From 7202f9e091c4b2687e03af683ecc6346d044ab7c Mon Sep 17 00:00:00 2001
From: Matthew Parkinson <mattpark@microsoft.com>
Date: Tue, 6 Apr 2021 09:58:50 +0100
Subject: [PATCH] Handle UB that was not exercised originally.

---
 src/ds/bits.h              | 7 ++++++-
 src/test/func/bits/bits.cc | 2 +-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/src/ds/bits.h b/src/ds/bits.h
index 1fc90e4..ec32e3c 100644
--- a/src/ds/bits.h
+++ b/src/ds/bits.h
@@ -55,6 +55,7 @@ namespace snmalloc
 
     SNMALLOC_FAST_PATH size_t clz(size_t x)
     {
+      SNMALLOC_ASSERT(x != 0); // Calling with 0 is UB on some implementations
 #if defined(_MSC_VER)
 #  ifdef USE_LZCNT
 #    ifdef SNMALLOC_VA_BITS_64
@@ -148,6 +149,8 @@ namespace snmalloc
 
     inline size_t ctz(size_t x)
     {
+      SNMALLOC_ASSERT(x != 0); // Calling with 0 is UB on some implementations
+
 #if defined(_MSC_VER)
 #  ifdef SNMALLOC_VA_BITS_64
       return _tzcnt_u64(x);
@@ -229,7 +232,9 @@ namespace snmalloc
     inline size_t next_pow2_bits(size_t x)
     {
       // Correct for numbers [1..MAX_SIZE].
-      // Returns 64 for 0. Approximately 2 cycles.
+      if (x == 1)
+        return 0;
+
       return BITS - clz(x - 1);
     }
 
diff --git a/src/test/func/bits/bits.cc b/src/test/func/bits/bits.cc
index 26c5ae7..f78be0b 100644
--- a/src/test/func/bits/bits.cc
+++ b/src/test/func/bits/bits.cc
@@ -2,8 +2,8 @@
  * Unit tests for operations in bits.h
  */
 
-#include <ds/bits.h>
 #include <iostream>
+#include <snmalloc.h>
 #include <test/setup.h>
 
 void test_ctz()