[CMake] compiler-rt: Add COMPILER_RT_BUILTINS_ENABLE_PIC

[JamesNagurne]

Summary

The configuration for -fPIC in the builtins library when built standalone
is unconditional, stating that the flags would "normally be added... by
the llvm cmake step"

This is untrue, as the llvm cmake step checks LLVM_ENABLE_PIC, which allows
a client to turn off -fPIC.

I've added an option when compiler-rt builtins are configured standalone, such
as when built as part of the LLVM runtimes system, to guard the application of
-fPIC for users that want it.

[phosek]

compiler-rt/lib/builtins/CMakeLists.txt

589–591

I think this changes the existing behavior. COMPILER_RT_STANDALONE_BUILD is set when compiler-rt is being built as a standalone project, which is not only when being built as part of runtimes, but also when you run CMake in compiler-rt as the source directory. In that case, COMPILER_RT_BUILTINS_ENABLE_PIC` will be unset because the new option is only defined when compiler-rt/lib/builtins is the root source directory, and -fPIC will be dropped in that case. This new option has to be defined in this block as well: https://github.com/llvm/llvm-project/blob/master/compiler-rt/CMakeLists.txt#L76 to avoid changing the current behavior in the standalone case.

[JamesNagurne]

compiler-rt/lib/builtins/CMakeLists.txt

589–591

Adding the option to the top-level block is undesirable, as that splits the option origin into two different places, since we need it both when builtins is the top-level, and when compiler-rt is the top-level.

Would adding the option in a block guarded by COMPILER_RT_STANDALONE_BUILD outside and just after of the SOURCE_DIR check it's currently in be a good alternative? That should fire in all cases of COMPILER_RT_STANDALONE_BUILD.

I'll upload the change when I get a chance, in case my description is confusing.

[JamesNagurne]

Moved option into a block guarded by COMPILER_RT_STANDLONE_BUILD, to indicate that it is an option that is available when not utilizing a linked LLVM toolchain and its configuration.

[phosek]

I think this makes sense, LGTM.

[JamesNagurne]

Thanks @phosek

compiler-rt/lib/builtins/CMakeLists.txt

if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
 
   if(APPLE)
     include(CompilerRTDarwinUtils)
   endif()
   if(CMAKE_HOST_APPLE AND APPLE)
     include(UseLibtool)
   endif()
   include(AddCompilerRT)
+
+endif()
+# When compiler-rt is being built standalone, possibly as a cross-compilation
+# target, the target may or may not want position independent code. This
+# option provides an avenue through which the flag may be controlled when an
+# LLVM configuration is not being utilized.
+if (COMPILER_RT_STANDALONE_BUILD)
+  option(COMPILER_RT_BUILTINS_ENABLE_PIC
+    "Turns on or off -fPIC for the builtin library source"
+    ON)
 endif()
 
 include(builtin-config-ix)
 
 # TODO: Need to add a mechanism for logging errors when builtin source files are
 # added to a sub-directory and not this CMakeLists file.
 set(GENERIC_SOURCES
   absvdi2.c

 else ()
   set(BUILTIN_CFLAGS "")
 
   append_list_if(COMPILER_RT_HAS_STD_C11_FLAG -std=c11 BUILTIN_CFLAGS)
 
   # These flags would normally be added to CMAKE_C_FLAGS by the llvm
   # cmake step. Add them manually if this is a standalone build.
   if(COMPILER_RT_STANDALONE_BUILD)
+    if(COMPILER_RT_BUILTINS_ENABLE_PIC)
       append_list_if(COMPILER_RT_HAS_FPIC_FLAG -fPIC BUILTIN_CFLAGS)
+    endif()
     append_list_if(COMPILER_RT_HAS_FNO_BUILTIN_FLAG -fno-builtin BUILTIN_CFLAGS)
     if(NOT ANDROID)
       append_list_if(COMPILER_RT_HAS_VISIBILITY_HIDDEN_FLAG -fvisibility=hidden BUILTIN_CFLAGS)
     endif()
     if(NOT COMPILER_RT_DEBUG)
       append_list_if(COMPILER_RT_HAS_OMIT_FRAME_POINTER_FLAG -fomit-frame-pointer BUILTIN_CFLAGS)
     endif()
   endif()