[CMake][ELF] Disable -fno-semantic-interposition on debug builds

[ljmf00]

Summary

When -fPIC is enabled via LLVM_ENABLE_PIC, -fno-semantic-interposition
optimization flag is added on both Release and Debug builds. This patch enables
it only on release builds.

[MaskRay]

The exact issue is -fsanitize=fuzzer-no-link -fno-semantic-interposition -fPIC. The -O level does not matter.

[ljmf00]

In D117144#3239523, @MaskRay wrote:

The exact issue is -fsanitize=fuzzer-no-link -fno-semantic-interposition -fPIC. The -O level does not matter.

Correct, although, it is still an optimization flag, at least according to GCC documentation about optimization options (https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html) and as described by the above comments in the CMake configuration. Looking into the discussion on D102453, shouldn't we prefer reliability on debug builds rather than speed? Another point to take into consideration is build/link times, does this affect it?

EDIT: GCC disables -fsemantic-interposition when -Ofast is present.

[MaskRay]

This may be abandoned :)

llvm/cmake/modules/HandleLLVMOptions.cmake

else()
     # Enable interprocedural optimizations for non-inline functions which would
     # otherwise be disabled due to GCC -fPIC's default.
     # Note: GCC<10.3 has a bug on SystemZ.
     #
     # Note: Clang allows IPO for -fPIC so this optimization is less effective.
     # Older Clang may support -fno-semantic-interposition but it used local
     # aliases to optimize global variables, which is incompatible with copy
     # relocations due to -fno-pic.
-    if ((CMAKE_COMPILER_IS_GNUCXX AND
+    if (NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG" AND ((CMAKE_COMPILER_IS_GNUCXX AND
          NOT (LLVM_NATIVE_ARCH STREQUAL "SystemZ" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 10.3))
-       OR (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND CMAKE_CXX_COMPILER_VERSION GREATER_EQUAL 13))
+       OR (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND CMAKE_CXX_COMPILER_VERSION GREATER_EQUAL 13)))
       add_flag_if_supported("-fno-semantic-interposition" FNO_SEMANTIC_INTERPOSITION)
     endif()
   endif()
   # GCC for MIPS can miscompile LLVM due to PR37701.
   if(CMAKE_COMPILER_IS_GNUCXX AND LLVM_NATIVE_ARCH STREQUAL "Mips" AND
          NOT Uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG")
     add_flag_or_print_warning("-fno-shrink-wrap" FNO_SHRINK_WRAP)
   endif()