[PATCH] libatomic: x86_64: Always try ifunc

[PATCH] libatomic: x86_64: Always try ifunc

[Xi Ruoyao]

We used to skip ifunc check when CX16 is available.  But now we use
CX16+AVX+Intel/AMD for the "perfect" 16b load implementation, so CX16
alone is not a sufficient reason not to use ifunc (see PR104688).

This causes a subtle and annoying issue: when GCC is built with a
higher -march= setting in CFLAGS_FOR_TARGET, ifunc is disabled and
the worst (locked) implementation of __atomic_load_16 is always used.

There seems no good way to check if the CPU is Intel or AMD from
the built-in macros (maybe we can check every known model like __skylake,
__bdver2, ..., but it will be very error-prune and require an update
whenever we add the support for a new x86 model).  The best thing we can
do seems "always try ifunc" here.

Bootstrapped and tested on x86_64-linux-gnu.  Ok for trunk?

libatomic/ChangeLog:

	* configure.tgt: For x86_64, always set try_ifunc=yes.
---
 libatomic/configure.tgt | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/libatomic/configure.tgt b/libatomic/configure.tgt
index a92ae9e8309..39dd5686f2e 100644
--- a/libatomic/configure.tgt
+++ b/libatomic/configure.tgt
@@ -100,9 +100,7 @@ EOF
 	fi
 	cat > conftestx.c <<EOF
 #ifdef __x86_64__
-#ifndef __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16
-#error need -mcx16
-#endif
+#error ifunc is always wanted for 16B atomic load
 #else
 #ifndef __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8
 #error need -march=i686
-- 
2.41.0

Re: [PATCH] libatomic: x86_64: Always try ifunc

[Bernhard Reutner-Fischer]

On 3 June 2023 13:25:32 CEST, Xi Ruoyao via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:

>There seems no good way to check if the CPU is Intel or AMD from
>the built-in macros (maybe we can check every known model like __skylake,
>__bdver2, ..., but it will be very error-prune and require an update
>whenever we add the support for a new x86 model).  The best thing we can
>do seems "always try ifunc" here.

IIRC there is __builtin_cpu_is (after initialisation) -- A couple of days ago, we wondered if it would be handy to lower that even in fortran without going through C, so i am pretty sure I don't make that up.. ;-)

Just a thought,

Re: [PATCH] libatomic: x86_64: Always try ifunc

[Xi Ruoyao]

On Sat, 2023-06-03 at 14:53 +0200, Bernhard Reutner-Fischer wrote:
> On 3 June 2023 13:25:32 CEST, Xi Ruoyao via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> 
> > There seems no good way to check if the CPU is Intel or AMD from
> > the built-in macros (maybe we can check every known model like
> > __skylake,
> > __bdver2, ..., but it will be very error-prune and require an update
> > whenever we add the support for a new x86 model).  The best thing we
> > can
> > do seems "always try ifunc" here.
> 
> IIRC there is __builtin_cpu_is (after initialisation) -- A couple of
> days ago, we wondered if it would be handy to lower that even in
> fortran without going through C, so i am pretty sure I don't make that
> up.. ;-)

Unfortunately __builtin_cpu_is performs CPU detection on runtime, not
compile time.

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University

Re: [PATCH] libatomic: x86_64: Always try ifunc

[Bernhard Reutner-Fischer]

On 3 June 2023 15:46:02 CEST, Xi Ruoyao <xry111@xry111.site> wrote:

>Unfortunately __builtin_cpu_is performs CPU detection on runtime, not
>compile time.

Right, you were talking about configure, sorry.

Ping: [PATCH] libatomic: x86_64: Always try ifunc

[Xi Ruoyao]

Ping (in hopes that someone can review before the weekend).

On Sat, 2023-06-03 at 19:25 +0800, Xi Ruoyao wrote:
> We used to skip ifunc check when CX16 is available.  But now we use
> CX16+AVX+Intel/AMD for the "perfect" 16b load implementation, so CX16
> alone is not a sufficient reason not to use ifunc (see PR104688).
> 
> This causes a subtle and annoying issue: when GCC is built with a
> higher -march= setting in CFLAGS_FOR_TARGET, ifunc is disabled and
> the worst (locked) implementation of __atomic_load_16 is always used.
> 
> There seems no good way to check if the CPU is Intel or AMD from
> the built-in macros (maybe we can check every known model like
> __skylake,
> __bdver2, ..., but it will be very error-prune and require an update
> whenever we add the support for a new x86 model).  The best thing we
> can
> do seems "always try ifunc" here.
> 
> Bootstrapped and tested on x86_64-linux-gnu.  Ok for trunk?
> 
> libatomic/ChangeLog:
> 
>         * configure.tgt: For x86_64, always set try_ifunc=yes.
> ---
>  libatomic/configure.tgt | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/libatomic/configure.tgt b/libatomic/configure.tgt
> index a92ae9e8309..39dd5686f2e 100644
> --- a/libatomic/configure.tgt
> +++ b/libatomic/configure.tgt
> @@ -100,9 +100,7 @@ EOF
>         fi
>         cat > conftestx.c <<EOF
>  #ifdef __x86_64__
> -#ifndef __GCC_HAVE_SYNC_COMPARE_AND_SWAP_16
> -#error need -mcx16
> -#endif
> +#error ifunc is always wanted for 16B atomic load
>  #else
>  #ifndef __GCC_HAVE_SYNC_COMPARE_AND_SWAP_8
>  #error need -march=i686

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University

Re: Ping: [PATCH] libatomic: x86_64: Always try ifunc

[Jakub Jelinek]

On Fri, Jun 09, 2023 at 08:37:20PM +0800, Xi Ruoyao wrote:
> Ping (in hopes that someone can review before the weekend).
> 
> On Sat, 2023-06-03 at 19:25 +0800, Xi Ruoyao wrote:
> > We used to skip ifunc check when CX16 is available.  But now we use
> > CX16+AVX+Intel/AMD for the "perfect" 16b load implementation, so CX16
> > alone is not a sufficient reason not to use ifunc (see PR104688).
> > 
> > This causes a subtle and annoying issue: when GCC is built with a
> > higher -march= setting in CFLAGS_FOR_TARGET, ifunc is disabled and
> > the worst (locked) implementation of __atomic_load_16 is always used.
> > 
> > There seems no good way to check if the CPU is Intel or AMD from
> > the built-in macros (maybe we can check every known model like
> > __skylake,
> > __bdver2, ..., but it will be very error-prune and require an update
> > whenever we add the support for a new x86 model).  The best thing we
> > can
> > do seems "always try ifunc" here.
> > 
> > Bootstrapped and tested on x86_64-linux-gnu.  Ok for trunk?
> > 
> > libatomic/ChangeLog:
> > 
> >         * configure.tgt: For x86_64, always set try_ifunc=yes.

Ok, thanks.

	Jakub