LoongArch: Use current_stack_pointer in current_pt_regs()

The current implementation of current_pt_regs() relies on the compiler
__builtin_frame_address(0). This introduces an unnecessary dependency
on the frame pointer register, which forces the compiler to generate
redundant prologue and epilogue code, create a larger stack frame, and
perform redundant memory operations to preserve the frame pointer even
in functions where it is otherwise unnecessary.

Optimize this by switching to current_stack_pointer, which explicitly
maps to the hardware stack pointer register. This allows the compiler
to compute the stack alignment directly from the natively maintained
"$sp" register, completely eliminating the overhead of preserving and
restoring the frame pointer on the stack memory.

As a prominent example, this optimization improves the hot-path function
copy_thread(). A disassembly comparison of copy_thread() illustrates the
elimination of the frame pointer, the reduction of stack frame size from
48 bytes down to 32 bytes, and a more compact epilogue path:

Before:

00000000000004f0 <copy_thread>:
 4f0:   02ff4063        addi.d          $sp, $sp, -48
 4f4:   29c08076        st.d            $fp, $sp, 32
 4f8:   29c06077        st.d            $s0, $sp, 24
 4fc:   29c0a061        st.d            $ra, $sp, 40
 500:   02c0c076        addi.d          $fp, $sp, 48
 ...
 54c:   1400006e        lu12i.w         $t2, 3
 ...
 55c:   03bffdce        ori             $t2, $t2, 0xfff
 560:   00153ace        or              $t2, $fp, $t2
 564:   02fb05cd        addi.d          $t1, $t2, -319
 ...
 628:   28c0a061        ld.d            $ra, $sp, 40
 62c:   28c08076        ld.d            $fp, $sp, 32
 630:   28c06077        ld.d            $s0, $sp, 24
 634:   00150004        move            $a0, $zero
 638:   02c0c063        addi.d          $sp, $sp, 48
 63c:   4c000020        ret

After:

00000000000004f0 <copy_thread>:
 4f0:   02ff8063        addi.d          $sp, $sp, -32
 4f4:   29c04077        st.d            $s0, $sp, 16
 4f8:   29c06061        st.d            $ra, $sp, 24
 [ prologue st.d and addi.d for $fp are completely eliminated ]
 ...
 544:   1400006e        lu12i.w         $t2, 3
 ...
 554:   03bffdce        ori             $t2, $t2, 0xfff
 558:   0015386e        or              $t2, $sp, $t2
 55c:   02fb05cd        addi.d          $t1, $t2, -319
 ...
 620:   28c06061        ld.d            $ra, $sp, 24
 624:   28c04077        ld.d            $s0, $sp, 16
 628:   00150004        move            $a0, $zero
 [ epilogue ld.d for $fp is eliminated; exit path is shortened ]
 62c:   02c08063        addi.d          $sp, $sp, 32
 630:   4c000020        ret

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

0 files changed, 0 insertions, 0 deletions