-fno-builtin wont works if -O0

[Duiccni]

The code I tried to compile is given in the bottom used clang version 20.1.5 Target: x86_64-pc-windows-msvc
without -O flag it gives error:

test-0bfdf5.o : error LNK2019: unresolved external symbol memset referenced in function _entry
CCC\test.exe : fatal error LNK1120: 1 unresolved externals
clang: error: linker command failed with exit code 1120 (use -v to see invocation)

which i believe it shoudnt becouse of -fno-builtin flag i used
with -O flag it works just fine

#undef UNICODE

#include <Windows.h>

HWND hwnd;
HINSTANCE hInstance;
char* className = "MyWindowClass";

LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam) {
   if (uMsg == WM_CLOSE) {
      return -1;
   }
   return DefWindowProc(hwnd, uMsg, wParam, lParam);
}

void _entry() {
   hInstance = GetModuleHandle(NULL);

   WNDCLASS wc = {0};
   wc.lpfnWndProc = WindowProc;
   wc.hInstance = hInstance;
   wc.lpszClassName = className;
   RegisterClassA(&wc);
   hwnd = CreateWindowExA(0, className, "My Window", WS_VISIBLE | WS_SYSMENU | WS_CAPTION, 0, 0, 320, 320, NULL, NULL, hInstance, NULL);

   MSG msg;
loop:
   while (PeekMessageA(&msg, hwnd /*NULL*/, 0, 0, PM_REMOVE)) {
      DWORD r = DispatchMessageA(&msg);
      if (r == -1) {
         MessageBoxA(NULL, "WM_CLOSE", "Info", 0); // I expect it not to be work because hwnd set to hwnd not null
         goto end;
      }
   }

   Sleep(500);
   goto loop;
end:

   DestroyWindow(hwnd);
   UnregisterClassA(className, hInstance);

   ExitProcess(0);
}```

[AaronBallman]

That's not what -fno-builtin means; that flag means that memset cannot be replaced automatically by an inline implementation. The reason -O0 matters is because other optimization modes decide they don't need to call memset to initialize global memory. LLVM (and by extension Clang) require that memset, memcpy, and memmove must be available, even in freestanding mode: https://clang.llvm.org/docs/UsersManual.html#freestanding-builds

https://godbolt.org/z/Pr7rTrja4 shows an example of how that flag works in practice.

Closing the issue as this behavior is by design.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Egemen Yalın (Duiccni)

The code I tried to compile is given in the bottom used clang version 20.1.5 Target: x86_64-pc-windows-msvc without -O flag it gives error:

test-0bfdf5.o : error LNK2019: unresolved external symbol memset referenced in function _entry
CCC\test.exe : fatal error LNK1120: 1 unresolved externals
clang: error: linker command failed with exit code 1120 (use -v to see invocation)

which i believe it shoudnt becouse of -fno-builtin flag i used
with -O flag it works just fine

#undef UNICODE

#include <Windows.h>

HWND hwnd;
HINSTANCE hInstance;
char* className = "MyWindowClass";

LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam) {
   if (uMsg == WM_CLOSE) {
      return -1;
   }
   return DefWindowProc(hwnd, uMsg, wParam, lParam);
}

void _entry() {
   hInstance = GetModuleHandle(NULL);

   WNDCLASS wc = {0};
   wc.lpfnWndProc = WindowProc;
   wc.hInstance = hInstance;
   wc.lpszClassName = className;
   RegisterClassA(&wc);
   hwnd = CreateWindowExA(0, className, "My Window", WS_VISIBLE | WS_SYSMENU | WS_CAPTION, 0, 0, 320, 320, NULL, NULL, hInstance, NULL);

   MSG msg;
loop:
   while (PeekMessageA(&msg, hwnd /*NULL*/, 0, 0, PM_REMOVE)) {
      DWORD r = DispatchMessageA(&msg);
      if (r == -1) {
         MessageBoxA(NULL, "WM_CLOSE", "Info", 0); // I expect it not to be work because hwnd set to hwnd not null
         goto end;
      }
   }

   Sleep(500);
   goto loop;
end:

   DestroyWindow(hwnd);
   UnregisterClassA(className, hInstance);

   ExitProcess(0);
}```
</details>