[cmake] Add LLVM_UBSAN_FLAGS, to allow overriding UBSan flags

[vsk]

Summary

Allow overriding the default set of flags used to enable UBSan when
building llvm.

This can be used to test new checks or opt out of certain checks.

[kastiglione]

llvm/CMakeLists.txt

490

should this be option()? I think this will overwrite -DLLVM_UBSAN_FLAGS=<value>.

[vsk]

llvm/CMakeLists.txt

490

From what I gather, set(... CACHE ...) is appropriate here. This does not overwrite a -D command line option (otherwise -DLLVM_USE_SANITIZER would not work). Also, technically option doesn't support non-BOOL types, although in practice I'm not sure whether the distinction matters.

[kastiglione]

thanks for correcting my misunderstanding

llvm/CMakeLists.txt

if( LLVM_USE_PERF )
   if( NOT CMAKE_SYSTEM_NAME MATCHES "Linux" )
     message(FATAL_ERROR "perf support is available on Linux only.")
   endif( NOT CMAKE_SYSTEM_NAME MATCHES "Linux" )
 endif( LLVM_USE_PERF )
 
 set(LLVM_USE_SANITIZER "" CACHE STRING
   "Define the sanitizer used to build binaries and tests.")
 option(LLVM_OPTIMIZE_SANITIZED_BUILDS "Pass -O1 on debug sanitizer builds" ON)
+set(LLVM_UBSAN_FLAGS
+    "-fsanitize=undefined -fno-sanitize=vptr,function -fno-sanitize-recover=all"
+    CACHE STRING
+    "Compile flags set to enable UBSan. Only used if LLVM_USE_SANITIZER contains 'Undefined'.")
 set(LLVM_LIB_FUZZING_ENGINE "" CACHE PATH
   "Path to fuzzing library for linking with fuzz targets")
 
 option(LLVM_USE_SPLIT_DWARF
   "Use -gsplit-dwarf when compiling llvm." OFF)
 
 # Define an option controlling whether we should build for 32-bit on 64-bit
 # platforms, where supported.

llvm/cmake/modules/HandleLLVMOptions.cmake

if (LLVM_ON_UNIX)
     elseif (LLVM_USE_SANITIZER MATCHES "Memory(WithOrigins)?")
       append_common_sanitizer_flags()
       append("-fsanitize=memory" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
       if(LLVM_USE_SANITIZER STREQUAL "MemoryWithOrigins")
         append("-fsanitize-memory-track-origins" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
       endif()
     elseif (LLVM_USE_SANITIZER STREQUAL "Undefined")
       append_common_sanitizer_flags()
-      append("-fsanitize=undefined -fno-sanitize=vptr,function -fno-sanitize-recover=all"
-              CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+      append("${LLVM_UBSAN_FLAGS}" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
     elseif (LLVM_USE_SANITIZER STREQUAL "Thread")
       append_common_sanitizer_flags()
       append("-fsanitize=thread" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
     elseif (LLVM_USE_SANITIZER STREQUAL "DataFlow")
       append("-fsanitize=dataflow" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
     elseif (LLVM_USE_SANITIZER STREQUAL "Address;Undefined" OR
             LLVM_USE_SANITIZER STREQUAL "Undefined;Address")
       append_common_sanitizer_flags()
-      append("-fsanitize=address,undefined -fno-sanitize=vptr,function -fno-sanitize-recover=all"
-              CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+      append("-fsanitize=address" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+      append("${LLVM_UBSAN_FLAGS}" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
     elseif (LLVM_USE_SANITIZER STREQUAL "Leaks")
       append_common_sanitizer_flags()
       append("-fsanitize=leak" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
     else()
       message(FATAL_ERROR "Unsupported value of LLVM_USE_SANITIZER: ${LLVM_USE_SANITIZER}")
     endif()
   elseif(MSVC)
     if (LLVM_USE_SANITIZER STREQUAL "Address")

llvm/docs/CMake.rst

**LLVM_ENABLE_DIA_SDK**:BOOL
   Enable building with MSVC DIA SDK for PDB debugging support. Available
   only with MSVC. Defaults to ON.
 
 **LLVM_USE_SANITIZER**:STRING
   Define the sanitizer used to build LLVM binaries and tests. Possible values
   are ``Address``, ``Memory``, ``MemoryWithOrigins``, ``Undefined``, ``Thread``,
   ``DataFlow``, and ``Address;Undefined``. Defaults to empty string.
 
+**LLVM_UBSAN_FLAGS**:STRING
+  Defines the set of compile flags used to enable UBSan. Only used if
+  ``LLVM_USE_SANITIZER`` contains ``Undefined``. This can be used to override
+  the default set of UBSan flags.
+
 **LLVM_ENABLE_LTO**:STRING
   Add ``-flto`` or ``-flto=`` flags to the compile and link command
   lines, enabling link-time optimization. Possible values are ``Off``,
   ``On``, ``Thin`` and ``Full``. Defaults to OFF.
 
 **LLVM_USE_LINKER**:STRING
   Add ``-fuse-ld={name}`` to the link invocation. The possible value depend on
   your compiler, for clang the value can be an absolute path to your custom