Atsushi Nemoto <[EMAIL PROTECTED]> writes:
> If I compile this code with gcc 4.1.1 (-O2),
>
> extern __thread int x;
> int foo(int arg)
> {
> if (arg)
> return x;
> return 0;
> }
>
> I got this output.
>
> foo:
> .frame $sp,0,$31 # vars= 0, regs= 0/0, args= 0, gp= 0
> .mask 0x00000000,0
> .fmask 0x00000000,0
> .set noreorder
> .cpload $25
> .set nomacro
>
> lw $2,%gottprel(x)($28)
> .set push
> .set mips32r2
> rdhwr $3,$29
> .set pop
> addu $2,$2,$3
> beq $4,$0,$L4
> move $3,$0
>
> lw $3,0($2)
> $L4:
> j $31
> move $2,$3
>
> The RDHWR is executed _before_ evaluating the "arg" value. For arg ==
> 0 case, the RDHWR has no point but just a overhead. Without -O2, the
> RDHWR is executed _after_ the evaluation, so gcc's optimizer reorder
> the RDHWR instruction.
The computation of the address of x was moved outside the
conditional--that is, both the rdhwr and the addu moved. You'll have
to figure out why. gcc shouldn't move instructions outside of a
conditional unless they are cheap and don't trap. This instruction
doesn't trap, but it's not cheap.
Ian
