[libc] Add inline assembly implementation of flsl() using BSR instruction

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/include/bits/strings.h

@@ -0,0 +1,31 @@
+#ifndef _BITS_STRINGS_H
+#define _BITS_STRINGS_H
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+/**
+ * Find last (i.e. most significant) set bit
+ *
+ * @v value		Value
+ * @ret msb		Most significant bit set in value (LSB=1), or zero
+ */
+static inline __attribute__ (( always_inline )) int __flsl ( long value ) {
+	long msb_minus_one;
+
+	/* If the input value is zero, the BSR instruction returns
+	 * ZF=1 and leaves an undefined value in the output register.
+	 * Perform this check in C rather than asm so that it can be
+	 * omitted in cases where the compiler is able to prove that
+	 * the input is non-zero.
+	 */
+	if ( value ) {
+		__asm__ ( "bsr %1, %0"
+			  : "=r" ( msb_minus_one )
+			  : "rm" ( value ) );
+		return ( msb_minus_one + 1 );
+	} else {
+		return 0;
+	}
+}
+
+#endif /* _BITS_STRINGS_H */