RISC-V calling conventions

Register conventions

Preserved across calls?

字面意思是Preserve across calls,什么意思呢,也就是标记为YES的寄存器,It needs to remain unchanged before and after the subroutine is executed.如何做到这一点呢,That's where the subroutine uses theseYESbefore deposit,These registers need to be pushed onto the stack,The values ​​of these registers are restored from the stack after use.
标记为NO的呢,It doesn't care when the subroutine is executed,如果NO寄存器,Subroutines are not desired(callee)before and after execution are changed,That requires the caller of the program(caller)Push the stack before the subroutine is executed,After the subroutine is executed, pop the stack to restore the register value.
总结：标记为YES的需要callee保存(Subroutines are saved by themselves),标记为NO的需要caller保存(调用者保存).

注意：

The way to keep the register value unchanged is not necessarily the only way to push and pop the stack,比入SP是YESHold after subroutine callSP的值不变,Then you can put it at the beginning of the subroutine
SP减去一个值,at the end of the subroutineSP加上一个值,Keep the pre- and post-call unchanged.As shown in the following program fragment

示例：

(gdb) disass main
Dump of assembler code for function main:
   0x0000000000010158 <+0>:     addi       sp,sp,-32 #Open up the stack size here
   0x000000000001015a <+2>:     sd         ra,24(sp)
   0x000000000001015c <+4>:     sd         s0,16(sp)
   0x000000000001015e <+6>:     addi       s0,sp,32
   0x0000000000010160 <+8>:     li         a5,1
   0x0000000000010162 <+10>:    sw         a5,-20(s0)
   0x0000000000010166 <+14>:    li         a5,2
   0x0000000000010168 <+16>:    sw         a5,-24(s0)
   0x000000000001016c <+20>:    lw         a4,-20(s0)
   0x0000000000010170 <+24>:    lw         a5,-24(s0)
   0x0000000000010174 <+28>:    addw       a5,a5,a4
   0x0000000000010176 <+30>:    sw         a5,-28(s0)
   0x000000000001017a <+34>:    lw         a5,-28(s0)
   0x000000000001017e <+38>:    mv         a1,a5
   0x0000000000010180 <+40>:    lui        a5,0x1c
   0x0000000000010182 <+42>:    addi       a0,a5,176 # 0x1c0b0
   0x0000000000010186 <+46>:    jal        ra,0x10332 <printf>
   0x000000000001018a <+50>:    li         a5,0
   0x000000000001018c <+52>:    mv         a0,a5
   0x000000000001018e <+54>:    ld         ra,24(sp)
   0x0000000000010190 <+56>:    ld         s0,16(sp)
   0x0000000000010192 <+58>:    addi       sp,sp,32 #Restore after use
   0x0000000000010194 <+60>:    ret
End of assembler dump.

参考：
CALLER AND CALLEE SAVED REGISTERS