Remove dependency on libatomic on Linux

    Due to some unfortunate interplay between clang and libstdc++, clang was
    not able to correctly identify to alignment of PoolRange and
    SideTableRefCountBits, causing it to emit library calls instead of
    inlining atomic operations. This was fixed by adding the appropriate
    alignment to those types. In addition to that the march for the Linux
    target was set to 'core2', which is the earliest architecture to support
    cx16, which is necessary for the atomic operations on PoolRange.

Author: drexin

cmake/modules/AddSwift.cmake

@@ -363,6 +363,13 @@ function(_add_variant_c_compile_flags)
    endif()
   endif()
 
+  if("${CFLAGS_SDK}" STREQUAL "LINUX")
+    if(${CFLAGS_ARCH} STREQUAL x86_64)
+      # this is the minimum architecture that supports 16 byte CAS, which is necessary to avoid a dependency to libatomic
+      list(APPEND result "-march=core2")
+    endif()
+  endif()
+
   set("${CFLAGS_RESULT_VAR_NAME}" "${result}" PARENT_SCOPE)
 endfunction()
 
@@ -460,7 +467,7 @@ function(_add_variant_link_flags)
     RESULT_VAR_NAME result)
 
   if("${LFLAGS_SDK}" STREQUAL "LINUX")
-    list(APPEND link_libraries "pthread" "atomic" "dl")
+    list(APPEND link_libraries "pthread" "dl")
   elseif("${LFLAGS_SDK}" STREQUAL "FREEBSD")
     list(APPEND link_libraries "pthread")
   elseif("${LFLAGS_SDK}" STREQUAL "CYGWIN")

cmake/modules/AddSwiftUnittests.cmake

@@ -48,8 +48,10 @@ function(add_swift_unittest test_dirname)
       "${android_system_libs}")
     set_property(TARGET "${test_dirname}" APPEND PROPERTY LINK_LIBRARIES "log")
   elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux")
-    set_property(TARGET "${test_dirname}" APPEND PROPERTY LINK_LIBRARIES
-      "atomic")
+    if(CMAKE_SYSTEM_PROCESSOR MATCHES "x86_64|AMD64")
+      target_compile_options(${test_dirname} PRIVATE
+        -march=core2)
+    endif()
   endif()
 
   find_program(LDLLD_PATH "ld.lld")

stdlib/public/SwiftShims/RefCount.h

@@ -589,7 +589,7 @@ class RefCountBitsT {
 
 typedef RefCountBitsT<RefCountIsInline> InlineRefCountBits;
 
-class SideTableRefCountBits : public RefCountBitsT<RefCountNotInline>
+class alignas(sizeof(void*) * 2) SideTableRefCountBits : public RefCountBitsT<RefCountNotInline>
 {
   uint32_t weakBits;
 

stdlib/public/runtime/Metadata.cpp

@@ -5054,7 +5054,7 @@ void swift::checkMetadataDependencyCycle(const Metadata *startMetadata,
 /***************************************************************************/
 
 namespace {
-  struct PoolRange {
+  struct alignas(sizeof(uintptr_t) * 2) PoolRange {
     static constexpr uintptr_t PageSize = 16 * 1024;
     static constexpr uintptr_t MaxPoolAllocationSize = PageSize / 2;