On Mon, Aug 15, 2011 at 5:25 PM, Michael Matz <m...@suse.de> wrote:
>> > > .LFB0:
>> > > .cfi_startproc
>> > > movsd .LC0(%rip), %xmm2
>> > > movapd %xmm0, %xmm1
>> > > andpd %xmm2, %xmm1
>> > > andnpd %xmm0, %xmm2
>> > > addsd .LC1(%rip), %xmm1
>> > > roundsd $1, %xmm1, %xmm1
>> > > orpd %xmm2, %xmm1
>> > > movapd %xmm1, %xmm0
>> > > ret
>> >
>> > Hm, why do we need the sign-copy? If I read the docs correctly
>> > we can simply use roundsd directly, no?
>>
>> round-half-away-from-zero breaks your neck. round[ps][sd] only supports
>> the usual four IEEE rounding modes.
>
> But, you should be able to apply the sign to the 0.5, which wouldn't
> require building the absolute value of input:
>
> round(x) = trunc(x + (copysign (0.5, x)))
>
> which should roughly be expanded to:
>
> movsd signbits(%rip), %xmm1
> andpd %xmm0, %xmm1
> movsd nextof0.5(%rip), %xmm2
> orpd %xmm1, %xmm2
> addpd %xmm2, %xmm0
> roundsd $1, %xmm0, %xmm0
> ret
>
> Which has one logical operation less (and one move because I chose a more
> optimal register assignment).
x86 logical insns can load constants directly from the memory, so your
proposal creates even better code:
myround:
.LFB0:
.cfi_startproc
movapd %xmm0, %xmm1
andpd .LC1(%rip), %xmm1
orpd .LC0(%rip), %xmm1
addsd %xmm0, %xmm1
roundsd $3, %xmm1, %xmm0
ret
Please also note that copysign expander has special code path for op0
being constant.
I am testing following patch:
2011-08-20 Uros Bizjak <ubiz...@gmail.com>
Michael Matz <m...@suse.de>
* config/i386/i386.c (ix86_expand_round_sse4): Expand as
trunc (a + copysign (nextafter (0.5, 0.0), a)).
Uros.
Index: i386/i386.c
===================================================================
--- i386/i386.c (revision 177925)
+++ i386/i386.c (working copy)
@@ -32700,42 +32700,44 @@ void
ix86_expand_round_sse4 (rtx op0, rtx op1)
{
enum machine_mode mode = GET_MODE (op0);
- rtx e1, e2, e3, res, half, mask;
+ rtx e1, e2, res, half, mask;
const struct real_format *fmt;
REAL_VALUE_TYPE pred_half, half_minus_pred_half;
+ rtx (*gen_copysign) (rtx, rtx, rtx);
rtx (*gen_round) (rtx, rtx, rtx);
switch (mode)
{
case SFmode:
+ gen_copysign = gen_copysignsf3;
gen_round = gen_sse4_1_roundsf2;
break;
case DFmode:
+ gen_copysign = gen_copysigndf3;
gen_round = gen_sse4_1_rounddf2;
break;
default:
gcc_unreachable ();
}
- /* e1 = fabs(op1) */
- e1 = ix86_expand_sse_fabs (op1, &mask);
+ /* round (a) = trunc (a + copysign (0.5, a)) */
/* load nextafter (0.5, 0.0) */
fmt = REAL_MODE_FORMAT (mode);
real_2expN (&half_minus_pred_half, -(fmt->p) - 1, mode);
REAL_ARITHMETIC (pred_half, MINUS_EXPR, dconsthalf, half_minus_pred_half);
+ half = const_double_from_real_value (pred_half, mode);
- /* e2 = e1 + 0.5 */
- half = force_reg (mode, const_double_from_real_value (pred_half, mode));
- e2 = expand_simple_binop (mode, PLUS, e1, half, NULL_RTX, 0, OPTAB_DIRECT);
+ /* e1 = copysign (0.5, op1) */
+ e1 = gen_reg_rtx (mode);
+ emit_insn (gen_copysign (e1, half, op1));
- /* e3 = trunc(e2) */
- e3 = gen_reg_rtx (mode);
- emit_insn (gen_round (e3, e2, GEN_INT (ROUND_TRUNC)));
+ /* e2 = op1 + e1 */
+ e2 = expand_simple_binop (mode, PLUS, op1, e1, NULL_RTX, 0, OPTAB_DIRECT);
- /* res = copysign (e3, op1) */
+ /* res = trunc (e2) */
res = gen_reg_rtx (mode);
- ix86_sse_copysign_to_positive (res, e3, op1, mask);
+ emit_insn (gen_round (res, e2, GEN_INT (ROUND_TRUNC)));
emit_move_insn (op0, res);
}
