16:13 ` pinskia at gcc dot gnu.org
2023-01-09 16:14 ` ruilvo at ua dot pt
` (9 subsequent siblings)
11 siblings, 0 replies; 13+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-01-09 16:13 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108342
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Hmm: diff.cpp03.numerics
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug libstdc++/108342] std::complex: ignoring packed attribute because of unpacked non-POD field
2023-01-09 12:57 [Bug c++/108342] New: std::complex: ignoring packed attribute because of unpacked non-POD field ruilvo at ua dot pt
2023-01-09 16:08 ` [Bug c++/108342] " pinskia at gcc dot gnu.org
2023-01-09 16:13 ` [Bug libstdc++/108342] " pinskia at gcc dot gnu.org
@ 2023-01-09 16:14 ` ruilvo at ua dot pt
2023-01-09 16:16 ` ruilvo at ua dot pt
` (8 subsequent siblings)
11 siblings, 0 replies; 13+ messages in thread
From: ruilvo at ua dot pt @ 2023-01-09 16:14 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108342
--- Comment #3 from Rui Oliveira <ruilvo at ua dot pt> ---
(In reply to Andrew Pinski from comment #1)
>
> I know about _Atomic and std::atomic but not std::complex and _Complex.
> Because std::complex was part of C++98 which was done before C99's _Complex
> ...
[complex.numbers.general] mentions:
If z is an lvalue of type cv complex<T> then:
the expression reinterpret_cast<cv T(&)[2]>(z) is well-formed,
https://eel.is/c++draft/complex.numbers.general
Basically stating that std::complex<T> is layouted exactly the same as T[2].
C standard says something similar iirc.
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug libstdc++/108342] std::complex: ignoring packed attribute because of unpacked non-POD field
2023-01-09 12:57 [Bug c++/108342] New: std::complex: ignoring packed attribute because of unpacked non-POD field ruilvo at ua dot pt
` (2 preceding siblings ...)
2023-01-09 16:14 ` ruilvo at ua dot pt
@ 2023-01-09 16:16 ` ruilvo at ua dot pt
2023-01-09 16:17 ` redi at gcc dot gnu.org
` (7 subsequent siblings)
11 siblings, 0 replies; 13+ messages in thread
From: ruilvo at ua dot pt @ 2023-01-09 16:16 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108342
--- Comment #4 from Rui Oliveira <ruilvo at ua dot pt> ---
(In reply to Andrew Pinski from comment #2)
> Hmm: diff.cpp03.numerics
I saw you moved the bug to libstdc++ but is the problem libstdc++, or should
g++ just accept packing when it encounters it?
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug libstdc++/108342] std::complex: ignoring packed attribute because of unpacked non-POD field
2023-01-09 12:57 [Bug c++/108342] New: std::complex: ignoring packed attribute because of unpacked non-POD field ruilvo at ua dot pt
` (3 preceding siblings ...)
2023-01-09 16:16 ` ruilvo at ua dot pt
@ 2023-01-09 16:17 ` redi at gcc dot gnu.org
2023-01-09 16:28 ` [Bug c++/108342] " pinskia at gcc dot gnu.org
` (6 subsequent siblings)
11 siblings, 0 replies; 13+ messages in thread
From: redi at gcc dot gnu.org @ 2023-01-09 16:17 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108342
--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I don't think there's anything the library can do here. The layout of
std::complex is fixed, as stated above. And the fact it's a non-POD is also
fixed.
If the front-end warns about trying to pack a non-POD, then the library can't
stop it warning about std::complex<float>.
I don't know whether clang allows packing non-PODs, or just doesn't ever warn
for them, or has a special case for std::complex, or does something smarter
like not warn for types that have no padding bytes anyway (so that packing them
would be a no-op).
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug c++/108342] std::complex: ignoring packed attribute because of unpacked non-POD field
2023-01-09 12:57 [Bug c++/108342] New: std::complex: ignoring packed attribute because of unpacked non-POD field ruilvo at ua dot pt
` (4 preceding siblings ...)
