[llvm-bugs] [Bug 44278] New: Default --gc-sections breaks cmake test

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44278

            Bug ID: 44278
           Summary: Default --gc-sections breaks cmake test
           Product: lld
           Version: unspecified
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: wasm
          Assignee: unassignedbugs at nondot.org
          Reporter: bob at pepin.io
                CC: llvm-bugs at lists.llvm.org, sbc at chromium.org

This could be addressed either on the lld or the cmake side, I'll let you
coordinate with the cmake maintainers if necessary.

The cmake CMAKE_SIZEOF_VOID_P macro compiles a temporary file with the size of
a pointer encoded in a string in one of the sections of the binary (see below).
It then tries to extract the result from the compiler output. However, with
--gc-sections enabled, the section containing the string in question is garbage
collected by the linker. 
This breaks in particular the compilation of LLVM compiler-rt with wasm32 and
the default linker options.

To reproduce:
CMakeLists.txt:
message("CMAKE_SIZEOF_VOID_P ${CMAKE_SIZEOF_VOID_P}")

toolchain.cmake:
set(CMAKE_SYSTEM_NAME WASM32)
set(CMAKE_SYSTEM_VERSION 1.0)
set(CMAKE_SYSTEM_PROCESSOR unknown)
set(triple wasm32)
set(CMAKE_C_COMPILER /usr/local/opt/llvm/bin/clang)
set(CMAKE_C_COMPILER_TARGET ${triple})
set(CMAKE_CXX_COMPILER /usr/local/opt/llvm/bin/clang++)
set(CMAKE_CXX_COMPILER_TARGET ${triple})
# This gives CMAKE_SIZEOF_VOID_P 4
#set(CMAKE_EXE_LINKER_FLAGS_INIT "-nostdlib -Wl,--no-entry
-Wl,--no-gc-sections")
# This gives an empty CMAKE_SIZEOF_VOID_P
set(CMAKE_EXE_LINKER_FLAGS_INIT "-nostdlib -Wl,--no-entry")


The cmake code in question is at the top of
https://github.com/Kitware/CMake/blob/master/Modules/CMakeCompilerABI.h

/* Size of a pointer-to-data in bytes.  */
#define SIZEOF_DPTR (sizeof(void*))
const char info_sizeof_dptr[] = {
  /* clang-format off */
  'I', 'N', 'F', 'O', ':', 's', 'i', 'z', 'e', 'o', 'f', '_', 'd', 'p', 't',
  'r', '[', ('0' + ((SIZEOF_DPTR / 10) % 10)), ('0' + (SIZEOF_DPTR % 10)), ']',
  '\0'
  /* clang-format on */
};

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