diff --git a/include/jemalloc/internal/bit_util.h b/include/jemalloc/internal/bit_util.h
index c045eb86..258fd978 100644
--- a/include/jemalloc/internal/bit_util.h
+++ b/include/jemalloc/internal/bit_util.h
@@ -11,20 +11,29 @@
 #  error JEMALLOC_INTERNAL_FFS{,L,LL} should have been defined by configure
 #endif
 
-
+/*
+ * Unlike the builtins and posix ffs functions, our ffs requires a non-zero
+ * input, and returns the position of the lowest bit set (as opposed to the
+ * posix versions, which return 1 larger than that position and use a return
+ * value of zero as a sentinel.  This tends to simplify logic in callers, and
+ * allows for consistency with the builtins we build fls on top of.
+ */
 BIT_UTIL_INLINE unsigned
-ffs_llu(unsigned long long bitmap) {
-	return JEMALLOC_INTERNAL_FFSLL(bitmap);
+ffs_llu(unsigned long long x) {
+	util_assume(x != 0);
+	return JEMALLOC_INTERNAL_FFSLL(x) - 1;
 }
 
 BIT_UTIL_INLINE unsigned
-ffs_lu(unsigned long bitmap) {
-	return JEMALLOC_INTERNAL_FFSL(bitmap);
+ffs_lu(unsigned long x) {
+	util_assume(x != 0);
+	return JEMALLOC_INTERNAL_FFSL(x) - 1;
 }
 
 BIT_UTIL_INLINE unsigned
-ffs_u(unsigned bitmap) {
-	return JEMALLOC_INTERNAL_FFS(bitmap);
+ffs_u(unsigned x) {
+	util_assume(x != 0);
+	return JEMALLOC_INTERNAL_FFS(x) - 1;
 }
 
 #ifdef JEMALLOC_INTERNAL_POPCOUNTL
@@ -41,7 +50,8 @@ popcount_lu(unsigned long bitmap) {
 
 BIT_UTIL_INLINE size_t
 cfs_lu(unsigned long* bitmap) {
-	size_t bit = ffs_lu(*bitmap) - 1;
+	util_assume(*bitmap != 0);
+	size_t bit = ffs_lu(*bitmap);
 	*bitmap ^= ZU(1) << bit;
 	return bit;
 }
@@ -209,7 +219,7 @@ lg_floor(size_t x) {
 		return (8 << LG_SIZEOF_PTR) - 1;
 	}
 	x++;
-	return ffs_zu(x) - 2;
+	return ffs_zu(x) - 1;
 }
 #endif
 
diff --git a/include/jemalloc/internal/bitmap.h b/include/jemalloc/internal/bitmap.h
index f7152a6a..dc19454d 100644
--- a/include/jemalloc/internal/bitmap.h
+++ b/include/jemalloc/internal/bitmap.h
@@ -272,7 +272,7 @@ bitmap_ffu(const bitmap_t *bitmap, const bitmap_info_t *binfo, size_t min_bit) {
 			}
 			return bitmap_ffu(bitmap, binfo, sib_base);
 		}
-		bit += ((size_t)(ffs_lu(group_masked) - 1)) <<
+		bit += ((size_t)ffs_lu(group_masked)) <<
 		    (lg_bits_per_group - LG_BITMAP_GROUP_NBITS);
 	}
 	assert(bit >= min_bit);
@@ -284,9 +284,9 @@ bitmap_ffu(const bitmap_t *bitmap, const bitmap_info_t *binfo, size_t min_bit) {
 	    - 1);
 	size_t bit;
 	do {
-		bit = ffs_lu(g);
-		if (bit != 0) {
-			return (i << LG_BITMAP_GROUP_NBITS) + (bit - 1);
+		if (g != 0) {
+			bit = ffs_lu(g);
+			return (i << LG_BITMAP_GROUP_NBITS) + bit;
 		}
 		i++;
 		g = bitmap[i];
@@ -307,20 +307,20 @@ bitmap_sfu(bitmap_t *bitmap, const bitmap_info_t *binfo) {
 #ifdef BITMAP_USE_TREE
 	i = binfo->nlevels - 1;
 	g = bitmap[binfo->levels[i].group_offset];
-	bit = ffs_lu(g) - 1;
+	bit = ffs_lu(g);
 	while (i > 0) {
 		i--;
 		g = bitmap[binfo->levels[i].group_offset + bit];
-		bit = (bit << LG_BITMAP_GROUP_NBITS) + (ffs_lu(g) - 1);
+		bit = (bit << LG_BITMAP_GROUP_NBITS) + ffs_lu(g);
 	}
 #else
 	i = 0;
 	g = bitmap[0];
-	while ((bit = ffs_lu(g)) == 0) {
+	while (g == 0) {
 		i++;
 		g = bitmap[i];
 	}
-	bit = (i << LG_BITMAP_GROUP_NBITS) + (bit - 1);
+	bit = (i << LG_BITMAP_GROUP_NBITS) + ffs_lu(g);
 #endif
 	bitmap_set(bitmap, binfo, bit);
 	return bit;
diff --git a/include/jemalloc/internal/prng.h b/include/jemalloc/internal/prng.h
index 15cc2d18..12380b41 100644
--- a/include/jemalloc/internal/prng.h
+++ b/include/jemalloc/internal/prng.h
@@ -136,7 +136,7 @@ prng_range_u32(atomic_u32_t *state, uint32_t range, bool atomic) {
 	assert(range > 1);
 
