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From 1d9f99ce1b3788d1897cb53a76d57e973111b8fe Mon Sep 17 00:00:00 2001
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From: Naohiro Tamura <naohirot@fujitsu.com>
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Date: Fri, 27 Aug 2021 05:03:04 +0000
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Subject: [PATCH] AArch64: Update A64FX memset not to degrade at 16KB
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This patch updates unroll8 code so as not to degrade at the peak
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performance 16KB for both FX1000 and FX700.
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Inserted 2 instructions at the beginning of the unroll8 loop,
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cmp and branch, are a workaround that is found heuristically.
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Reviewed-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
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---
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sysdeps/aarch64/multiarch/memset_a64fx.S | 9 ++++++++-
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1 file changed, 8 insertions(+), 1 deletion(-)
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diff --git a/sysdeps/aarch64/multiarch/memset_a64fx.S b/sysdeps/aarch64/multiarch/memset_a64fx.S
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index 7bf759b6a7..f7dfdaace7 100644
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--- a/sysdeps/aarch64/multiarch/memset_a64fx.S
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+++ b/sysdeps/aarch64/multiarch/memset_a64fx.S
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@@ -96,7 +96,14 @@ L(vl_agnostic): // VL Agnostic
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L(unroll8):
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sub count, count, tmp1
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.p2align 4
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-1: st1b_unroll 0, 7
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+ // The 2 instructions at the beginning of the following loop,
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+ // cmp and branch, are a workaround so as not to degrade at
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+ // the peak performance 16KB.
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+ // It is found heuristically and the branch condition, b.ne,
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+ // is chosen intentionally never to jump.
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+1: cmp xzr, xzr
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+ b.ne 1b
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+ st1b_unroll 0, 7
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add dst, dst, tmp1
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subs count, count, tmp1
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b.hi 1b
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--
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2.31.1
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