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* [gcc r14-1686] libatomic: x86_64: Always try ifunc
@ 2023-06-10 13:45 Xi Ruoyao
0 siblings, 0 replies; only message in thread
From: Xi Ruoyao @ 2023-06-10 13:45 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:db80262475d28ae074a2cda7a1a6c2ea525791b9
commit r14-1686-gdb80262475d28ae074a2cda7a1a6c2ea525791b9
Author: Xi Ruoyao <xry111@xry111.site>
Date: Sat Jun 3 02:25:44 2023 +0800
libatomic: x86_64: Always try ifunc
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.
libatomic/ChangeLog:
* configure.tgt: For x86_64, always set try_ifunc=yes.
Diff:
---
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
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