[libc] Add ffs(), ffsl(), and ffsll()

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

1 files changed, 37 insertions, 1 deletions

diff --git a/src/arch/x86_64/include/bits/strings.h b/src/arch/x86_64/include/bits/strings.h
index a56a911df..3b7911f3b 100644
--- a/src/arch/x86_64/include/bits/strings.h
+++ b/src/arch/x86_64/include/bits/strings.h
@@ -4,6 +4,42 @@
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 /**
+ * Find first (i.e. least significant) set bit
+ *
+ * @v value		Value
+ * @ret lsb		Least significant bit set in value (LSB=1), or zero
+ */
+static inline __attribute__ (( always_inline )) int __ffsll ( long long value ){
+	long long lsb_minus_one;
+
+	/* If the input value is zero, the BSF instruction returns
+	 * ZF=0 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__ ( "bsfq %1, %0"
+			  : "=r" ( lsb_minus_one )
+			  : "rm" ( value ) );
+		return ( lsb_minus_one + 1 );
+	} else {
+		return 0;
+	}
+}
+
+/**
+ * Find first (i.e. least significant) set bit
+ *
+ * @v value		Value
+ * @ret lsb		Least significant bit set in value (LSB=1), or zero
+ */
+static inline __attribute__ (( always_inline )) int __ffsl ( long value ) {
+
+	return __ffsll ( value );
+}
+
+/**
  * Find last (i.e. most significant) set bit
  *
  * @v value		Value
@@ -13,7 +49,7 @@ static inline __attribute__ (( always_inline )) int __flsll ( long long value ){
 	long 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.
+	 * ZF=0 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.