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Date: Mon, 26 Oct 2009 09:28:21 +0100 From: Andi Kleen <andi@...stfloor.org> To: "H. Peter Anvin" <hpa@...or.com> Cc: Eric Dumazet <eric.dumazet@...il.com>, Jan Beulich <JBeulich@...ell.com>, mingo@...e.hu, tglx@...utronix.de, linux-kernel@...r.kernel.org Subject: Re: [PATCH] x86-64: adjust ia32entry branches for better static prediction "H. Peter Anvin" <hpa@...or.com> writes: > On 10/20/2009 04:21 PM, Eric Dumazet wrote: >>> >>> --- linux-2.6.32-rc5/arch/x86/ia32/ia32entry.S 2009-10-19 13:12:43.000000000 +0200 >>> +++ 2.6.32-rc5-x86_64-ia32-syscall-trace-branch-dir/arch/x86/ia32/ia32entry.S 2009-10-19 09:36:39.000000000 +0200 >>> @@ -249,9 +249,9 @@ sysenter_tracesys: >>> call syscall_trace_enter >>> LOAD_ARGS32 ARGOFFSET /* reload args from stack in case ptrace changed it */ >>> RESTORE_REST >>> - cmpl $(IA32_NR_syscalls-1),%eax >>> - ja int_ret_from_sys_call /* sysenter_tracesys has set RAX(%rsp) */ >>> - jmp sysenter_do_call >>> + cmpl $IA32_NR_syscalls,%eax >>> + jb sysenter_do_call >>> + jmp int_ret_from_sys_call /* sysenter_tracesys has set RAX(%rsp) */ >>> CFI_ENDPROC >> >> on cmov enabled build, we could use cmov instruction to load eax with number >> that calls enosys_syscall(), to avoid loading -ENOSYS,RAX >> and these branches... >> > > It's x86-64, cmov is guaranteed. But it's also not necessarily faster. -Andi -- ak@...ux.intel.com -- Speaking for myself only. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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