[glibc/dczva] aarch64: Disable using DC ZVA in emag memset

[Feng Xue <fxue at sourceware dot org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=23e6b0233dec987aaa456f39af6db433d8136c90

commit 23e6b0233dec987aaa456f39af6db433d8136c90
Author: Feng Xue <feng.xue@amperecomputing.com>
Date:   Wed Aug 14 10:48:05 2019 +0800

    aarch64: Disable using DC ZVA in emag memset
    
        * sysdeps/aarch64/multiarch/memset_base64.S (DC_ZVA_THRESHOLD):
        Disable DC ZVA code if this macro is defined as zero.
        * sysdeps/aarch64/multiarch/memset_emag.S (DC_ZVA_THRESHOLD):
        Change to zero to disable using DC ZVA.

Diff:
---
 ChangeLog                                 |  7 +++++++
 sysdeps/aarch64/multiarch/memset_base64.S | 12 ++++++++++--
 sysdeps/aarch64/multiarch/memset_emag.S   | 12 +++++++-----
 3 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index be6c9b3..b6c92d4 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2019-08-14  Feng Xue  <fxue@os.amperecomputing.com>
+
+	* sysdeps/aarch64/multiarch/memset_base64.S (DC_ZVA_THRESHOLD):
+	Disable DC ZVA code if this macro is defined as zero.
+	* sysdeps/aarch64/multiarch/memset_emag.S (DC_ZVA_THRESHOLD):
+	Change to zero to disable using DC ZVA.
+
 2019-08-13  Joseph Myers  <joseph@codesourcery.com>
 
 	* bits/libc-header-start.h (__GLIBC_USE_IEC_60559_FUNCS_EXT):
diff --git a/sysdeps/aarch64/multiarch/memset_base64.S b/sysdeps/aarch64/multiarch/memset_base64.S
index 9a62325..c0cccba 100644
--- a/sysdeps/aarch64/multiarch/memset_base64.S
+++ b/sysdeps/aarch64/multiarch/memset_base64.S
@@ -23,6 +23,7 @@
 # define MEMSET __memset_base64
 #endif
 
+/* To disable DC ZVA, set this threshold to 0. */
 #ifndef DC_ZVA_THRESHOLD
 # define DC_ZVA_THRESHOLD 512
 #endif
@@ -91,11 +92,12 @@ L(set96):
 	.p2align 4
 L(set_long):
 	stp	val, val, [dstin]
+	bic	dst, dstin, 15
+#if DC_ZVA_THRESHOLD
 	cmp	count, DC_ZVA_THRESHOLD
 	ccmp	val, 0, 0, cs
-	bic	dst, dstin, 15
 	b.eq	L(zva_64)
-
+#endif
 	/* Small-size or non-zero memset does not use DC ZVA. */
 	sub	count, dstend, dst
 
@@ -105,7 +107,11 @@ L(set_long):
 	 * count is less than 33 bytes, so as to bypass 2 unneccesary stps.
 	 */
 	sub	count, count, 64+16+1
+
+#if DC_ZVA_THRESHOLD
+	/* Align loop on 16-byte boundary, this might be friendly to i-cache. */
 	nop
+#endif
 
 1:	stp	val, val, [dst, 16]
 	stp	val, val, [dst, 32]
@@ -121,6 +127,7 @@ L(set_long):
 	stp	val, val, [dstend, -16]
 	ret
 
+#if DC_ZVA_THRESHOLD
 	.p2align 3
 L(zva_64):
 	stp	val, val, [dst, 16]
@@ -173,6 +180,7 @@ L(zva_64):
 1:	stp	val, val, [dstend, -32]
 	stp	val, val, [dstend, -16]
 	ret
+#endif
 
 END (MEMSET)
 libc_hidden_builtin_def (MEMSET)
diff --git a/sysdeps/aarch64/multiarch/memset_emag.S b/sysdeps/aarch64/multiarch/memset_emag.S
index 1c1fabc..c2aed62 100644
--- a/sysdeps/aarch64/multiarch/memset_emag.S
+++ b/sysdeps/aarch64/multiarch/memset_emag.S
@@ -21,12 +21,14 @@
 # define MEMSET __memset_emag
 
 /*
- * Using dc zva to zero memory does not produce better performance if
+ * Using DC ZVA to zero memory does not produce better performance if
  * memory size is not very large, especially when there are multiple
- * processes/threads contending memory/cache. Here we use a somewhat
- * large threshold to trigger usage of dc zva.
-*/
-# define DC_ZVA_THRESHOLD 1024
+ * processes/threads contending memory/cache. Here we set threshold to
+ * zero to disable using DC ZVA, which is good for multi-process/thread
+ * workloads.
+ */
+
+# define DC_ZVA_THRESHOLD 0
 
 # include "./memset_base64.S"
 #endif