[libc] Don't pass -fpie/-ffreestanding on Windows

[caitlyncano]

Summary

The current compile options function hardcodes the -fpie and
-ffreestanding flags, which don't exist on Windows. This patch sets the
compilation flags conditionally based on the OS specifics.

[aeubanks]

libc/cmake/modules/LLVMLibCObjectRules.cmake

5

missing a space

7

I'd recommend setting the common flags outside the if, then adding -fpie -ffreestanding / /Qfreestanding in the if

I think

set(${output_var} ${output_var} -extraflags)

should append flags

[caitlyncano]

[libc] Fixing space and shifting set configuration

Moving outside the common flags outside the if.

[caitlyncano]

[libc] Fixing flag overwrites

Previous amendment overwrote the output by not including the flags set outside
the if block

[aeubanks]

the commit title should be imperative, see https://chris.beams.io/posts/git-commit/ for good git commit tips

and I'd be clearer and say something like "Don't pass -fpie/-ffreestanding on Windows"

libc/cmake/modules/LLVMLibCObjectRules.cmake

6

actually I don't see this available in clang-cl or msvc, where did you find this?

[caitlyncano]

libc/cmake/modules/LLVMLibCObjectRules.cmake

6

This is the for the Intel compiler and since the Linux flag syntax was the same I figured I could use the Windows equivalent? Not sure (https://software.intel.com/content/www/us/en/develop/documentation/cpp-compiler-developer-guide-and-reference/top/compiler-reference/compiler-options/compiler-option-details/advanced-optimization-options/ffreestanding-qfreestanding.html)

[aeubanks]

libc/cmake/modules/LLVMLibCObjectRules.cmake

6

that looks like it only works on the intel C++ compiler, it won't work with msvc or clang which is what we care about on windows

let's try not adding any alternatives for -fpie/-ffreestanding for now on windows

8

nit: spacing

[caitlyncano]

libc/cmake/modules/LLVMLibCObjectRules.cmake

6

But it seems like in this article (https://nullprogram.com/blog/2016/01/31/) -ffreestanding is available on MinGW so maybe that's where we'd have to build llvmlibc to ensure the freestanding environment?

[caitlyncano]

[libc] Don't include -ffreestanding equivalent on Windows

/Qfreestanding is only available on Intel C++ compiler, so it won't affect a
compilation in clang/msvc.

[aeubanks]

libc/cmake/modules/LLVMLibCObjectRules.cmake

6

/Qfreestanding is only available on the intel C++ compiler for windows

-ffreestanding is available on gcc/clang/intel for Linux

mingw is similar to Linux (even it's on windows) and uses gcc, but we currently don't care about mingw, at least for llvm libc

6

we should skip this if block since it's not doing anything

[caitlyncano]

[libc] Simplify conditional

Unecessary inclusion of an else case removed since Windows will not use any
flags outside the standard compiler flags.

[aeubanks]

lgtm

libc/cmake/modules/LLVMLibCObjectRules.cmake

 set(OBJECT_LIBRARY_TARGET_TYPE "OBJECT_LIBRARY")
 
 function(_get_common_compile_options output_var)
+  if(${LIBC_TARGET_OS} STREQUAL "windows") 
+    set(${output_var}${LLVM_CXX_STD_default} /Qfreestanding ${LIBC_COMPILE_OPTIONS_DEFAULT} ${ARGN} PARENT_SCOPE)
+  else()
    set(${output_var} -fpie ${LLVM_CXX_STD_default} -ffreestanding ${LIBC_COMPILE_OPTIONS_DEFAULT} ${ARGN} PARENT_SCOPE)
+  endif()
 endfunction()
 
 # Rule which is essentially a wrapper over add_library to compile a set of
 # sources to object files.
 # Usage:
 #     add_object_library(
 #       <target_name>
 #       HDRS <list of header files>