Messages in this thread | | | Subject | Re: [PATCH] x86-64: adjust ia32entry branches for better static prediction | From | Andi Kleen <> | Date | Mon, 26 Oct 2009 09:28:21 +0100 |
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"H. Peter Anvin" <hpa@zytor.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@linux.intel.com -- Speaking for myself only.
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