From 1b721fc4146c033e3f8136cd49978aa785321cf8 Mon Sep 17 00:00:00 2001
From: Paul Bone <pbone@mozilla.com>
Date: Thu, 21 Oct 2021 06:03:45 +0000
Subject: [PATCH] Bug 1735715 - Handle gMaxSubPageClass == 0 properly
 r=glandium

Differential Revision: https://phabricator.services.mozilla.com/D128448
---
 memory/build/Utils.h         | 14 --------------
 memory/build/mozjemalloc.cpp | 14 ++++++++------
 mfbt/MathAlgorithms.h        |  2 +-
 3 files changed, 9 insertions(+), 21 deletions(-)

diff --git a/memory/build/Utils.h b/memory/build/Utils.h
index 76d180d22eb8..6032bf23d0c6 100644
--- a/memory/build/Utils.h
+++ b/memory/build/Utils.h
@@ -20,20 +20,6 @@ struct Log2 : mozilla::tl::CeilingLog2<N> {
 };
 #define LOG2(N) Log2<N>::value
 
-// Like Log2, but ignores 0.
-template <size_t N>
-struct Log2Or0 : mozilla::tl::CeilingLog2<N> {
-  using mozilla::tl::CeilingLog2<N>::value;
-  static_assert(1ULL << value == N, "Number is not a power of 2");
-};
-template <>
-struct Log2Or0<0> {
-  // This makes no sense but neither does any other value.  It's just enough
-  // that this can be used on the unused side of a conditional expression.
-  static const size_t value = 0;
-};
-#define LOG2_OR_0(N) Log2Or0<N>::value
-
 enum class Order {
   eLess = -1,
   eEqual = 0,
diff --git a/memory/build/mozjemalloc.cpp b/memory/build/mozjemalloc.cpp
index 022332399c36..583f5570079c 100644
--- a/memory/build/mozjemalloc.cpp
+++ b/memory/build/mozjemalloc.cpp
@@ -483,21 +483,21 @@ static size_t gPageSize;
 #  define END_GLOBALS
 #  define DEFINE_GLOBAL(type) static const type
 #  define GLOBAL_LOG2 LOG2
-#  define GLOBAL_LOG2_OR_0 LOG2_OR_0
 #  define GLOBAL_ASSERT_HELPER1(x) static_assert(x, #  x)
 #  define GLOBAL_ASSERT_HELPER2(x, y) static_assert(x, y)
 #  define GLOBAL_ASSERT(...)                                               \
     MACRO_CALL(                                                            \
         MOZ_PASTE_PREFIX_AND_ARG_COUNT(GLOBAL_ASSERT_HELPER, __VA_ARGS__), \
         (__VA_ARGS__))
+#  define GLOBAL_CONSTEXPR constexpr
 #else
 #  define DECLARE_GLOBAL(type, name) static type name;
 #  define DEFINE_GLOBALS static void DefineGlobals() {
 #  define END_GLOBALS }
 #  define DEFINE_GLOBAL(type)
 #  define GLOBAL_LOG2 FloorLog2
-#  define GLOBAL_LOG2_OR_0 FloorLog2
 #  define GLOBAL_ASSERT MOZ_RELEASE_ASSERT
+#  define GLOBAL_CONSTEXPR
 #endif
 
 DECLARE_GLOBAL(size_t, gMaxSubPageClass)
@@ -520,10 +520,12 @@ gMaxSubPageClass = gPageSize / 2 >= kMinSubPageClass ? gPageSize / 2 : 0;
 
 // Number of sub-page bins.
 DEFINE_GLOBAL(uint8_t)
-gNumSubPageClasses =
-    static_cast<uint8_t>(gMaxSubPageClass ? GLOBAL_LOG2_OR_0(gMaxSubPageClass) -
-                                                LOG2(kMinSubPageClass) + 1
-                                          : 0);
+gNumSubPageClasses = []() GLOBAL_CONSTEXPR -> uint8_t {
+  if GLOBAL_CONSTEXPR (gMaxSubPageClass != 0) {
+    return FloorLog2(gMaxSubPageClass) - LOG2(kMinSubPageClass) + 1;
+  }
+  return 0;
+}();
 
 DEFINE_GLOBAL(uint8_t) gPageSize2Pow = GLOBAL_LOG2(gPageSize);
 DEFINE_GLOBAL(size_t) gPageSizeMask = gPageSize - 1;
diff --git a/mfbt/MathAlgorithms.h b/mfbt/MathAlgorithms.h
index 8c64685ef1d8..b60dbfa12e6e 100644
--- a/mfbt/MathAlgorithms.h
+++ b/mfbt/MathAlgorithms.h
@@ -375,7 +375,7 @@ class FloorLog2<T, 8> {
  * FloorLog2(8..15) is 3; and so on.
  */
 template <typename T>
-inline uint_fast8_t FloorLog2(const T aValue) {
+inline constexpr uint_fast8_t FloorLog2(const T aValue) {
   return detail::FloorLog2<T>::compute(aValue);
 }