Fix the GCC build (#40)

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

CMakeLists.txt

@@ -52,7 +52,10 @@ if(MSVC)
   set(CMAKE_EXE_LINKER_FLAGS_RELEASE "${CMAKE_EXE_LINKER_FLAGS_RELEASE} /DEBUG")
 else()
   find_package(Threads REQUIRED)
-  add_compile_options(-mcx16 -march=native -Wall -Wextra -Werror -Wsign-conversion -g -fno-exceptions -fno-rtti)
+  if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+    add_compile_options(-Wsign-conversion)
+  endif ()
+  add_compile_options(-mcx16 -march=native -Wall -Wextra -Werror -g -fno-exceptions -fno-rtti)
 endif()
 
 set(CMAKE_CXX_STANDARD 17)

README.md

@@ -36,8 +36,10 @@ using the Visual Studio compiler.
 # Building on macOS, Linux or FreeBSD
 Snmalloc has very few dependencies, CMake, Ninja, Clang 6.0 or later and a C++17
 standard library.
-Building with GCC is currently not supported because GCC lacks support for the
-`selectany` attribute to specify variables in a COMDAT.
+Building with GCC is currently not recommended because GCC lacks support for the
+`selectany` attribute to specify variables in a COMDAT.  
+It will, however, build with GCC-7, but some of global variables will be 
+preemptible.
 
 To build a debug configuration:
 ```

azure-pipelines.yml

@@ -92,6 +92,50 @@ phases:
     failOnStderr: true
     displayName: 'Test'
 
+- phase: LinuxGCC
+  queue:
+    name: 'Hosted Ubuntu 1604'
+    parallel: 2
+    matrix:
+      Debug:
+        BuildType: Debug
+      Release:
+        BuildType: Release
+
+  steps:
+  - script: |
+      sudo add-apt-repository ppa:ubuntu-toolchain-r/test
+      sudo apt-get update
+      sudo apt-get install -y ninja-build libc++-dev libc++abi-dev libc++abi1 libstdc++-7-dev gcc-7
+      # sudo apt-get install clang-6.0 clang++-6.0 
+
+      sudo update-alternatives --install /usr/bin/cc cc /usr/bin/gcc-7 100
+      sudo update-alternatives --set cc /usr/bin/gcc-7
+
+      sudo update-alternatives --install /usr/bin/c++ c++ /usr/bin/g++-7 100
+      sudo update-alternatives --set c++ /usr/bin/g++-7
+
+    displayName: 'Install Build Dependencies'
+
+  - task: CMake@1
+    displayName: 'CMake .. -GNinja -DCMAKE_BUILD_TYPE=$(BuildType) -DCMAKE_CXX_FLAGS="-std=c++17"'
+    inputs:
+      cmakeArgs: '.. -GNinja -DCMAKE_BUILD_TYPE=$(BuildType) -DCMAKE_CXX_FLAGS="-std=c++17"'
+
+  - script: |
+      ninja
+
+    workingDirectory: build
+    failOnStderr: true
+    displayName: 'Compile'
+
+  - script: |
+      ctest -j 4 --output-on-failure -C $(BuildType)
+
+    workingDirectory: build
+    failOnStderr: true
+    displayName: 'Test'
+
 - phase: Windows64bit
   queue:
     name: 'Hosted VS2017'

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; \
-  }
+  }