[PATCH] libcpp: Use [[likely]] conditionally

[PATCH] libcpp: Use [[likely]] conditionally

[Marek Polacek]

Let's hide [[likely]] behind a macro, to suppress warnings if the
compiler doesn't support it.

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

	PR preprocessor/103355

libcpp/ChangeLog:

	* lex.c: Use ATTR_LIKELY instead of [[likely]].
	* system.h (ATTR_LIKELY): Define.
---
 libcpp/lex.c    |  2 +-
 libcpp/system.h | 10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/libcpp/lex.c b/libcpp/lex.c
index 94c36f0d014..9c27d8b5a08 100644
--- a/libcpp/lex.c
+++ b/libcpp/lex.c
@@ -1286,7 +1286,7 @@ namespace bidi {
       case kind::RTL:
 	/* These aren't popped by a PDF/PDI.  */
 	break;
-      [[likely]] case kind::NONE:
+      ATTR_LIKELY case kind::NONE:
 	break;
       default:
 	abort ();
diff --git a/libcpp/system.h b/libcpp/system.h
index ee5fbe28889..f6fc583ab80 100644
--- a/libcpp/system.h
+++ b/libcpp/system.h
@@ -422,6 +422,16 @@ extern void fancy_abort (const char *, int, const char *) ATTRIBUTE_NORETURN;
 #define gcc_checking_assert(EXPR) ((void)(0 && (EXPR)))
 #endif
 
+#ifdef __has_cpp_attribute
+# if __has_cpp_attribute(likely)
+#  define ATTR_LIKELY [[likely]]
+# elif __has_cpp_attribute(__likely__)
+#  define ATTR_LIKELY [[__likely__]]
+# else
+#  define ATTR_LIKELY
+# endif
+#endif
+
 /* Poison identifiers we do not want to use.  */
 #if (GCC_VERSION >= 3000)
 #undef calloc

base-commit: 1aedb3920a45bfe75db4514502b4e7f83e108f63
-- 
2.33.1

Re: [PATCH] libcpp: Use [[likely]] conditionally

[Jeff Law]

On 11/22/2021 10:22 AM, Marek Polacek via Gcc-patches wrote:
> Let's hide [[likely]] behind a macro, to suppress warnings if the
> compiler doesn't support it.
>
> Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> 	PR preprocessor/103355
>
> libcpp/ChangeLog:
>
> 	* lex.c: Use ATTR_LIKELY instead of [[likely]].
> 	* system.h (ATTR_LIKELY): Define.
OK
jeff

Re: [PATCH] libcpp: Use [[likely]] conditionally

[Christophe LYON]

Hi!

On 23/11/2021 01:26, Jeff Law via Gcc-patches wrote:
>
>
> On 11/22/2021 10:22 AM, Marek Polacek via Gcc-patches wrote:
>> Let's hide [[likely]] behind a macro, to suppress warnings if the
>> compiler doesn't support it.
>>
>> Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
>>
>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>>
>>     PR preprocessor/103355
>>
>> libcpp/ChangeLog:
>>
>>     * lex.c: Use ATTR_LIKELY instead of [[likely]].
>>     * system.h (ATTR_LIKELY): Define.
> OK
> jeff


This patch breaks the build when the host compiler is gcc-4.8.5, because 
__has_cpp_attribute is not defined.

Is this small patch OK with a proper ChangeLog?


diff --git a/libcpp/system.h b/libcpp/system.h
index f6fc583ab80..b78ab813d2f 100644
--- a/libcpp/system.h
+++ b/libcpp/system.h
@@ -430,6 +430,8 @@ extern void fancy_abort (const char *, int, const 
char *) ATTRIBUTE_NORETURN;
  # else
  #  define ATTR_LIKELY
  # endif
+#else
+# define ATTR_LIKELY
  #endif

  /* Poison identifiers we do not want to use.  */


Thanks,


Christophe

Re: [PATCH] libcpp: Use [[likely]] conditionally

[Marek Polacek]

On Tue, Nov 23, 2021 at 04:26:19PM +0100, Christophe LYON via Gcc-patches wrote:
> Hi!
> 
> On 23/11/2021 01:26, Jeff Law via Gcc-patches wrote:
> > 
> > 
> > On 11/22/2021 10:22 AM, Marek Polacek via Gcc-patches wrote:
> > > Let's hide [[likely]] behind a macro, to suppress warnings if the
> > > compiler doesn't support it.
> > > 
> > > Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
> > > 
> > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > > 
> > >     PR preprocessor/103355
> > > 
> > > libcpp/ChangeLog:
> > > 
> > >     * lex.c: Use ATTR_LIKELY instead of [[likely]].
> > >     * system.h (ATTR_LIKELY): Define.
> > OK
> > jeff
> 
> 
> This patch breaks the build when the host compiler is gcc-4.8.5, because
> __has_cpp_attribute is not defined.
 
Ah, of course.

> Is this small patch OK with a proper ChangeLog?

Yes, please.

