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Subject[RFC][PATCH] x86,ftrace: Shrink ftrace_regs_caller() by one byte
On Tue, Mar 31, 2020 at 11:31:36AM -0400, Steven Rostedt wrote:
> On Tue, 31 Mar 2020 13:16:52 +0200
> Peter Zijlstra <peterz@infradead.org> wrote:
>
> > @@ -235,8 +237,8 @@ SYM_INNER_LABEL(ftrace_regs_call, SYM_L_GLOBAL)
> >
> > /* If ORIG_RAX is anything but zero, make this a call to that */
> > movq ORIG_RAX(%rsp), %rax
> > - cmpq $0, %rax
> > - je 1f
> > + testq %rax, %rax
> > + jz 1f
> >
> > /* Swap the flags with orig_rax */
> > movq MCOUNT_REG_SIZE(%rsp), %rdi
>
> Hi Peter,
>
> Can you send this change as a separate patch as it has nothing to do with
> this current change, and is a clean up patch that stands on its own.

Sure. But then I have to like write a Changelog for it... :/

---
Subject: x86,ftrace: Shrink ftrace_regs_caller() by one byte

'Optimize' ftrace_regs_caller. Instead of comparing against an
immediate, the more natural way to test for zero on x86 is: 'test
%r,%r'.

48 83 f8 00 cmp $0x0,%rax
74 49 je 226 <ftrace_regs_call+0xa3>

48 85 c0 test %rax,%rax
74 49 je 225 <ftrace_regs_call+0xa2>

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/kernel/ftrace_64.S | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/ftrace_64.S b/arch/x86/kernel/ftrace_64.S
index 369e61faacfe..8e71c492d623 100644
--- a/arch/x86/kernel/ftrace_64.S
+++ b/arch/x86/kernel/ftrace_64.S
@@ -235,8 +235,8 @@ SYM_INNER_LABEL(ftrace_regs_call, SYM_L_GLOBAL)

/* If ORIG_RAX is anything but zero, make this a call to that */
movq ORIG_RAX(%rsp), %rax
- cmpq $0, %rax
- je 1f
+ testq %rax, %rax
+ jz 1f

/* Swap the flags with orig_rax */
movq MCOUNT_REG_SIZE(%rsp), %rdi
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