本文讲解了C语言运行在MIPS体系中结构体传参的情况

硬件平台：Loongson 3A3000

系统：uos

在《see MIPS run》中11.2.5中描述到，会将结构体成员压缩到各个arg寄存器中进行传递
“we have to pack the register with data to mimic the arrangement of data in memory.”

1. C语言代码：
   构造一个函数，参数为结构体，在main函数中调用函数，并传入结构体

	#include <stdio.h>
#include <stdint.h>struct shape{uint32_t a;uint16_t b
};void test(struct shape sp){uint32_t c = sp.a + sp.b;printf("%d \n", c); 
}void main()
{struct shape sp; sp.a = 12; sp.b = 32; test(sp);
}

2. 汇编代码

0000000000000b00 <test>:b00:   67bdffc0    daddiu  sp,sp,-64b04:   ffbf0038    sd  ra,56(sp)b08:   ffbe0030    sd  s8,48(sp)b0c:   ffbc0028    sd  gp,40(sp)b10:   03a0f025    move    s8,spb14:   3c1c0002    lui gp,0x2b18:   0399e02d    daddu   gp,gp,t9b1c:   679c8320    daddiu  gp,gp,-31968b20:   ffc40010    sd  a0,16(s8)b24:   8fc30010    lw  v1,16(s8)b28:   97c20014    lhu v0,20(s8)b2c:   00621021    addu    v0,v1,v00000000000000b74 <main>:b74:   67bdffd0    daddiu  sp,sp,-48b78:   ffbf0028    sd  ra,40(sp)b7c:   ffbe0020    sd  s8,32(sp)b80:   ffbc0018    sd  gp,24(sp)b84:   03a0f025    move    s8,spb88:   3c1c0002    lui gp,0x2b8c:   0399e02d    daddu   gp,gp,t9b90:   679c82ac    daddiu  gp,gp,-32084b94:   2402000c    li  v0,12		//加载立即数到V0b98:   afc20000    sw  v0,0(s8)		//保存到栈中b9c:   24020020    li  v0,32		//加载立即数到V0ba0:   a7c20004    sh  v0,4(s8)	//保存到栈中ba4:   dfc40000    ld  a0,0(s8)ba8:   df828058    ld  v0,-32680(gp)bac:   0040c825    move    t9,v0bb0:   0411ffd3    bal b00 <test>

• 在ba0代码执行完毕后栈内数据情况如下
  
• ba4 ld a0,0(s8)； 把栈中8个字节加载到了a0寄存器，即图中0~8中的数据，即结构体中的a、b
• b20 sd a0,16(s8); 把a0，放到栈中16offset
• b24 lw v1,16(s8)； 从栈中16offset读取前4个字节
• b28 lhu v0,20(s8)；从栈中20offset读取剩下两个字节