Simon Kagstrom <simon.kagstrom@xxxxxx> writes:
> When disassembling the generated code, this generates the following
> MIPS assembly code, with my comments
>
> 00000000 <problem>:
> 0: 27bdffe8 addiu sp,sp,-24
> 4: afbf0010 sw ra,16(sp)
> 8: 24020001 li v0,1 # Result of the inline assembly piece
> c: 0c000000 jal overwriter # Overwrites v0
> 10: 00000000 nop # jal delay slot - v0 should have been saved here!
> 14: 00402021 move a0,v0 # GCC thinks v0 still contains the result of the inline assembly
> 18: 0c000000 jal tst
> 1c: 00402821 move a1,v0 # ... *and* the result of overwriter()!
> 20: 8fbf0010 lw ra,16(sp)
> 24: 00000000 nop
> 28: 03e00008 jr ra
> 2c: 27bd0018 addiu sp,sp,24
This is not a bug. It is a documented restriction on local register
variables:
Local register variables in specific registers do not reserve the
registers, except at the point where they are used as input or
output operands in an asm statement and the asm statement itself
is not deleted.
There are several ways to avoid this problem. Simply returning the
register variable doesn't help, but it should work to store the value
in a different register. Or change the asm to explicitly copy the
value to a variable with the "d" constraint.
Ian
