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Re: [PATCH] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs

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* Denys Vlasenko <dvlasenk@redhat.com> wrote:

> Use of a temporary R8 register here seems to be unnecessary.
> 
> "push %r8" is a two-byte insn (it needs REX prefix to specify R8),
> "push $0" is two-byte too. It seems just using the latter would be
> no worse.
> 
> Thus, code had an unnecessary "xorq %r8,%r8" insn.
> It probably costs nothing in execution time here since we are probably
> limited by store bandwidth at this point, but still.

Note that the 3 fewer instruction in the image also shrink the code by 16 bytes:

arch/x86/entry/entry_64_compat.o:

   text    data     bss     dec     hex filename
    380       0       0     380     17c entry_64_compat.o.before
    364       0       0     364     16c entry_64_compat.o.after

because (at least in this defconfig build) one of these functions shrunk below a 
16-byte boundary. So cache footprint got denser.

Thanks,

	Ingo

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[PATCH] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs Denys Vlasenko <dvlasenk@redhat.com> - 2016-03-11 18:00 +0100
  Re: [PATCH] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs Ingo Molnar <mingo@kernel.org> - 2016-03-12 16:40 +0100
    Re: [PATCH] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs Denys Vlasenko <dvlasenk@redhat.com> - 2016-03-12 19:00 +0100
      Re: [PATCH] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs Andy Lutomirski <luto@amacapital.net> - 2016-03-12 19:10 +0100
        Re: [PATCH] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs Denys Vlasenko <dvlasenk@redhat.com> - 2016-03-12 22:50 +0100
  Re: [PATCH] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs Ingo Molnar <mingo@kernel.org> - 2016-03-12 16:50 +0100

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