Thread: cpluspluscheck complains about use of register
Hi,
When running cpluspluscheck I get many many complaints like
In file included from /tmp/pg-test-repo/src/include/port/atomics.h:70,
from /tmp/pg-test-repo/src/include/utils/dsa.h:17,
from /tmp/pg-test-repo/src/include/nodes/tidbitmap.h:26,
from /tmp/pg-test-repo/src/include/nodes/execnodes.h:24,
from /tmp/pg-test-repo/src/include/commands/trigger.h:17,
from /tmp/pg-test-repo/src/pl/plpgsql/src/plpgsql.h:21,
from /tmp/cpluspluscheck.qOi18T/test.cpp:3:
/tmp/pg-test-repo/src/include/port/atomics/arch-x86.h: In function ‘bool pg_atomic_test_set_flag_impl(volatile
pg_atomic_flag*)’:
/tmp/pg-test-repo/src/include/port/atomics/arch-x86.h:143:23: warning: ISO C++17 does not allow ‘register’ storage
classspecifier [-Wregister]
143 | register char _res = 1;
| ^~~~
It seems we should just remove the use of register? It's currently only used
in
src/include/storage/s_lock.h
src/include/port/atomics/arch-x86.h
From what I understand compilers essentially have been ignoring it for quite a
while...
Greetings,
Andres Freund
Andres Freund <andres@anarazel.de> writes:
> When running cpluspluscheck I get many many complaints like
> /tmp/pg-test-repo/src/include/port/atomics/arch-x86.h:143:23: warning: ISO C++17 does not allow ‘register’ storage
classspecifier [-Wregister]
Interesting, I don't see that here.
> It seems we should just remove the use of register?
I have a vague idea that it was once important to say "register" if
you are going to use the variable in an asm snippet that requires it
to be in a register. That might be wrong, or it might be obsolete
even if once true. We could try taking these out and seeing if the
buildfarm complains. (If so, maybe -Wno-register would help?)
regards, tom lane
Hi, On 2022-03-08 13:46:36 -0500, Tom Lane wrote: > Andres Freund <andres@anarazel.de> writes: > > When running cpluspluscheck I get many many complaints like > > /tmp/pg-test-repo/src/include/port/atomics/arch-x86.h:143:23: warning: ISO C++17 does not allow ‘register’ storage classspecifier [-Wregister] > > Interesting, I don't see that here. Probably a question of the gcc version. I think starting with 11 g++ defaults to C++ 17. > > It seems we should just remove the use of register? > > I have a vague idea that it was once important to say "register" if > you are going to use the variable in an asm snippet that requires it > to be in a register. That might be wrong, or it might be obsolete > even if once true. We could try taking these out and seeing if the > buildfarm complains. We have several inline asm statements not using register despite using variables in a register (e.g. pg_atomic_compare_exchange_u32_impl()), so I wouldn't expect a problem with compilers we support. Should we make configure test for -Wregister? There's at least one additional use of register that we'd have to change (pg_regexec). > (If so, maybe -Wno-register would help?) That's what I did to work around the flood of warnings locally, so it'd work. Greetings, Andres Freund
>>> It seems we should just remove the use of register?
>>
>> I have a vague idea that it was once important to say "register" if
>> you are going to use the variable in an asm snippet that requires it
>> to be in a register. That might be wrong, or it might be obsolete
>> even if once true. We could try taking these out and seeing if the
>> buildfarm complains.
>
> We have several inline asm statements not using register despite using
> variables in a register (e.g. pg_atomic_compare_exchange_u32_impl()), so I
> wouldn't expect a problem with compilers we support.
>
> Should we make configure test for -Wregister? There's at least one additional
> use of register that we'd have to change (pg_regexec).
From a compilation perspective, "register" tells the compiler that you
cannot have a pointer on a variable, i.e. it generates an error if someone
adds something like:
void * p = ®ister_variable;
Removing the "register" declaration means that such protection would be
removed, and creating such a pointer could reduce drastically compiler
optimization opportunities.
--
Fabien.
