fans-rt 任务调度-堆栈切换篇(4)tiny模型详细分析

优化后的Tiny模型代码：

;
; Copyright(C) 2013-2015, Fans-rt development team.
;
; All rights reserved.
;
; This is open source software.
; Learning and research can be unrestricted to modification, use and dissemination.
; If you need for commercial purposes, you should get the author's permission.
;
; Configuration:
; System global core stack NO
; The local core stack of general task YES
; The loacl core stack of kernel task YES
; The local user stack of general task NO
; Hardware supported task switch IRQ NO
; Hardware supported double stack NO
;
; date author notes
; 2015-06-25 JiangYong new file
; 2015-07-07 JiangYong rename to kboard_interrupt.s
; 2015-07-11 JiangYong code optimization
;
INCLUDE kirq_define_enum.inc

EXPORT UsageFault_Handler
EXPORT BusFault_Handler
EXPORT MemManage_Handler
EXPORT HardFault_Handler
EXPORT SysTick_Handler
EXPORT PendSV_Handler
EXPORT SVC_Handler
EXPORT CORE_Switch2UserMode

IMPORT CORE_EnterIRQ
IMPORT CORE_LeaveIRQ
IMPORT CORE_TickHandler
IMPORT CORE_TaskScheduling
IMPORT CORE_HandlerLPC
IMPORT CORE_SwitchTask
IMPORT CORE_SetTaskStackPosition
IMPORT CORE_GetTaskStackPosition
IMPORT CORE_GetCoreStackPosition
IMPORT CORE_CheckMustbeSchedule

PRESERVE8

AREA |.text|, CODE, READONLY
ALIGN 4
THUMB

CORE_Switch2UserMode PROC
MOV R0, #0 ;
MSR PRIMASK, R0 ; Enable IRQ
BX LR ; return
ENDP

PendSV_Handler PROC
BX LR ; Not support PendSV IRQ
ENDP

SVC_Handler     PROC
    CPSID   I                           ;  Why to disable IRQ ? Guess !
<span style="color:#ff0000;">    MOV     R0,     SP                  ;  R0 = Offset of {R0 - R3}
    PUSH    {LR, R12}                   ;  Why to push 12? Guess !
    MOV     R12,    R0                  ;  R12 = Offset of {R0 - R3}</span>
    BL      CORE_EnterIRQ               ;  Set current interrupt nest layer
    CPSIE   I                           ;  Enable IRQ
<span style="color:#ff0000;">    LDMFD   R12,    {R0-R3}             ;  Resume R0 - R3 to call service</span>
    BL      CORE_HandlerLPC             ;  Call system service
<span style="color:#ff0000;">    B       ST_L1                       ;  The next step same as system tick handler</span>
    ENDP

SysTick_Handler PROC
CPSID I ; Why to disable IRQ ? Guess !
<span style="color:#ff0000;"> PUSH {R12, LR} ; Why to push R12?</span>
BL CORE_EnterIRQ ; Set current interrupt nest layer
BL CORE_TickHandler ; Inc the system tick
CPSIE I ; Enable IRQ
BL CORE_TaskScheduling ; Find the new task will be scheduling
<span style="color:#ff0000;">ST_L1</span>
CPSID I ; Disable IRQ
BL CORE_LeaveIRQ ; Set and get current interrupt nest layer
CBNZ R0, ST_LE ; Nest layer != 0 then leave this interrupt
BL CORE_CheckMustbeSchedule ; Check need schedule
CBZ R0, ST_LE ; Must schedule = FALSE then leave this interrupt
<span style="color:#ff0000;"> PUSH {R4 - R11} ; nest layer = 0 and Must schedule = TRUE then scheduling</span>
MRS R0, MSP ; R0 = Core stack for old task
MRS R1, PSP ; R1 = User stack for old task
BL CORE_SwitchTask ; CORE_SwitchTask(CoreStack, UserStack);
; No need check the task permssion
BL CORE_GetTaskStackPosition ; R0 = User stack for new task(no need)
MOV R1, R0 ; R1 = User stack for new task(no need)
BL CORE_GetCoreStackPosition ; R0 = Core stack for new task
MSR PSP, R1 ; Update user stack(no need)
MSR MSP, R0 ; Update core stack
<span style="color:#ff0000;"> POP {R4 - R11} ; Restore new task registers</span>
ST_LE
POP {R12, LR} ; Resume break point
CPSIE I ; Enable IRQ
BX LR ; Return to task break point
ENDP

HardFault_Handler PROC
B .
ENDP

MemManage_Handler PROC
B .
ENDP
BusFault_Handler PROC
B .
ENDP
UsageFault_Handler PROC
B .
ENDP

