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.

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):

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)

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