Fix the GCC build (#40)

* Make it compile with GCC.  
* Add GCC to CI.
* Add warning to documentation about using GCC.

src/ds/bits.h

@@ -14,7 +14,13 @@
 #  include <emmintrin.h>
 #  define ALWAYSINLINE __attribute__((always_inline))
 #  define NOINLINE __attribute__((noinline))
-#  define HEADER_GLOBAL __attribute__((selectany))
+#  ifdef __clang__
+#    define HEADER_GLOBAL __attribute__((selectany))
+#  else
+//  GCC does not support selectany, weak is almost the correct
+//  attribute, but leaves the global variable preemptible.
+#    define HEADER_GLOBAL __attribute__((weak))
+#  endif
 #endif
 
 #if defined(__i386__) || defined(_M_IX86) || defined(_X86_) || \
@@ -299,7 +305,8 @@ namespace snmalloc
 #  endif
 #else
       size_t prod = x * y;
-      return y && (x > ((size_t)-1 / y));
+      overflow = y && (x > ((size_t)-1 / y));
+      return prod;
 #endif
     }
 

src/ds/mpmcstack.h

@@ -7,14 +7,14 @@ namespace snmalloc
   template<class T, Construction c = RequiresInit>
   class MPMCStack
   {
-    using ABA = ABA<T, c>;
+    using ABAT = ABA<T, c>;
 
   private:
     static_assert(
       std::is_same<decltype(T::next), std::atomic<T*>>::value,
       "T->next must be a std::atomic<T*>");
 
-    ABA stack;
+    ABAT stack;
 
   public:
     void push(T* item)
@@ -29,7 +29,7 @@ namespace snmalloc
 
       do
       {
-        T* top = ABA::load(cmp);
+        T* top = ABAT::load(cmp);
         last->next.store(top, std::memory_order_release);
       } while (!stack.compare_exchange(cmp, first));
     }
@@ -44,7 +44,7 @@ namespace snmalloc
 
       do
       {
-        top = ABA::load(cmp);
+        top = ABAT::load(cmp);
 
         if (top == nullptr)
           break;
@@ -63,7 +63,7 @@ namespace snmalloc
 
       do
       {
-        top = ABA::load(cmp);
+        top = ABAT::load(cmp);
 
         if (top == nullptr)
           break;

src/mem/largealloc.h

@@ -8,6 +8,8 @@
 #include "baseslab.h"
 #include "sizeclass.h"
 
+#include <new>
+
 namespace snmalloc
 {
   template<class PAL>

src/pal/pal_linux.h

@@ -44,6 +44,11 @@ namespace snmalloc
 
       if constexpr (zero_mem == YesZero)
         zero<true>(p, size);
+      else
+      {
+        UNUSED(p);
+        UNUSED(size);
+      }
     }
 
     /// OS specific function for zeroing memory

src/test/measuretime.h

@@ -11,4 +11,4 @@
     auto diff__ = finish__ - start__; \
     std::cout << name << ": " << std::setw(12) << diff__.count() << " ns" \
               << std::endl; \
-  }
+  }