Merge pull request #29581 from drexin/wip-fix-atomics

Remove dependency on libatomic on Linux

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -349,6 +349,13 @@ function(_add_variant_c_compile_flags)
     list(APPEND result "-D__ANDROID_API__=${SWIFT_ANDROID_API_LEVEL}")
   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()
 
@@ -466,7 +473,7 @@ function(_add_variant_link_flags)
     RESULT_VAR_NAME result
     MACCATALYST_BUILD_FLAVOR  "${LFLAGS_MACCATALYST_BUILD_FLAVOR}")
   if("${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

@@ -631,7 +631,7 @@ 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

@@ -5115,7 +5115,7 @@ 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;