AArch64: Update A64FX memset not to degrade at 16KB

This patch updates unroll8 code so as not to degrade at the peak
performance 16KB for both FX1000 and FX700.

Inserted 2 instructions at the beginning of the unroll8 loop,
cmp and branch, are a workaround that is found heuristically.

Reviewed-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com>

diff --git a/sysdeps/aarch64/multiarch/memset_a64fx.S b/sysdeps/aarch64/multiarch/memset_a64fx.S
index 7bf759b6a7530e76e2ec8f2d2f173eec0adead61..f7dfdaace7cfe74139d6d2261e88219035cbd6a1 100644 (file)
--- a/sysdeps/aarch64/multiarch/memset_a64fx.S
+++ b/sysdeps/aarch64/multiarch/memset_a64fx.S
@@ -96,7 +96,14 @@ L(vl_agnostic): // VL Agnostic
 L(unroll8):
        sub     count, count, tmp1
        .p2align 4
-1:     st1b_unroll 0, 7
+       // The 2 instructions at the beginning of the following loop,
+       // cmp and branch, are a workaround so as not to degrade at
+       // the peak performance 16KB.
+       // It is found heuristically and the branch condition, b.ne,
+       // is chosen intentionally never to jump.
+1:     cmp     xzr, xzr
+       b.ne    1b
+       st1b_unroll 0, 7
        add     dst, dst, tmp1
        subs    count, count, tmp1
        b.hi    1b