Hi, On 2022-03-08 10:59:02 -0800, Andres Freund wrote: > On 2022-03-08 13:46:36 -0500, Tom Lane wrote: > > Andres Freund <andres@anarazel.de> writes: > > > When running cpluspluscheck I get many many complaints like > > > /tmp/pg-test-repo/src/include/port/atomics/arch-x86.h:143:23: warning: ISO C++17 does not allow ‘register’ storageclass specifier [-Wregister] > > > > Interesting, I don't see that here. > > Probably a question of the gcc version. I think starting with 11 g++ defaults > to C++ 17. > > > > > It seems we should just remove the use of register? > > > > I have a vague idea that it was once important to say "register" if > > you are going to use the variable in an asm snippet that requires it > > to be in a register. That might be wrong, or it might be obsolete > > even if once true. We could try taking these out and seeing if the > > buildfarm complains. > > We have several inline asm statements not using register despite using > variables in a register (e.g. pg_atomic_compare_exchange_u32_impl()), so I > wouldn't expect a problem with compilers we support. > > Should we make configure test for -Wregister? There's at least one additional > use of register that we'd have to change (pg_regexec). > > > > (If so, maybe -Wno-register would help?) > > That's what I did to work around the flood of warnings locally, so it'd > work. I hit this again while porting cplupluscheck to be invoked by meson as well. ISTM that we should just remove the uses of register. Yes, some very old compilers might generate worse code without register, but I don't think we need to care about peak efficiency with neolithic compilers. Fabien raised the concern that removing register might lead to accidentally adding pointers to such variables - I don't find that convincing, because a) such code is typically inside a helper inline anyway b) we don't use register widely enough to ensure this. Attached is a patch removing uses of register. The use in regexec.c could remain, since we only try to keep headers C++ clean. But there really doesn't seem to be a good reason to use register in that spot. I tried to use -Wregister to keep us honest going forward, but unfortunately it only works with a C++ compiler... I tested this by redefining register to something else, and I grepped for non-comment uses of register. Entirely possible that I missed something. Greetings, Andres Freund
Attachment
On Sat, Sep 24, 2022 at 12:11 PM Andres Freund <andres@anarazel.de> wrote: > I hit this again while porting cplupluscheck to be invoked by meson as > well. ISTM that we should just remove the uses of register. Yes, some very old > compilers might generate worse code without register, but I don't think we > need to care about peak efficiency with neolithic compilers. +1. I seem to recall reading that the register keyword was basically useless as long as 15 years ago. -- Peter Geoghegan
Andres Freund <andres@anarazel.de> writes:
> I hit this again while porting cplupluscheck to be invoked by meson as
> well. ISTM that we should just remove the uses of register.
OK by me.
> I tried to use -Wregister to keep us honest going forward, but unfortunately
> it only works with a C++ compiler...
I think we only really care about stuff that cpluspluscheck would spot,
so I don't feel a need to mess with the standard compilation flags.
regards, tom lane
Hi, On 2022-09-24 16:01:25 -0400, Tom Lane wrote: > Andres Freund <andres@anarazel.de> writes: > > I hit this again while porting cplupluscheck to be invoked by meson as > > well. ISTM that we should just remove the uses of register. > > OK by me. Done. Thanks Tom, Peter.
Re: Tom Lane > > I hit this again while porting cplupluscheck to be invoked by meson as > > well. ISTM that we should just remove the uses of register. > > OK by me. > > > I tried to use -Wregister to keep us honest going forward, but unfortunately > > it only works with a C++ compiler... > > I think we only really care about stuff that cpluspluscheck would spot, > so I don't feel a need to mess with the standard compilation flags. This has started to hurt: postgresql-debversion (a Debian version number data type written in C++) failed to build against Postgresql <= 15 on Ubuntu's next LTS release (24.04): In file included from /usr/include/postgresql/15/server/port/atomics.h:70: /usr/include/postgresql/15/server/port/atomics/arch-x86.h:143:2: error: ISO C++17 does not allow 'register' storage classspecifier [-Wregister] 143 | register char _res = 1; I managed to work around it by putting `#define register` before including the PG headers. Should the removal of "register" be backported to support that better? Christoph
Christoph Berg <myon@debian.org> writes:
> Should the removal of "register" be backported to support that better?
Perhaps. It's early days yet, but nobody has complained that that
broke anything in v16, so I'm guessing it'd be fine.
regards, tom lane