> diff --git a/libcpp/system.h b/libcpp/system.h
> index f6fc583ab80..b78ab813d2f 100644
> --- a/libcpp/system.h
> +++ b/libcpp/system.h
> @@ -430,6 +430,8 @@ extern void fancy_abort (const char *, int, const char
> *) ATTRIBUTE_NORETURN;
>  # else
>  #  define ATTR_LIKELY
>  # endif
> +#else
> +# define ATTR_LIKELY
>  #endif
> 
>  /* Poison identifiers we do not want to use.  */
> 
> 
> Thanks,
> 
> 
> Christophe
> 
> 
> 

Marek

Re: [PATCH] libcpp: Use [[likely]] conditionally

[Jeff Law]

On 11/23/2021 8:26 AM, Christophe LYON via Gcc-patches wrote:
> Hi!
>
> On 23/11/2021 01:26, Jeff Law via Gcc-patches wrote:
>>
>>
>> On 11/22/2021 10:22 AM, Marek Polacek via Gcc-patches wrote:
>>> Let's hide [[likely]] behind a macro, to suppress warnings if the
>>> compiler doesn't support it.
>>>
>>> Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
>>>
>>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>>>
>>>     PR preprocessor/103355
>>>
>>> libcpp/ChangeLog:
>>>
>>>     * lex.c: Use ATTR_LIKELY instead of [[likely]].
>>>     * system.h (ATTR_LIKELY): Define.
>> OK
>> jeff
>
>
> This patch breaks the build when the host compiler is gcc-4.8.5, 
> because __has_cpp_attribute is not defined.
Sigh.  I'd like to move to a more recent prereq if we could.


>
> Is this small patch OK with a proper ChangeLog?
Yes.  Sorry about the breakage.
jeff

Re: [PATCH] libcpp: Use [[likely]] conditionally

[Christophe Lyon]

On Tue, Nov 23, 2021 at 4:41 PM Jeff Law via Gcc-patches <
gcc-patches@gcc.gnu.org> wrote:

>
>
> On 11/23/2021 8:26 AM, Christophe LYON via Gcc-patches wrote:
> > Hi!
> >
> > On 23/11/2021 01:26, Jeff Law via Gcc-patches wrote:
> >>
> >>
> >> On 11/22/2021 10:22 AM, Marek Polacek via Gcc-patches wrote:
> >>> Let's hide [[likely]] behind a macro, to suppress warnings if the
> >>> compiler doesn't support it.
> >>>
> >>> Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
> >>>
> >>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> >>>
> >>>     PR preprocessor/103355
> >>>
> >>> libcpp/ChangeLog:
> >>>
> >>>     * lex.c: Use ATTR_LIKELY instead of [[likely]].
> >>>     * system.h (ATTR_LIKELY): Define.
> >> OK
> >> jeff
> >
> >
> > This patch breaks the build when the host compiler is gcc-4.8.5,
> > because __has_cpp_attribute is not defined.
> Sigh.  I'd like to move to a more recent prereq if we could.
>

I don't know why we have such an old dependency indeed.
I am not requesting it, I just happen to have an old enough host
compiler so that I can check/complain when we accidentally
break the dependency :-)

Christophe



>
>
> >
> > Is this small patch OK with a proper ChangeLog?
> Yes.  Sorry about the breakage.
> jeff
>
>
>

Re: [PATCH] libcpp: Use [[likely]] conditionally

[Jakub Jelinek]

On Tue, Nov 23, 2021 at 09:34:04PM +0100, Christophe Lyon via Gcc-patches wrote:
> > > This patch breaks the build when the host compiler is gcc-4.8.5,
> > > because __has_cpp_attribute is not defined.
> > Sigh.  I'd like to move to a more recent prereq if we could.
> >
> 
> I don't know why we have such an old dependency indeed.
> I am not requesting it, I just happen to have an old enough host
> compiler so that I can check/complain when we accidentally
> break the dependency :-)

4.8.5 is still widely used and is the first one that supports C++11
reasonably well that it can be used.
__has_cpp_attribute has been added I think only in C++20, before that it was
in SD6, but even that is post C++11 I believe.
So provided we want to support C++11 (and IMHO we should, we can't afford to
be like Rust that can't build with a few days old compiler), we need to be
prepared that __has_cpp_attribute won't be defined.

	Jakub

Re: [PATCH] libcpp: Use [[likely]] conditionally

[Jeff Law]

On 11/23/2021 1:34 PM, Christophe Lyon wrote:
>
>
> On Tue, Nov 23, 2021 at 4:41 PM Jeff Law via Gcc-patches 
> <gcc-patches@gcc.gnu.org> wrote:
>
>
>
>     On 11/23/2021 8:26 AM, Christophe LYON via Gcc-patches wrote:
>     > Hi!
>     >
>     > On 23/11/2021 01:26, Jeff Law via Gcc-patches wrote:
>     >>
>     >>
>     >> On 11/22/2021 10:22 AM, Marek Polacek via Gcc-patches wrote:
>     >>> Let's hide [[likely]] behind a macro, to suppress warnings if the
>     >>> compiler doesn't support it.
>     >>>
>     >>> Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
>     >>>
>     >>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>     >>>
>     >>>     PR preprocessor/103355
>     >>>
>     >>> libcpp/ChangeLog:
>     >>>
>     >>>     * lex.c: Use ATTR_LIKELY instead of [[likely]].
>     >>>     * system.h (ATTR_LIKELY): Define.
>     >> OK
>     >> jeff
>     >
>     >
>     > This patch breaks the build when the host compiler is gcc-4.8.5,
>     > because __has_cpp_attribute is not defined.
>     Sigh.  I'd like to move to a more recent prereq if we could.
>
>
> I don't know why we have such an old dependency indeed.
> I am not requesting it, I just happen to have an old enough host
> compiler so that I can check/complain when we accidentally
> break the dependency :-)
Probably the enterprise distros.  I suspect we'll be able to roll 
forward in 2-3 years...

Jeff