[RFC][PATCH] x86,ftrace: Shrink ftrace_regs_caller() by one byte

[Peter Zijlstra]

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@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
---
 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