Use VirtualAlloc2 on Windows.

The newer API (Windows 10 and newer) allows the allocator to ask for
strongly aligned memory.

This is enabled only if the `WINVER` macro is set to target Windows 10
or newer.  There is now a CMake option to target older versions of
Windows, so we can test both code paths.

The Azure Pipelines config now includes a test of the compatibility
version.  This runs only the release build, because it's mainly there as
a sanity check - 99% of the code is the same as the default Windows
config.

CMakeLists.txt

@@ -24,10 +24,21 @@ macro(linklibs project)
     if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
       target_link_libraries(${project} atomic)
     endif()
+  else()
+    # VirtualAlloc2 is exposed by mincore.lib, not Kernel32.lib (as the
+    # documentation says)
+    target_link_libraries(${project} mincore)
   endif()
 endmacro()
 
 if(MSVC)
+  set(WIN8COMPAT FALSE CACHE BOOL "Avoid Windows 10 APIs")
+  if (WIN8COMPAT)
+    add_definitions(-DWINVER=0x0603)
+    message(STATUS "Avoiding Windows 10 APIs")
+  else()
+    message(STATUS "Using Windows 10 APIs")
+  endif()
   # Force to always compile with W4
   if(CMAKE_CXX_FLAGS MATCHES "/W[0-4]")
     string(REGEX REPLACE "/W[0-4]" "/W4" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")

azure-pipelines.yml

@@ -79,6 +79,30 @@ phases:
     workingDirectory: build
     displayName: 'Run Ctest'
 
+- phase: Windows8Compat
+  queue:
+    name: 'Hosted VS2017'
+    parallel: 2
+    matrix:
+      Release:
+        BuildType: Release
+
+  steps:
+  - task: CMake@1
+    displayName: 'CMake .. -G"Visual Studio 15 2017 Win64"'
+    inputs:
+      cmakeArgs: '.. -G"Visual Studio 15 2017 Win64" -DWIN8COMPAT=TRUE'
+
+  - task: MSBuild@1
+    displayName: 'Build solution build/snmalloc.sln'
+    inputs:
+      solution: build/snmalloc.sln
+      msbuildArguments: '/m /p:Configuration=$(BuildType)'
+
+  - script: 'ctest -j 4 --interactive-debug-mode 0 --output-on-failure'
+    workingDirectory: build
+    displayName: 'Run Ctest'
+
 - phase: Windows32bit
   queue:
     name: 'Hosted VS2017'

src/pal/pal_windows.h

@@ -68,17 +68,29 @@ namespace snmalloc
     template<bool committed>
     void* reserve(size_t* size, size_t align) noexcept
     {
+      DWORD flags = MEM_RESERVE;
+
+      if (committed)
+        flags |= MEM_COMMIT;
+
+#  if (WINVER >= _WIN32_WINNT_WIN10) && !defined(USE_SYSTEMATIC_TESTING)
+      // If we're on Windows 10 or newer, we can use the VirtualAlloc2
+      // function.  The FromApp variant is useable by UWP applications and
+      // cannot allocate executable memory.
+      MEM_ADDRESS_REQUIREMENTS addressReqs = {0};
+      MEM_EXTENDED_PARAMETER param = {0};
+      addressReqs.Alignment = align;
+      param.Type = MemExtendedParameterAddressRequirements;
+      param.Pointer = &addressReqs;
+      return VirtualAlloc2FromApp(
+        nullptr, nullptr, *size, flags, PAGE_READWRITE, &param, 1);
+#  else
       // Add align, so we can guarantee to provide at least size.
       size_t request = *size + align;
 
       // Alignment must be a power of 2.
       assert(align == bits::next_pow2(align));
 
-      DWORD flags = MEM_RESERVE;
-
-      if (committed)
-        flags |= MEM_COMMIT;
-
       void* p;
 #    ifdef USE_SYSTEMATIC_TESTING
       size_t retries = 1000;
@@ -105,6 +117,7 @@ namespace snmalloc
       }
       *size = request;
       return p;
+#  endif
     }
   };
 }