[PATCH] D58331: [compiler-rt] Passthrough RUNTIMES_BUILD to the custom libc++ build

[Petr Hosek via Phabricator via llvm-commits]

phosek created this revision.
phosek added reviewers: Hahnfeld, vitalybuka, morehouse.
Herald added subscribers: llvm-commits, Sanitizers, jdoerfert, christof, mgorny, dberris.
Herald added a reviewer: EricWF.
Herald added projects: LLVM, Sanitizers.

We use it to decide whether to include -nostdlib++ in the required
compiler flags. This addresses the issue introduced in rCRT354231 <https://reviews.llvm.org/rCRT354231>
which broke the runtimes build.


Repository:
  rCRT Compiler Runtime

https://reviews.llvm.org/D58331

Files:
  compiler-rt/cmake/Modules/CustomLibcxx/CMakeLists.txt


Index: compiler-rt/cmake/Modules/CustomLibcxx/CMakeLists.txt
===================================================================
--- compiler-rt/cmake/Modules/CustomLibcxx/CMakeLists.txt
+++ compiler-rt/cmake/Modules/CustomLibcxx/CMakeLists.txt
@@ -1,5 +1,24 @@
 cmake_minimum_required(VERSION 3.4.3)
 
+# TODO: We shouldn't need a working C++ library, we're building C++ library
+# using this file after all. However, libc++abi and libc++ in standalone mode
+# include LLVM's HandleLLVMOptions which checks whether the compiler support
+# C++ using check_cxx_compiler_flag which tries to link C++ library and that
+# breaks C++ library hasn't been built yet (e.g. in the runtimes build where
+# C++ library is being built using the just built compiler).
+#
+# We need to prevent CMake from trying to use the C++ library. One way to do
+# that is by using -nostdlib and then only including -lc plus runtimes which
+# requires handling runtimes compiler-rt builtins and libgcc which adds another
+# layer of complexity. We could use -nostdlib++ but that flag is only supported
+# by newer Clang versions.
+#
+# For now, we pass the RUNTIMES_BUILD option through to detect whether we're
+# performing the runtimes build, and if so we use the -nostdlib++ flag.
+if (RUNTIMES_BUILD)
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
+endif()
+
 # Build static libcxxabi.
 set(LIBCXXABI_STANDALONE_BUILD 1)
 set(LIBCXXABI_ENABLE_SHARED OFF CACHE BOOL "")


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