x86: Move and slightly improve memset_erms

Implementation wise:
    1. Remove the VZEROUPPER as memset_{impl}_unaligned_erms does not
       use the L(stosb) label that was previously defined.

    2. Don't give the hotpath (fallthrough) to zero size.

Code positioning wise:

Move memset_{chk}_erms to its own file.  Leaving it in between the
memset_{impl}_unaligned both adds unnecessary complexity to the
file and wastes space in a relatively hot cache section.

1 files changed, 44 insertions, 0 deletions

diff --git a/sysdeps/x86_64/multiarch/memset-erms.S b/sysdeps/x86_64/multiarch/memset-erms.S
new file mode 100644
index 0000000000..e83cccc731
--- /dev/null
+++ b/sysdeps/x86_64/multiarch/memset-erms.S
@@ -0,0 +1,44 @@
+/* memset implement with rep stosb
+   Copyright (C) 2022 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+
+#include <sysdep.h>
+
+#if defined USE_MULTIARCH && IS_IN (libc)
+	.text
+ENTRY (__memset_chk_erms)
+	cmp	%RDX_LP, %RCX_LP
+	jb	HIDDEN_JUMPTARGET (__chk_fail)
+END (__memset_chk_erms)
+
+/* Only used to measure performance of REP STOSB.  */
+ENTRY (__memset_erms)
+	/* Skip zero length.  */
+	test	%RDX_LP, %RDX_LP
+	jz	 L(stosb_return_zero)
+	mov	%RDX_LP, %RCX_LP
+	movzbl	%sil, %eax
+	mov	%RDI_LP, %RDX_LP
+	rep stosb
+	mov	%RDX_LP, %RAX_LP
+	ret
+L(stosb_return_zero):
+	movq	%rdi, %rax
+	ret
+END (__memset_erms)
+#endif