 	/* Compute the ceiling of lg(range). */
-	lg_range = ffs_u32(pow2_ceil_u32(range)) - 1;
+	lg_range = ffs_u32(pow2_ceil_u32(range));
 
 	/* Generate a result in [0..range) via repeated trial. */
 	do {
@@ -154,7 +154,7 @@ prng_range_u64(uint64_t *state, uint64_t range) {
 	assert(range > 1);
 
 	/* Compute the ceiling of lg(range). */
-	lg_range = ffs_u64(pow2_ceil_u64(range)) - 1;
+	lg_range = ffs_u64(pow2_ceil_u64(range));
 
 	/* Generate a result in [0..range) via repeated trial. */
 	do {
@@ -172,7 +172,7 @@ prng_range_zu(atomic_zu_t *state, size_t range, bool atomic) {
 	assert(range > 1);
 
 	/* Compute the ceiling of lg(range). */
-	lg_range = ffs_u64(pow2_ceil_u64(range)) - 1;
+	lg_range = ffs_u64(pow2_ceil_u64(range));
 
 	/* Generate a result in [0..range) via repeated trial. */
 	do {
diff --git a/src/pages.c b/src/pages.c
index 0ddc5ba0..05bbf728 100644
--- a/src/pages.c
+++ b/src/pages.c
@@ -211,8 +211,8 @@ pages_map(void *addr, size_t size, size_t alignment, bool *commit) {
 			flags |= MAP_FIXED | MAP_EXCL;
 		} else {
 			unsigned alignment_bits = ffs_zu(alignment);
-			assert(alignment_bits > 1);
-			flags |= MAP_ALIGNED(alignment_bits - 1);
+			assert(alignment_bits > 0);
+			flags |= MAP_ALIGNED(alignment_bits);
 		}
 
 		void *ret = mmap(addr, size, prot, flags, -1, 0);
@@ -600,7 +600,7 @@ init_thp_state(void) {
 #endif
 
         if (nread < 0) {
-		goto label_error; 
+		goto label_error;
         }
 
 	if (strncmp(buf, sys_state_madvise, (size_t)nread) == 0) {
diff --git a/test/unit/bit_util.c b/test/unit/bit_util.c
index 3eeb7a31..f3761fd7 100644
--- a/test/unit/bit_util.c
+++ b/test/unit/bit_util.c
@@ -101,11 +101,63 @@ TEST_BEGIN(test_lg_ceil_floor) {
 }
 TEST_END
 
+#define TEST_FFS(t, suf, test_suf, pri) do {				\
+	for (unsigned i = 0; i < sizeof(t) * 8; i++) {			\
+		for (unsigned j = 0; j <= i; j++) {			\
+			for (unsigned k = 0; k <= j; k++) {		\
+				t x = (t)1 << i;			\
+				x |= (t)1 << j;				\
+				x |= (t)1 << k;				\
+				expect_##test_suf##_eq(ffs_##suf(x), k,	\
+				    "Unexpected result, x=%"pri, x);	\
+			}						\
+		}							\
+	}								\
+} while(0)
+
+TEST_BEGIN(test_ffs_u) {
+	TEST_FFS(unsigned, u, u,"u");
+}
+TEST_END
+
+
+TEST_BEGIN(test_ffs_lu) {
+	TEST_FFS(unsigned long, lu, lu, "lu");
+}
+TEST_END
+
+TEST_BEGIN(test_ffs_llu) {
+	TEST_FFS(unsigned long long, llu, qd, "llu");
+}
+TEST_END
+
+TEST_BEGIN(test_ffs_u32) {
+	TEST_FFS(uint32_t, u32, u32, FMTu32);
+}
+TEST_END
+
+
+TEST_BEGIN(test_ffs_u64) {
+	TEST_FFS(uint64_t, u64, u64, FMTu64);
+}
+TEST_END
+
+TEST_BEGIN(test_ffs_zu) {
+	TEST_FFS(size_t, zu, zu, "zu");
+}
+TEST_END
+
 int
 main(void) {
 	return test(
 	    test_pow2_ceil_u64,
 	    test_pow2_ceil_u32,
 	    test_pow2_ceil_zu,
-	    test_lg_ceil_floor);
+	    test_lg_ceil_floor,
+	    test_ffs_u,
+	    test_ffs_lu,
+	    test_ffs_llu,
+	    test_ffs_u32,
+	    test_ffs_u64,
+	    test_ffs_zu);
 }