PIC code

[BenShen98]

1. Does the two instruction below means set up the stack pointer for position independent coding?

   lui    $28,%hi(__gnu_local_gp)
   addiu  $28,$28,%lo(__gnu_local_gp)

2. What does the following mean?

• .cprestore
• .reloc
• .cpload

foo:    .set    noreorder
        .cpload $25
        .set    reorder

        subu    $29, $29, 32
        .cprestore 16

.cprestore 16
.reloc 1f,R_MIPS_JALR,foo

Below are the complete code

testnif:
   .frame $fp,32,$31    # vars= 0, regs= 2/0, args= 16, gp= 8
   .mask  0xc0000000,-4
   .fmask 0x00000000,0
   .set   noreorder
   .set   nomacro
   addiu  $sp,$sp,-32
   sw $31,28($sp)
   sw $fp,24($sp)
   move   $fp,$sp
   lui    $28,%hi(__gnu_local_gp)
   addiu  $28,$28,%lo(__gnu_local_gp)
   .cprestore 16
   sw $4,32($fp)
   lw $2,%call16(foo)($28)
   move   $25,$2
   .reloc 1f,R_MIPS_JALR,foo
1: jalr   $25
   nop

   lw $28,16($fp)
   slt    $2,$2,5
   bne    $2,$0,$L2
   nop

   li $2,3         # 0x3
   b  $L3
   nop

$L2:
   li $2,1         # 0x1
$L3:
   move   $sp,$fp
   lw $31,28($sp)
   lw $fp,24($sp)
   addiu  $sp,$sp,32
   j  $31
   nop

   .set   macro
   .set   reorder
   .end   testnif
   .size  testnif, .-testnif
   .ident "GCC: (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609"

[johnwickerson]

Ah, this is one for Google/StackOverflow in the first instance. See here and here etc.

[BenShen98]

Hi John,
after reading the wiki, I still didn't quite get it.

So if a function is doing something that clobbers it, the value needs to be restored before returning.

Does it mean if I don't change $28, I don't need to save .cprestore?
But I still would get the warning of

Warning: no .cprestore pseudo-op used in PIC code

Also, does the 16 in .cprestore 16 have special meaning? Where is 16 calculated from?

[ymherklotz]

I believe if you do not change it you will not need it, but I do not think that it hurts to add it. The 16 stands for the offset that the $gp should be stored at so that it can be restored later (https://reviews.llvm.org/D6267).