Move the inline memcmp function currently only available in 'boot/string.c'
into the shared string function header <asm/shared/string.h> to be reused.

This is not done through <asm/string.h> to avoid pulling unnecessary code
in 'boot/string.c' that causes build errors in 'boot/compressed/string.c'
and 'purgatory/purgatory.ro'.

Signed-off-by: Mauricio Faria de Oliveira <mfo@igalia.com>

---

Thanks to David Laight for noticing the return value difference between
inline and regular memcmp().
---
 arch/x86/boot/string.c               |  6 ++----
 arch/x86/include/asm/shared/string.h | 20 ++++++++++++++++++++
 arch/x86/include/asm/string.h        |  2 ++
 3 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/arch/x86/boot/string.c b/arch/x86/boot/string.c
index ac0f900ebc47efa81c92e1bb2010ea41677899c4..be454a6864225f3a972c3e81826b77ed4e8a57fe 100644
--- a/arch/x86/boot/string.c
+++ b/arch/x86/boot/string.c
@@ -15,6 +15,7 @@
 #include <linux/errno.h>
 #include <linux/limits.h>
 #include <asm/asm.h>
+#include <asm/shared/string.h>
 #include "ctype.h"
 #include "string.h"
 
@@ -31,10 +32,7 @@
 
 int memcmp(const void *s1, const void *s2, size_t len)
 {
-	bool diff;
-	asm("repe cmpsb"
-	    : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len));
-	return diff;
+	return __inline_memcmp(s1, s2, len);
 }
 
 /*
diff --git a/arch/x86/include/asm/shared/string.h b/arch/x86/include/asm/shared/string.h
new file mode 100644
index 0000000000000000000000000000000000000000..02b92927553f7b8e1c87e6122bbaa70439e57ea7
--- /dev/null
+++ b/arch/x86/include/asm/shared/string.h
@@ -0,0 +1,20 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_SHARED_STRING_H
+#define _ASM_X86_SHARED_STRING_H
+
+/*
+ * This inline memcmp() returns 0 (equal) or 1 (not equal).
+ * The regular memcmp() returns <0 (less than), 0 (equal), or >0 (greater than)
+ * to indicate ordering as well.
+ */
+static __always_inline int __inline_memcmp(const void *s1, const void *s2, size_t len)
+{
+	bool diff;
+
+	asm("repe cmpsb"
+	    : "=@ccnz" (diff), "+D" (s1), "+S" (s2), "+c" (len));
+
+	return diff;
+}
+
+#endif /* _ASM_X86_SHARED_STRING_H */
diff --git a/arch/x86/include/asm/string.h b/arch/x86/include/asm/string.h
index 9cb5aae7fba9ffcf0f5af8f939d30467750ccaa9..8d271a451721d97f78a84009613381414f91f2a6 100644
--- a/arch/x86/include/asm/string.h
+++ b/arch/x86/include/asm/string.h
@@ -8,6 +8,8 @@
 # include <asm/string_64.h>
 #endif
 
+#include <asm/shared/string.h>
+
 static __always_inline void *__inline_memcpy(void *to, const void *from, size_t len)
 {
 	void *ret = to;

-- 
2.47.3