2023-01-09 16:17 ` redi at gcc dot gnu.org
@ 2023-01-09 16:28 ` pinskia at gcc dot gnu.org
2023-01-09 16:31 ` pinskia at gcc dot gnu.org
` (5 subsequent siblings)
11 siblings, 0 replies; 13+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-01-09 16:28 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108342
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Component|libstdc++ |c++
--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
clang:
typedef struct __attribute__((__packed__))
{
/** Every frame starts with BB_FRAME_MAGIC. */
uint32_t magic;
unsigned char t;
fcomplex_t bb_iq_samples[BB_FRAME_IQ_SAMPLES_COUNT];
} bb_frame_t;
int t = sizeof(bb_frame_t);
t:
.long 4101 # 0x1005
.size t, 4
it might be the case clang does not implement the ABI ....
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug c++/108342] std::complex: ignoring packed attribute because of unpacked non-POD field
2023-01-09 12:57 [Bug c++/108342] New: std::complex: ignoring packed attribute because of unpacked non-POD field ruilvo at ua dot pt
` (5 preceding siblings ...)
2023-01-09 16:28 ` [Bug c++/108342] " pinskia at gcc dot gnu.org
@ 2023-01-09 16:31 ` pinskia at gcc dot gnu.org
2023-01-09 16:37 ` pinskia at gcc dot gnu.org
` (4 subsequent siblings)
11 siblings, 0 replies; 13+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-01-09 16:31 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108342
--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #5)
> I don't know whether clang allows packing non-PODs, or just doesn't ever
> warn for them, or has a special case for std::complex, or does something
> smarter like not warn for types that have no padding bytes anyway (so that
> packing them would be a no-op).
clang allows packing of non-PODs:
```
class f
{
public:
f();
private:
int t[2];
};
typedef struct __attribute__((__packed__))
{
/** Every frame starts with BB_FRAME_MAGIC. */
unsigned magic;
unsigned char t;
f bb_iq_samples[100];
} bb_frame_t;
```
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug c++/108342] std::complex: ignoring packed attribute because of unpacked non-POD field
2023-01-09 12:57 [Bug c++/108342] New: std::complex: ignoring packed attribute because of unpacked non-POD field ruilvo at ua dot pt
` (6 preceding siblings ...)
2023-01-09 16:31 ` pinskia at gcc dot gnu.org
@ 2023-01-09 16:37 ` pinskia at gcc dot gnu.org
2023-01-09 16:39 ` pinskia at gcc dot gnu.org
` (3 subsequent siblings)
11 siblings, 0 replies; 13+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-01-09 16:37 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108342
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
See Also| |https://gcc.gnu.org/bugzill
| |a/show_bug.cgi?id=13983,
| |https://gcc.gnu.org/bugzill
| |a/show_bug.cgi?id=17519
--- Comment #8 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
r0-51814-ge0d1297c4320ae added the warning and started to ignore the packed
(back in 2003).
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug c++/108342] std::complex: ignoring packed attribute because of unpacked non-POD field
2023-01-09 12:57 [Bug c++/108342] New: std::complex: ignoring packed attribute because of unpacked non-POD field ruilvo at ua dot pt
` (7 preceding siblings ...)
2023-01-09 16:37 ` pinskia at gcc dot gnu.org
@ 2023-01-09 16:39 ` pinskia at gcc dot gnu.org
2023-01-09 21:23 ` ruilvo at ua dot pt
` (2 subsequent siblings)
11 siblings, 0 replies; 13+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-01-09 16:39 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108342
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |INVALID
Status|UNCONFIRMED |RESOLVED
--- Comment #9 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
https://gcc.gnu.org/pipermail/gcc-patches/2003-July/110087.html
This was done on purpose. GCC also rejects References too while clang accepts
that (but I am not sure it gives the correct code for references of packed
either which is why GCC started to reject it).
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug c++/108342] std::complex: ignoring packed attribute because of unpacked non-POD field
2023-01-09 12:57 [Bug c++/108342] New: std::complex: ignoring packed attribute because of unpacked non-POD field ruilvo at ua dot pt
` (8 preceding siblings ...)
