[llvm-bugs] [Bug 39712] New: clang-cl generates more static initializers than VC++ does

via llvm-bugs llvm-bugs at lists.llvm.org
Mon Nov 19 13:50:31 PST 2018


            Bug ID: 39712
           Summary: clang-cl generates more static initializers than VC++
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: brucedawson at chromium.org
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

This is an example of code that VC++ optimizes better than clang-cl. With VC++
there is no static initializer whereas with clang there is:

struct my_id {
  my_id() : v_(0) {}
  int v_;
template <typename T> struct Foo {
  static my_id id;
template <typename T> my_id Foo<T>::id;
int main() { Foo<char>::id.v_++; }

So, this is a missing optimization in LLVM. LLVM's globalopt pass is supposed
to handle this, but it gives up too easily. The COMDAT scheme that Microsoft
uses is clever and allows the compiler to optimize this to static
initialization without changing the ABI.

This was reported in crbug.com/782027 and it leads to eleven static
initializers in clang-cl builds of Chrome (chrome.exe, chrome.dll, and
chrome_child.dll) that are not present in VC++ builds, coming from the VC++ STL
header files.

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