[PATCH v4 23/23] powerpc/syscall: Avoid storing 'current' in another pointer

[Christophe Leroy]

By saving the pointer pointing to thread_info.flags, gcc copies r2
in a non-volatile register.

We know 'current' doesn't change, so avoid that intermediaite pointer.

Reduces null_syscall benchmark by 2 cycles (322 => 320 cycles)

On PPC64, gcc seems to know that 'current' is not changing, and it keeps
it in a non volatile register to avoid multiple read of 'current' in paca.

Signed-off-by: Christophe Leroy <christophe.le...@csgroup.eu>
---
 arch/powerpc/kernel/syscall.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/kernel/syscall.c b/arch/powerpc/kernel/syscall.c
index 47ae55f94d1c..72e0b18b88d8 100644
--- a/arch/powerpc/kernel/syscall.c
+++ b/arch/powerpc/kernel/syscall.c
@@ -186,7 +186,6 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
                                           struct pt_regs *regs,
                                           long scv)
 {
-       unsigned long *ti_flagsp = &current_thread_info()->flags;
        unsigned long ti_flags;
        unsigned long ret = 0;
 
@@ -202,7 +201,7 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
        /* Check whether the syscall is issued inside a restartable sequence */
        rseq_syscall(regs);
 
-       ti_flags = *ti_flagsp;
+       ti_flags = current_thread_info()->flags;
 
        if (unlikely(r3 >= (unsigned long)-MAX_ERRNO) && !scv) {
                if (likely(!(ti_flags & (_TIF_NOERROR | _TIF_RESTOREALL)))) {
@@ -216,7 +215,7 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
                        ret = _TIF_RESTOREALL;
                else
                        regs->gpr[3] = r3;
-               clear_bits(_TIF_PERSYSCALL_MASK, ti_flagsp);
+               clear_bits(_TIF_PERSYSCALL_MASK, &current_thread_info()->flags);
        } else {
                regs->gpr[3] = r3;
        }
@@ -228,7 +227,7 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
 
 again:
        local_irq_disable();
-       ti_flags = READ_ONCE(*ti_flagsp);
+       ti_flags = READ_ONCE(current_thread_info()->flags);
        while (unlikely(ti_flags & (_TIF_USER_WORK_MASK & ~_TIF_RESTORE_TM))) {
                local_irq_enable();
                if (ti_flags & _TIF_NEED_RESCHED) {
@@ -244,7 +243,7 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
                        do_notify_resume(regs, ti_flags);
                }
                local_irq_disable();
-               ti_flags = READ_ONCE(*ti_flagsp);
+               ti_flags = READ_ONCE(current_thread_info()->flags);
        }
 
        if (IS_ENABLED(CONFIG_PPC_BOOK3S) && IS_ENABLED(CONFIG_PPC_FPU)) {
-- 
2.25.0