Hello,
I'm just preparing to switch from GCC 4.3.2 towards GCC 4.4.1 for MIPS platform and found the following issue.
We're making use of global register variables, which work fine in GCC version 4.3.2. GCC 4.4.1 however produces slightly different code, which seems to be incorrect.
A brief example is:
//---------------test_register_call.c-------------------
register unsigned long long *g_reg_counter asm("$20");
void register_add (void)
{
*g_reg_counter++;
}
//---------------test_register_call.c-------------------
Compiled with:
mips-elf-gcc.exe -save-temps test_register_call.c
This produces the following assembler file using GCC 4.3.2
//---------------test_register_call_4.3.2.s-------------
.file 1 "test_register_call.c"
.section .mdebug.abi32
.previous
.gnu_attribute 4, 1
.text
.align 2
.globl register_add
.set nomips16
.ent register_add
register_add:
.frame $fp,8,$31 # vars= 0, regs= 1/0, args= 0, gp= 0
.mask 0x40000000,-4
.fmask 0x00000000,0
.set noreorder
.set nomacro
addiu $sp,$sp,-8
sw $fp,4($sp)
move $fp,$sp
move $2,$20
addiu $2,$2,8
move $20,$2
move $sp,$fp
lw $fp,4($sp)
addiu $sp,$sp,8
j $31
nop
.set macro
.set reorder
.end register_add
.size register_add, .-register_add
.ident "GCC: 4.3.2"
//---------------test_register_call_4.3.2.s-------------
GCC 4.4.1 produces different assembly code (lines are marked):
//---------------test_register_call_4.4.1.s-------------
.file 1 "test_register_call.c"
.section .mdebug.abi32
.previous
.gnu_attribute 4, 1
.text
.align 2
.globl register_add
.set nomips16
.ent register_add
.type register_add, @function
register_add:
.frame $fp,8,$31 # vars= 0, regs= 2/0, args= 0, gp= 0
.mask 0x40100000,-4
.fmask 0x00000000,0
.set noreorder
.set nomacro
addiu $sp,$sp,-8
sw $fp,4($sp)
sw $20,0($sp) # <== $20 is stored to stack
move $fp,$sp
move $2,$20
addiu $2,$2,8
move $20,$2
move $sp,$fp
lw $fp,4($sp)
lw $20,0($sp) # <== $20 is restored from stack
addiu $sp,$sp,8
j $31
nop
.set macro
.set reorder
.end register_add
.size register_add, .-register_add
.ident "GCC: 4.4.1"
//---------------test_register_call_4.4.1.s-------------
As can be seen in the example, MIPS register $20 is the global variable register. The variable is modified in the function and the register should therefore not be saved and restored. GCC 4.3.2 seems to be correct here. GCC 4.4.1, however, stores and restores the register $20 content. In my understanding this behaviour is incorrect.
Is this a known bug or am I missing something? Can someone reproduce this behaviour?
Thanks, Steffen
