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

From: Ingo Molnar
Date: Sat Mar 12 2016 - 10:45:22 EST

* Denys Vlasenko <dvlasenk@xxxxxxxxxx> 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:


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.