ins instruction is not produced in the assembly code (mips32r2)

[fanqifei <fanqifei@xxxxxxxxx>]

Hello,
I am tring to compile a small c file using gcc4.1.2.
The source code:
struct test_foo {
unsigned int a:18;
unsigned int b:2;
unsigned int c:12;
};

struct test_foo x;
unsigned int foo()
{
    unsigned int a=x.b;
    x.b=2;
    return a;
}

the command: mipsel-linux-gcc -mips32r2 -O2 -c -Wall foo.c -o foo.o

The corresponding disassembly code:
foo.o:     file format elf32-tradlittlemips
Disassembly of section .text:
00000000 <foo>:
   0:   3c1c0000    lui gp,0x0
   4:   279c0000    addiu   gp,gp,0
   8:   0399e021    addu    gp,gp,t9
   c:   8f850000    lw  a1,0(gp)
  10:   3c03fff3    lui v1,0xfff3
  14:   3463ffff    ori v1,v1,0xffff
  18:   8ca20000    lw  v0,0(a1)
  1c:   3c040008    lui a0,0x8
  20:   00431824    and v1,v0,v1
  24:   00641825    or  v1,v1,a0
  28:   7c420c80    ext v0,v0,0x12,0x2
  2c:   03e00008    jr  ra
  30:   aca30000    sw  v1,0(a1)

The statement x.b=2 can be simply translated to lb and ins
instructions( ins is in mips32r2).

I wonder how ins instrution can be produced.
More specifically, how can gcc generates the insv pattern in RTL?
Is it related to the coding style -- using bit filed or logical
operations(shift, and ...)?

Thanks.
Qifei