为什么 MIPS GCC 在 function 调用（GOT 指针）之后从 16（$fp）重新加载 28 美元，而没有先存储到 16（$fp）？

Question

I think I need to paste the full code although it looks long.

I write a simple code for test.

#include <stdio.h>

int funadd(int a, int b){
  int x = 0;

  x = a + b;

  return x;
}

int fun(int a, int b){
  int y = 17;
  int returnvalue = 0;

  returnvalue = funadd(a, b);
  returnvalue = returnvalue - y;

  return returnvalue;
}

int main(){

  int a = 32;
  int b = 24;

  int c = 0;

  c = fun(a, b);

  printf("%d\n", c);

  return c;

}

After assembly:

    .file   1 "testfuncall.c"
    .section .mdebug.abi32
    .previous
    .nan    legacy
    .module fp=xx
    .module nooddspreg
    .abicalls
    .text
    .align  2
    .globl  funadd
    .set    nomips16
    .set    nomicromips
    .ent    funadd
    .type   funadd, @function
funadd:
    .frame  $fp,24,$31      # vars= 8, regs= 1/0, args= 0, gp= 8
    .mask   0x40000000,-4
    .fmask  0x00000000,0
    .set    noreorder
    .set    nomacro
    addiu   $sp,$sp,-24
    sw  $fp,20($sp)
    move    $fp,$sp
    sw  $4,24($fp)
    sw  $5,28($fp)
    sw  $0,8($fp)
    lw  $3,24($fp)
    lw  $2,28($fp)
    addu    $2,$3,$2
    sw  $2,8($fp)
    lw  $2,8($fp)
    move    $sp,$fp
    lw  $fp,20($sp)
    addiu   $sp,$sp,24
    jr  $31
    nop

    .set    macro
    .set    reorder
    .end    funadd
    .size   funadd, .-funadd
    .align  2
    .globl  fun
    .set    nomips16
    .set    nomicromips
    .ent    fun
    .type   fun, @function
fun:
    .frame  $fp,40,$31      # vars= 8, regs= 2/0, args= 16, gp= 8
    .mask   0xc0000000,-4
    .fmask  0x00000000,0
    .set    noreorder
    .cpload $25
    .set    nomacro
    addiu   $sp,$sp,-40
    sw  $31,36($sp)
    sw  $fp,32($sp)
    move    $fp,$sp
    .cprestore  16
    sw  $4,40($fp)
    sw  $5,44($fp)
    li  $2,17           # 0x11
    sw  $2,24($fp)
    sw  $0,28($fp)
    lw  $5,44($fp)
    lw  $4,40($fp)
    lw  $2,%got(funadd)($28)
    move    $25,$2
    .reloc  1f,R_MIPS_JALR,funadd
1:  jalr    $25
    nop

    lw  $28,16($fp)
    sw  $2,28($fp)
    lw  $3,28($fp)
    lw  $2,24($fp)
    subu    $2,$3,$2
    sw  $2,28($fp)
    lw  $2,28($fp)
    move    $sp,$fp
    lw  $31,36($sp)
    lw  $fp,32($sp)
    addiu   $sp,$sp,40
    jr  $31
    nop

    .set    macro
    .set    reorder
    .end    fun
    .size   fun, .-fun
    .rdata
    .align  2
$LC0:
    .ascii  "%d\012\000"
    .text
    .align  2
    .globl  main
    .set    nomips16
    .set    nomicromips
    .ent    main
    .type   main, @function
main:
    .frame  $fp,48,$31      # vars= 16, regs= 2/0, args= 16, gp= 8
    .mask   0xc0000000,-4
    .fmask  0x00000000,0
    .set    noreorder
    .cpload $25
    .set    nomacro
    addiu   $sp,$sp,-48
    sw  $31,44($sp)
    sw  $fp,40($sp)
    move    $fp,$sp
    .cprestore  16
    li  $2,32           # 0x20
    sw  $2,24($fp)
    li  $2,24           # 0x18
    sw  $2,28($fp)
    sw  $0,32($fp)
    lw  $5,28($fp)
    lw  $4,24($fp)
    lw  $2,%got(fun)($28)
    move    $25,$2
    .reloc  1f,R_MIPS_JALR,fun
1:  jalr    $25
    nop

    lw  $28,16($fp)
    sw  $2,32($fp)
    lw  $5,32($fp)
    lw  $2,%got($LC0)($28)
    addiu   $4,$2,%lo($LC0)
    lw  $2,%call16(printf)($28)
    move    $25,$2
    .reloc  1f,R_MIPS_JALR,printf
1:  jalr    $25
    nop

    lw  $28,16($fp)
    lw  $2,32($fp)
    move    $sp,$fp
    lw  $31,44($sp)
    lw  $fp,40($sp)
    addiu   $sp,$sp,48
    jr  $31
    nop

    .set    macro
    .set    reorder
    .end    main
    .size   main, .-main
    .ident  "GCC: (Debian 6.3.0-18+deb9u1) 6.3.0 20170516"

I realize after each function call, there is a lw $28,16($fp) instruction. But I don't see any code that would have stored a value there first in either the caller or callee.

I can read MIPS assembly. I know that lw is load word, and how $fp and $sp are frame pointer and stack pointer.

I just can't understand how it makes sense to load anything from 16($fp) ; it seems there is an uninitialized space.

I know $28 is $gp , and can see it being used as a GOT pointer to load function addresses before calls, but it seems nothing initializes that register either before its used in functions.

Does the MIPS calling convention require $28 to already be pointing at the GOT on function entry?

Answer 1

lw $28,16($fp)

lw is a "load word" instruction -- it loads a word (4 bytes or 32 bits) from memory into a register. $28 is the destination register (also sometimes referred to a $gp ), and 16($fp) is the address to load from -- 16 bytes into the frame ( $fp is the frame pointer register, and 16 is added to it to get the address to load from).

The "frame" is generally used to hold local variables of the function --when a function starts, it allocates a frame on the stack by subtracting a constant from $sp , then stores the caller's $fp value somewhere in there and copies $sp to $fp so it points at this newly allocated frame. It then reads and writes local data into/from the frame with load( l ) and store( s ) instructions.

If you had compiled with optimization, GCC would keep locals in registers when possible instead of wasting huge numbers of instructions storing/reloading them to the stack. And would access stack memory relative to the stack pointer, instead of spending an instruction to set up $fp as a traditional frame pointer. Un-optimized code doesn't look anything like what a human would write by hand, but optimized code sometimes does.

Answer 2

I think I know why.

I have ignored the .cprestore 16 automatically because I regard it as useless. But it turns out to emit actual instructions, not just metadata, related to the $gp register.

.cprestore 16 will expand to sw $gp,16(sp) . It is used together with .cpload $25 and other necessary code. Specifically, you can read the link https://www.linux-mips.org/wiki/PIC_code