syscall exit path optimization

Follow up on previous discussion, this patch optimize how we handle
r8/r10 in syscall return path.  If there are no pending work to be
done, we will skip storing/loading r8/r10, cutting out 4 memory
references in the fast path.  This resulted a net of 4 cycles saving.

Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
Signed-off-by: Rohit Seth <rohit.seth@intel.com>


--- linux-ia64-release/arch/ia64/kernel/entry.S.orig	2005-01-26 11:41:24.000000000 -0800
+++ linux-ia64-release/arch/ia64/kernel/entry.S	2005-01-26 12:31:52.000000000 -0800
@@ -558,7 +558,8 @@ GLOBAL_ENTRY(ia64_trace_syscall)
 .mem.offset 0,0; st8.spill [r2]=r8		// store return value in slot for r8
 .mem.offset 8,0; st8.spill [r3]=r10		// clear error indication in slot for r10
 	br.call.sptk.many rp=syscall_trace_leave // give parent a chance to catch return value
-.ret3:	br.cond.sptk ia64_leave_syscall
+	cmp.eq p9,p8=r0,r0
+	br.cond.sptk ia64_leave_syscall

 strace_error:
 	ld8 r3=[r2]				// load pt_regs.r8
@@ -619,12 +620,10 @@ END(ia64_ret_from_clone)
 	// fall through
 GLOBAL_ENTRY(ia64_ret_from_syscall)
 	PT_REGS_UNWIND_INFO(0)
+	cmp.eq p8,p9=r0,r0	// p8: ret val in live reg, p9: ret val in pt_regs
 	cmp.ge p6,p7=r8,r0			// syscall executed successfully?
 	adds r2=PT(R8)+16,sp			// r2 = &pt_regs.r8
-	adds r3=PT(R10)+16,sp			// r3 = &pt_regs.r10
-	;;
-(p6)	st8 [r2]=r8				// store return value in slot for r8
-(p6)	st8 [r3]=r0				// clear error indication in slot for r10
+	mov r10=r0				// clear error indication in r10
 (p7)	br.cond.spnt handle_syscall_error	// handle potential syscall failure
 END(ia64_ret_from_syscall)
 	// fall through
@@ -715,10 +714,10 @@ ENTRY(ia64_leave_syscall)
 	;;
 	mov r16=ar.bsp				// M2  get existing backing store pointer
 (p6)	cmp4.ne.unc p6,p0=r15, r0		// any special work pending?
-(p6)	br.cond.spnt .work_pending
+(p6)	br.cond.spnt .work_pending_syscall
 	;;
 	// start restoring the state saved on the kernel stack (struct pt_regs):
-	ld8 r8=[r2],16
+(p9)	ld8 r8=[r2],16
 	ld8 r9=[r3],16
 	mov f6=f0		// clear f6
 	;;
@@ -726,9 +725,10 @@ ENTRY(ia64_leave_syscall)
 	rsm psr.i | psr.ic	// M2 initiate turning off of interrupt and interruption collection
 	mov f9=f0		// clear f9

-	ld8 r10=[r2],16
+	.pred.rel.mutex p8,p9
+(p9)	ld8 r10=[r2],16
 	ld8 r11=[r3],16
-	mov f7=f0		// clear f7
+(p8)	add r2=32,r2
 	;;
 	ld8 r29=[r2],16		// load cr.ipsr
 	ld8 r28=[r3],16			// load cr.iip
@@ -760,7 +760,7 @@ ENTRY(ia64_leave_syscall)
 	;;
 	srlz.d			// M0  ensure interruption collection is off
 	ld8.fill r13=[r3],16
-	nop.i 0
+	mov f7=f0		// clear f7
 	;;
 	ld8.fill r12=[r2]	// restore r12 (sp)
 	ld8.fill r15=[r3]	// restore r15
@@ -770,8 +770,8 @@ ENTRY(ia64_leave_syscall)
 (pUStk) st1 [r14]=r17
 	mov b6=r18		// I0  restore b6
 	;;
-	shr.u r18=r19,16	// I0|1 get byte size of existing "dirty" partition
 	mov r14=r0		// clear r14
+	shr.u r18=r19,16	// I0|1 get byte size of existing "dirty" partition
 (pKStk) br.cond.dpnt.many skip_rbs_switch

 	mov.m ar.ccv=r0		// clear ar.ccv
@@ -1083,6 +1083,10 @@ skip_rbs_switch:
 	 * On exit:
 	 *	p6 = TRUE if work-pending-check needs to be redone
 	 */
+.work_pending_syscall:
+(p8)	st8 [r2]=r8,16
+	;;
+(p8)	st8 [r2]=r10,16
 .work_pending:
 	tbit.nz p6,p0=r31,TIF_SIGDELAYED		// signal delayed from  MCA/INIT/NMI/PMI context?
 (p6)	br.cond.sptk.few .sigdelayed
@@ -1104,12 +1108,14 @@ skip_rbs_switch:
 	;;
 (pKStk)	st4 [r20]=r0		// preempt_count() <- 0
 #endif
+	cmp.eq p9,p8=r0,r0
 (pLvSys)br.cond.sptk.many .work_processed_syscall	// re-check
 	br.cond.sptk.many .work_processed_kernel	// re-check

 .notify:
 (pUStk)	br.call.spnt.many rp=notify_resume_user
 .ret10:	cmp.ne p6,p0=r0,r0				// p6 <- 0
+	cmp.eq p9,p8=r0,r0
 (pLvSys)br.cond.sptk.many .work_processed_syscall	// don't re-check
 	br.cond.sptk.many .work_processed_kernel	// don't re-check

@@ -1121,6 +1127,7 @@ skip_rbs_switch:
 .sigdelayed:
 	br.call.sptk.many rp=do_sigdelayed
 	cmp.eq p6,p0=r0,r0				// p6 <- 1, always re-check
+	cmp.eq p9,p8=r0,r0
 (pLvSys)br.cond.sptk.many .work_processed_syscall	// re-check
 	br.cond.sptk.many .work_processed_kernel	// re-check

@@ -1135,17 +1142,11 @@ ENTRY(handle_syscall_error)
 	 */
 	PT_REGS_UNWIND_INFO(0)
 	ld8 r3=[r2]		// load pt_regs.r8
-	sub r9=0,r8		// negate return value to get errno
 	;;
-	mov r10=-1		// return -1 in pt_regs.r10 to indicate error
 	cmp.eq p6,p7=r3,r0	// is pt_regs.r8==0?
-	adds r3=16,r2		// r3=&pt_regs.r10
-	;;
-(p6)	mov r9=r8
-(p6)	mov r10=0
 	;;
-	st8 [r2]=r9		// store errno in pt_regs.r8
-	st8 [r3]=r10		// store error indication in pt_regs.r10
+(p7)	mov r10=-1
+(p7)	sub r8=0,r8		// negate return value to get errno
 	br.cond.sptk ia64_leave_syscall
 END(handle_syscall_error)



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