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    SubjectRe: [PATCH] x86-64: adjust ia32entry branches for better static prediction
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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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