This rewrites the iteration step of swdivsf to be more register
efficient (two registers instead of four, no load of a FP constant).
This matches how ICC emits the rcp sequence and causes no overall loss
of precision (Micha might still remember the exact details). The patch is
fallout of the work trying to fix PR47989.
Bootstrapped and tested on x86_64-unknown-linux-gnu, ok for 4.7?
Thanks,
Richard.
2011-03-14 Richard Guenther <rguent...@suse.de>
* config/i386/i386.c (ix86_emit_swdivsf): Implement more
efficiently.
Index: trunk/gcc/config/i386/i386.c
===================================================================
--- trunk.orig/gcc/config/i386/i386.c 2011-03-09 11:52:21.000000000 +0100
+++ trunk/gcc/config/i386/i386.c 2011-03-10 15:43:47.000000000 +0100
@@ -31747,38 +31747,38 @@ void ix86_emit_i387_log1p (rtx op0, rtx
void ix86_emit_swdivsf (rtx res, rtx a, rtx b, enum machine_mode mode)
{
- rtx x0, x1, e0, e1, two;
+ rtx x0, x1, e0, e1;
x0 = gen_reg_rtx (mode);
e0 = gen_reg_rtx (mode);
e1 = gen_reg_rtx (mode);
x1 = gen_reg_rtx (mode);
- two = CONST_DOUBLE_FROM_REAL_VALUE (dconst2, SFmode);
-
- if (VECTOR_MODE_P (mode))
- two = ix86_build_const_vector (mode, true, two);
-
- two = force_reg (mode, two);
-
- /* a / b = a * rcp(b) * (2.0 - b * rcp(b)) */
+ /* a / b = a * ((rcp(b) + rcp(b)) - (b * rcp(b) * rcp (b))) */
/* x0 = rcp(b) estimate */
emit_insn (gen_rtx_SET (VOIDmode, x0,
gen_rtx_UNSPEC (mode, gen_rtvec (1, b),
UNSPEC_RCP)));
- /* e0 = x0 * a */
+ /* e0 = x0 * b */
emit_insn (gen_rtx_SET (VOIDmode, e0,
- gen_rtx_MULT (mode, x0, a)));
- /* e1 = x0 * b */
- emit_insn (gen_rtx_SET (VOIDmode, e1,
gen_rtx_MULT (mode, x0, b)));
- /* x1 = 2. - e1 */
+
+ /* e0 = x0 * e0 */
+ emit_insn (gen_rtx_SET (VOIDmode, e0,
+ gen_rtx_MULT (mode, x0, e0)));
+
+ /* e1 = x0 + x0 */
+ emit_insn (gen_rtx_SET (VOIDmode, e1,
+ gen_rtx_PLUS (mode, x0, x0)));
+
+ /* x1 = e1 - e0 */
emit_insn (gen_rtx_SET (VOIDmode, x1,
- gen_rtx_MINUS (mode, two, e1)));
- /* res = e0 * x1 */
+ gen_rtx_MINUS (mode, e1, e0)));
+
+ /* res = a * x1 */
emit_insn (gen_rtx_SET (VOIDmode, res,
- gen_rtx_MULT (mode, e0, x1)));
+ gen_rtx_MULT (mode, a, x1)));
}
/* Output code to perform a Newton-Rhapson approximation of a