ALIGN
END
代码中，红色部分为第一个版本与优化后的主要差异点，为什么这么优化呢？在我们分析完代码执行过程中的堆栈环境就明白了。Tiny模型所有任务只有一个堆栈，本文将所有任务堆栈按照内核堆栈进行分析。
首先，进入中断时的堆栈状态：

; |-----------------------|
; | ........ |
; |-----------------------|
; | xPSR | <<= SP + 0x1C
; |-----------------------|
; | PC | <<= SP + 0x18
; |-----------------------|
; | LR | <<= SP + 0x14
; |-----------------------|
; | RC | <<= SP + 0x10
; |-----------------------|
; | R3 | <<= SP + 0x0C
; |-----------------------|
; | R2 | <<= SP + 0x08
; |-----------------------|
; | R1 | <<= SP + 0x04
; |-----------------------|
; | R0 | <<= SP + 0x00
; |-----------------------|
; | ........ |
; |-----------------------|
; 中断入口的堆栈映像，由CPU自动保存

从堆栈映像图可以看出，SP 刚好指向保存 R0 的位置, 所以 SVC_Handler 中断入口在执行
    <span style="color:#ff0000;">MOV     R0,     SP                  ;  R0 = Offset of {R0 - R3}</span>
    PUSH    {LR, R12}                   ;  Why to push 12? Guess !
    <span style="color:#ff0000;">MOV     R12,    R0                  ;  R12 = Offset of {R0 - R3}</span>
后， R12就指向堆栈中保存R0-R3寄存器的首地址，在调用 CORE_HandlerLPC之前，通过

<span style="color:#ff0000;"> LDMFD R12, {R0-R3} ; Resume R0 - R3 to call service</span>
BL CORE_HandlerLPC ; Call system service即可恢复R0-R3寄存器，以便作为参数传递给 CORE_HandlerLPC。

当代码执行完 LR 和 R12 寄存器的入栈操作后堆栈如下：

; |-----------------------|
; | ........ |
; |-----------------------|
; | xPSR | <<= SP + 0x24
; |-----------------------|
; | PC | <<= SP + 0x20
; |-----------------------|
; | LR | <<= SP + 0x1C
; |-----------------------|
; | RC | <<= SP + 0x18
; |-----------------------|
; | R3 | <<= SP + 0x14
; |-----------------------|
; | R2 | <<= SP + 0x10
; |-----------------------|
; | R1 | <<= SP + 0x0C
; |-----------------------|
; | R0 | <<= SP + 0x08
; |-----------------------|
; | LR | <<= SP + 0x04
; |-----------------------|
; | RC | <<= SP + 0x00
; |-----------------------|
; | ........ |
; |-----------------------|
; LR 和 R12 入栈后修改前的代码SVC_Handler和SysTick_Handler分别对R4和R0入栈，并未对R12入栈。为什么修改为对R12入栈呢？因为任务切换时需要构造断点堆栈映像如下：
; |-----------------------|
; | ........ |
; |-----------------------|
; | xPSR | <<= SP + 0x54
; |-----------------------|
; | PC | <<= SP + 0x50
; |-----------------------|
;
af2e
| LR | <<= SP + 0x4C
; |-----------------------|
; | RC | <<= SP + 0x48
; |-----------------------|
; | R3 | <<= SP + 0x44
; |-----------------------|
; | R2 | <<= SP + 0x40
; |-----------------------|
; | R1 | <<= SP + 0x3C
; |-----------------------|
; | R0 | <<= SP + 0x38
; |-----------------------|
; | LR | <<= SP + 0x34
; |-----------------------|
; | RC | <<= SP + 0x30
; |-----------------------|
; | RB | <<= SP + 0x2C
; |-----------------------|
; | RA | <<= SP + 0x28
; |-----------------------|
; | R9 | <<= SP + 0x24
; |-----------------------|
; | ........ |
; |-----------------------|
; | R0 | <<= SP + 0x00 (It's the old task break point)
; |-----------------------|
; | ........ |
; |-----------------------|
; 断点堆栈映像在确认需要调度后，会对 R4-R11进行入栈以构造断点堆栈映像，而已入栈的R12刚好在断点堆栈映像中正确的位置。所以，相比修改前，节省了R4/R0的入栈和出栈动作。
优化点：

   1.中断入口的入栈由R4/R0修改为R12，减少R0/R4的入栈和出栈消耗

   2.在 SVC_Handler 中，对R0-R3的入栈和出栈修改为由R12寻址的LDMFD指令，通过两次寄存器访问来减少对内存的4次访问

   3.减少冗余代码，SVC_Handler和SysTick_Handler共用中断下半部代码。