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From: Konstantin Belousov <kib@FreeBSD.org>
Date: Thu, 15 Sep 2011 09:53:04 +0000 (UTC)
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Subject: svn commit: r225575 - head/sys/amd64/amd64
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Author: kib
Date: Thu Sep 15 09:53:04 2011
New Revision: 225575
URL: http://svn.freebsd.org/changeset/base/225575

Log:
  Microoptimize the return path for the fast syscalls on amd64. Arrange
  the code to have the fall-through path to follow the likely target.
  Do not use intermediate register to reload user %rsp.
  
  Proposed by:	alc
  Reviewed by:	alc, jhb
  Approved by:	re (bz)
  MFC after:	2 weeks

Modified:
  head/sys/amd64/amd64/exception.S

Modified: head/sys/amd64/amd64/exception.S
==============================================================================
--- head/sys/amd64/amd64/exception.S	Thu Sep 15 09:22:49 2011	(r225574)
+++ head/sys/amd64/amd64/exception.S	Thu Sep 15 09:53:04 2011	(r225575)
@@ -393,14 +393,8 @@ IDTVEC(fast_syscall)
 	/* Check for and handle AST's on return to userland. */
 	movq	PCPU(CURTHREAD),%rax
 	testl	$TDF_ASTPENDING | TDF_NEEDRESCHED,TD_FLAGS(%rax)
-	je	2f
-	sti
-	movq	%rsp, %rdi
-	call	ast
-	jmp	1b
-
-	.align	16
-2:	/* Restore preserved registers. */
+	jne	2f
+	/* Restore preserved registers. */
 	MEXITCOUNT
 	movq	TF_RDI(%rsp),%rdi	/* bonus; preserve arg 1 */
 	movq	TF_RSI(%rsp),%rsi	/* bonus: preserve arg 2 */
@@ -408,10 +402,16 @@ IDTVEC(fast_syscall)
 	movq	TF_RAX(%rsp),%rax	/* return value 1 */
 	movq	TF_RFLAGS(%rsp),%r11	/* original %rflags */
 	movq	TF_RIP(%rsp),%rcx	/* original %rip */
-	movq	TF_RSP(%rsp),%r9	/* user stack pointer */
-	movq	%r9,%rsp		/* original %rsp */
+	movq	TF_RSP(%rsp),%rsp	/* user stack pointer */
 	swapgs
 	sysretq
+
+2:	/* AST scheduled. */
+	sti
+	movq	%rsp,%rdi
+	call	ast
+	jmp	1b
+
 3:	/* Requested full context restore, use doreti for that. */
 	MEXITCOUNT
 	jmp	doreti