linux-kernel - [PATCH] x86-64: adjust ia32entry branches for better static prediction

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Date:	Tue, 20 Oct 2009 08:10:04 +0100
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	<mingo@...e.hu>, <tglx@...utronix.de>, <hpa@...or.com>
Cc:	<linux-kernel@...r.kernel.org>
Subject: [PATCH] x86-64: adjust ia32entry branches for better static
	 prediction

Swap branch pairs so that the static branch prediction logic would
consider the syscall-number-in-range case the taken path.

Signed-off-by: Jan Beulich <jbeulich@...ell.com>

---
 arch/x86/ia32/ia32entry.S |   18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

--- linux-2.6.32-rc5/arch/x86/ia32/ia32entry.S	2009-10-19 13:12:43.000000000 +0200
+++ 2.6.32-rc5-x86_64-ia32-syscall-trace-branch-dir/arch/x86/ia32/ia32entry.S	2009-10-19 09:36:39.000000000 +0200
@@ -249,9 +249,9 @@ sysenter_tracesys:
 	call	syscall_trace_enter
 	LOAD_ARGS32 ARGOFFSET  /* reload args from stack in case ptrace changed it */
 	RESTORE_REST
-	cmpl	$(IA32_NR_syscalls-1),%eax
-	ja	int_ret_from_sys_call /* sysenter_tracesys has set RAX(%rsp) */
-	jmp	sysenter_do_call
+	cmpl	$IA32_NR_syscalls,%eax
+	jb	sysenter_do_call
+	jmp	int_ret_from_sys_call /* sysenter_tracesys has set RAX(%rsp) */
 	CFI_ENDPROC
 ENDPROC(ia32_sysenter_target)
 
@@ -368,9 +368,9 @@ cstar_tracesys:
 	LOAD_ARGS32 ARGOFFSET, 1  /* reload args from stack in case ptrace changed it */
 	RESTORE_REST
 	xchgl %ebp,%r9d
-	cmpl $(IA32_NR_syscalls-1),%eax
-	ja int_ret_from_sys_call /* cstar_tracesys has set RAX(%rsp) */
-	jmp cstar_do_call
+	cmpl $IA32_NR_syscalls,%eax
+	jb cstar_do_call
+	jmp int_ret_from_sys_call /* cstar_tracesys has set RAX(%rsp) */
 END(ia32_cstar_target)
 				
 ia32_badarg:
@@ -445,9 +445,9 @@ ia32_tracesys:			 
 	call syscall_trace_enter
 	LOAD_ARGS32 ARGOFFSET  /* reload args from stack in case ptrace changed it */
 	RESTORE_REST
-	cmpl $(IA32_NR_syscalls-1),%eax
-	ja  int_ret_from_sys_call	/* ia32_tracesys has set RAX(%rsp) */
-	jmp ia32_do_call
+	cmpl $IA32_NR_syscalls,%eax
+	jb ia32_do_call
+	jmp int_ret_from_sys_call	/* ia32_tracesys has set RAX(%rsp) */
 END(ia32_syscall)
 
 ia32_badsys:



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