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

Ingo Molnar
2016-03-12T15:45:15Z

* 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