Re: mips RDHWR instruction in glibc

[Atsushi Nemoto <anemo at mba dot ocn dot ne dot jp>]

On Thu, 15 Jun 2006 11:32:52 -0400, Daniel Jacobowitz <dan@debian.org> wrote:
> > I also found a "rdhwr" in gcc's mips.md file ("tls_get_tp_<mode>").
> > Is this the origin?  MD is a very foreign language for me...
> 
> Yes.  Compile something like this with -O2 but without -fpic:
> 
> __thread int x;
> int foo() { return x; }
> 
> It should use the IE model, which will generate a rdhwr.

Thanks.  So this must be a gcc issue, not glibc issue.

extern __thread int x;
int foo(int arg)
{
	if (arg)
		return x;
	return 0;
}

If I compiled this program with -O2 I got:

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

It looks too bad for arg == 0 case.  I should ask on gcc list.

---
Atsushi Nemoto