math/big: improve performance of mulAddVWW on ppc64x

This changes the assembly implementation on ppc64x
to improve performance by reordering some instructions.
It also eliminates an unnecessary move by changing an
ADDZE to use the correct target register.

Improvement on power9:

MulAddVWW/1         6.89ns ± 0%    7.30ns ± 0%   +5.95%  (p=1.000 n=1+1)
MulAddVWW/2         8.04ns ± 0%    8.06ns ± 0%   +0.25%  (p=1.000 n=1+1)
MulAddVWW/3         9.39ns ± 0%    9.39ns ± 0%     ~     (all equal)
MulAddVWW/4         9.76ns ± 0%    9.48ns ± 0%   -2.87%  (p=1.000 n=1+1)
MulAddVWW/5         10.5ns ± 0%    10.3ns ± 0%   -1.90%  (p=1.000 n=1+1)
MulAddVWW/10        15.4ns ± 0%    14.9ns ± 0%   -3.25%  (p=1.000 n=1+1)
MulAddVWW/100        149ns ± 0%     125ns ± 0%  -16.11%  (p=1.000 n=1+1)
MulAddVWW/1000      1.42µs ± 0%    1.28µs ± 0%   -9.74%  (p=1.000 n=1+1)
MulAddVWW/10000     14.2µs ± 0%    12.8µs ± 0%   -9.73%  (p=1.000 n=1+1)
MulAddVWW/100000     144µs ± 0%     129µs ± 0%  -10.10%  (p=1.000 n=1+1)

Change-Id: I0ae7002a69783ca19d7a4e3e42042ae75dc60069
Reviewed-on: https://go-review.googlesource.com/c/go/+/248721
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Carlos Eduardo Seo <cseo@linux.vnet.ibm.com>
Reviewed-by: Paul Murphy <murp@ibm.com>

diff --git a/src/math/big/arith_ppc64x.s b/src/math/big/arith_ppc64x.s
index dbb168a37692c1aa62fd4e0520f7ce017845586c..409e10ab48bd4741c168998be6839d5f2fef2e75 100644 (file)
--- a/src/math/big/arith_ppc64x.s
+++ b/src/math/big/arith_ppc64x.s
@@ -394,17 +394,16 @@ loop:
        ADDZE   R21
        MULLD   R9, R22, R26
        MULHDU  R9, R22, R22
-       ADDC    R21, R26
-       ADDZE   R22
        MULLD   R9, R23, R27
        MULHDU  R9, R23, R23
-       ADDC    R22, R27
-       ADDZE   R23
+       ADDC    R21, R26
+       ADDZE   R22
        MOVD    R24, 8(R10)       // z[i]
        MOVD    R25, 16(R10)      // z[i+1]
+       ADDC    R22, R27
+       ADDZE   R23,R4            // update carry
        MOVD    R26, 24(R10)      // z[i+2]
        MOVDU   R27, 32(R10)      // z[i+3]
-       MOVD    R23, R4           // R4 = c
        ADD     $-4, R11          // R11 = z_len - 4
        BC      16, 0, loop       // bdnz