[RFC PATCH v1 3/5] tools/nolibc: x86-64: Use `rep cmpsb` for `memcmp()`

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

Simplify memcmp() on the x86-64 arch.

The x86-64 arch has a 'rep cmpsb' instruction, which can be used to
implement the memcmp() function.

    %rdi = source 1
    %rsi = source 2
    %rcx = length

Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
---
 tools/include/nolibc/arch-x86_64.h | 19 +++++++++++++++++++
 tools/include/nolibc/string.h      |  2 ++
 2 files changed, 21 insertions(+)

diff --git a/tools/include/nolibc/arch-x86_64.h b/tools/include/nolibc/arch-x86_64.h
index 42f2674ad1ecdd64..6c1b54ba9f774e7b 100644
--- a/tools/include/nolibc/arch-x86_64.h
+++ b/tools/include/nolibc/arch-x86_64.h
@@ -214,4 +214,23 @@ __asm__ (
 	"retq\n"
 );
 
+#define NOLIBC_ARCH_HAS_MEMCMP
+static int memcmp(const void *s1, const void *s2, size_t n)
+{
+	const unsigned char *p1 = s1;
+	const unsigned char *p2 = s2;
+
+	if (!n)
+		return 0;
+
+	__asm__ volatile (
+		"rep cmpsb"
+		: "+D"(p2), "+S"(p1), "+c"(n)
+		: "m"(*(const unsigned char (*)[n])s1),
+		  "m"(*(const unsigned char (*)[n])s2)
+	);
+
+	return p1[-1] - p2[-1];
+}
+
 #endif /* _NOLIBC_ARCH_X86_64_H */
diff --git a/tools/include/nolibc/string.h b/tools/include/nolibc/string.h
index 1bad6121ef8c4ab5..3c941289d5dd0985 100644
--- a/tools/include/nolibc/string.h
+++ b/tools/include/nolibc/string.h
@@ -15,6 +15,7 @@ static void *malloc(size_t len);
  * As much as possible, please keep functions alphabetically sorted.
  */
 
+#ifndef NOLIBC_ARCH_HAS_MEMCMP
 static __attribute__((unused))
 int memcmp(const void *s1, const void *s2, size_t n)
 {
@@ -26,6 +27,7 @@ int memcmp(const void *s1, const void *s2, size_t n)
 	}
 	return c1;
 }
+#endif /* #ifndef NOLIBC_ARCH_HAS_MEMCMP */
 
 static __attribute__((unused))
 void *_nolibc_memcpy_up(void *dst, const void *src, size_t len)
-- 
Ammar Faizi