2023-01-09 16:39 ` pinskia at gcc dot gnu.org
@ 2023-01-09 21:23 ` ruilvo at ua dot pt
2023-01-09 21:34 ` jakub at gcc dot gnu.org
2023-01-09 21:40 ` ruilvo at ua dot pt
11 siblings, 0 replies; 13+ messages in thread
From: ruilvo at ua dot pt @ 2023-01-09 21:23 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108342
--- Comment #10 from Rui Oliveira <ruilvo at ua dot pt> ---
So my options are to create like a placeholder, say
```c
typedef struct __attribute__((__packed__)) // Packed isn't really necessary
here I think?
{
float re, im;
} ph_fcomplex_t
```
To silence the warning and get packing to work, and trust
[complex.numbers.general] for a reinterpret_cast into std::complex<float> I
guess.
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug c++/108342] std::complex: ignoring packed attribute because of unpacked non-POD field
2023-01-09 12:57 [Bug c++/108342] New: std::complex: ignoring packed attribute because of unpacked non-POD field ruilvo at ua dot pt
` (9 preceding siblings ...)
2023-01-09 21:23 ` ruilvo at ua dot pt
@ 2023-01-09 21:34 ` jakub at gcc dot gnu.org
2023-01-09 21:40 ` ruilvo at ua dot pt
11 siblings, 0 replies; 13+ messages in thread
From: jakub at gcc dot gnu.org @ 2023-01-09 21:34 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108342
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |jakub at gcc dot gnu.org
--- Comment #11 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Rui Oliveira from comment #10)
> So my options are to create like a placeholder, say
>
> ```c
> typedef struct __attribute__((__packed__)) // Packed isn't really necessary
> here I think?
> {
> float re, im;
> } ph_fcomplex_t
>
> ```
>
> To silence the warning and get packing to work, and trust
> [complex.numbers.general] for a reinterpret_cast into std::complex<float> I
> guess.
No, if you have the packed ph_fcomplex_t not aligned at alignof (float), you
need
to copy it to a properly aligned variable before trying to reinterpret_cast it.
^ permalink raw reply [flat|nested] 13+ messages in thread
* [Bug c++/108342] std::complex: ignoring packed attribute because of unpacked non-POD field
2023-01-09 12:57 [Bug c++/108342] New: std::complex: ignoring packed attribute because of unpacked non-POD field ruilvo at ua dot pt
` (10 preceding siblings ...)
2023-01-09 21:34 ` jakub at gcc dot gnu.org
@ 2023-01-09 21:40 ` ruilvo at ua dot pt
11 siblings, 0 replies; 13+ messages in thread
From: ruilvo at ua dot pt @ 2023-01-09 21:40 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108342
--- Comment #12 from Rui Oliveira <ruilvo at ua dot pt> ---
(In reply to Jakub Jelinek from comment #11)
> No, if you have the packed ph_fcomplex_t not aligned at alignof (float), you
> need
> to copy it to a properly aligned variable before trying to reinterpret_cast
> it.
Some `if constexpr` comparing of the remainder between
alignof(std::complex<float>) and (alignof(bb_frame_t) +
offsetof(bb_iq_samples)) could perhaps make one avoid that. But that's just a
side idea to think of.
Main point is, the code is de-serializing a serial stream. I do not expect to
find the "magic" word at the right aligment for `bb_frame_t` anyway, so
generous memcpy'ing to properly aligned variables will be required anyway.
^ permalink raw reply [flat|nested] 13+ messages in thread
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2023-01-09 12:57 [Bug c++/108342] New: std::complex: ignoring packed attribute because of unpacked non-POD field ruilvo at ua dot pt
2023-01-09 16:08 ` [Bug c++/108342] " pinskia at gcc dot gnu.org
2023-01-09 16:13 ` [Bug libstdc++/108342] " pinskia at gcc dot gnu.org
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2023-01-09 16:16 ` ruilvo at ua dot pt
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