diff --git a/libc/intrin/bsf.c b/libc/intrin/bsf.c
index da6eca51e..70989f7ca 100644
--- a/libc/intrin/bsf.c
+++ b/libc/intrin/bsf.c
@@ -32,11 +32,20 @@
  *     0x08000000       27       27       28       27        4
  *     0xffffffff        0        0        1       31        0
  *
- * @param x is a 32-bit integer
- * @return number in range 0..31 or undefined if 𝑥 is 0
+ * @param 𝑥 is a 64-bit integer
+ * @return number in range 0..63 or undefined if 𝑥 is 0
  */
-int(_bsf)(int x) {
-  return _bsf(x);
+int(_bsfl)(long x) {
+  unsigned l, r;
+  x &= -x;
+  l = x | x >> 32;
+  r = !!(x >> 32), r <<= 1;
+  r += !!((l & 0xffff0000)), r <<= 1;
+  r += !!((l & 0xff00ff00)), r <<= 1;
+  r += !!((l & 0xf0f0f0f0)), r <<= 1;
+  r += !!((l & 0xcccccccc)), r <<= 1;
+  r += !!((l & 0xaaaaaaaa));
+  return r;
 }
 
 /**
@@ -53,11 +62,11 @@ int(_bsf)(int x) {
  *     0x08000000       27       27       28       27        4
  *     0xffffffff        0        0        1       31        0
  *
- * @param 𝑥 is a 64-bit integer
- * @return number in range 0..63 or undefined if 𝑥 is 0
+ * @param x is a 32-bit integer
+ * @return number in range 0..31 or undefined if 𝑥 is 0
  */
-int(_bsfl)(long x) {
-  return _bsfl(x);
+int(_bsf)(int x) {
+  return _bsf((unsigned)x);
 }
 
 __weak_reference(_bsfl, _bsfll);
diff --git a/libc/intrin/bsr.c b/libc/intrin/bsr.c
index 31e25879e..b8d8646dc 100644
--- a/libc/intrin/bsr.c
+++ b/libc/intrin/bsr.c
@@ -18,26 +18,12 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/intrin/bsr.h"
 
-/**
- * Returns binary logarithm of 𝑥.
- *
- *                           ctz(𝑥)         31^clz(𝑥)   clz(𝑥)
- *       uint32 𝑥  _bsf(𝑥) tzcnt(𝑥)   ffs(𝑥)  _bsr(𝑥) lzcnt(𝑥)
- *     0x00000000      wut       32        0      wut       32
- *     0x00000001        0        0        1        0       31
- *     0x80000001        0        0        1       31        0
- *     0x80000000       31       31       32       31        0
- *     0x00000010        4        4        5        4       27
- *     0x08000010        4        4        5       27        4
- *     0x08000000       27       27       28       27        4
- *     0xffffffff        0        0        1       31        0
- *
- * @param x is a 32-bit integer
- * @return number in range 0..31 or undefined if 𝑥 is 0
- */
-int(_bsr)(int x) {
-  return _bsr(x);
-}
+static const char kDebruijn[64] = {
+    0,  47, 1,  56, 48, 27, 2,  60, 57, 49, 41, 37, 28, 16, 3,  61,
+    54, 58, 35, 52, 50, 42, 21, 44, 38, 32, 29, 23, 17, 11, 4,  62,
+    46, 55, 26, 59, 40, 36, 15, 53, 34, 51, 20, 43, 31, 22, 10, 45,
+    25, 39, 14, 33, 19, 30, 9,  24, 13, 18, 8,  12, 7,  6,  5,  63,
+};
 
 /**
  * Returns binary logarithm of 𝑥.
@@ -57,7 +43,34 @@ int(_bsr)(int x) {
  * @return number in range 0..63 or undefined if 𝑥 is 0
  */
 int(_bsrl)(long x) {
-  return _bsrl(x);
+  x |= x >> 1;
+  x |= x >> 2;
+  x |= x >> 4;
+  x |= x >> 8;
+  x |= x >> 16;
+  x |= x >> 32;
+  return kDebruijn[(x * 0x03f79d71b4cb0a89) >> 58];
+}
+
+/**
+ * Returns binary logarithm of 𝑥.
+ *
+ *                           ctz(𝑥)         31^clz(𝑥)   clz(𝑥)
+ *       uint32 𝑥  _bsf(𝑥) tzcnt(𝑥)   ffs(𝑥)  _bsr(𝑥) lzcnt(𝑥)
+ *     0x00000000      wut       32        0      wut       32
+ *     0x00000001        0        0        1        0       31
+ *     0x80000001        0        0        1       31        0
+ *     0x80000000       31       31       32       31        0
+ *     0x00000010        4        4        5        4       27
+ *     0x08000010        4        4        5       27        4
+ *     0x08000000       27       27       28       27        4
+ *     0xffffffff        0        0        1       31        0
+ *
+ * @param x is a 32-bit integer
+ * @return number in range 0..31 or undefined if 𝑥 is 0
+ */
+int(_bsr)(int x) {
+  return _bsrl((unsigned)x);
 }
 
 __weak_reference(_bsrl, _bsrll);