llvmPackages_16: Make the pkgsLLVM.stdenv work

What changed:

* Fixed crtbeginS.o and crtendS.o missing
  (they may or may not be called crt{begin_end},{,_shared}.

* Fixed implicit function declaration causing build errors for various
  builds by supplying -Wno-implicit-function-declaration.

* Fixed __cxxabi_config.h missing, by adding -I${cxxabi}/include/c++/v1
  in the wrapper.

* Fixed libcxx failing to build due to missing libunwind symbols by
  making libunwind a propagated build input from libcxxabi, and setting
  -DLIBCXX_ADDITIONAL_LIBRARIES=unwind for stdenv.hostPlatform.useLLVM == true.

* libcxxabi wants to find libunwind at libunwind_shared.so, so symlink
  it there in libunwind.

What was tested:

* x86_64-linux, aarch64-linux, the stdenv builds.
  * Additionally I was able to get nix to build, with an overlay to fix
    a couple of minor issues in downstream packages (overlay supplied in
    PR #246577.
* aarch64-darwin fails spuriously in a single LLVM test
  strip-preserve-atime.test checking atime timestamps.

pkgs/development/compilers/llvm/16/compiler-rt/default.nix

@@ -114,6 +114,12 @@ stdenv.mkDerivation {
   '' + lib.optionalString (useLLVM) ''
     ln -s $out/lib/*/clang_rt.crtbegin-*.o $out/lib/crtbegin.o
     ln -s $out/lib/*/clang_rt.crtend-*.o $out/lib/crtend.o
+    # Note the history of crt{begin,end}S in previous versions of llvm in nixpkg:
+    # The presence of crtbegin_shared has been added and removed; it's possible
+    # people have added/removed it to get it working on their platforms.
+    # Try each in turn for now.
+    ln -s $out/lib/*/clang_rt.crtbegin-*.o $out/lib/crtbeginS.o
+    ln -s $out/lib/*/clang_rt.crtend-*.o $out/lib/crtendS.o
     ln -s $out/lib/*/clang_rt.crtbegin_shared-*.o $out/lib/crtbeginS.o
     ln -s $out/lib/*/clang_rt.crtend_shared-*.o $out/lib/crtendS.o
   '' + lib.optionalString doFakeLibgcc ''

pkgs/development/compilers/llvm/16/default.nix

@@ -253,7 +253,18 @@ in let
       nixSupport.cc-cflags =
         [ "-rtlib=compiler-rt"
           "-Wno-unused-command-line-argument"
+          # The compiler got stricter about this, so a number of downstream
+          # things seem to fail with it switched on.
+          "-Wno-implicit-function-declaration"
           "-B${targetLlvmLibraries.compiler-rt}/lib"
+
+          # Combat "__cxxabi_config.h not found". Maybe this could be fixed by
+          # copying these headers into libcxx? Note that building libcxx
+          # outside of monorepo isn't supported anymore, might be related to
+          # https://github.com/llvm/llvm-project/issues/55632
+          # ("16.0.3 libcxx, libcxxabi: circular build dependencies")
+          # Looks like the machinery changed in https://reviews.llvm.org/D120727.
+          "-I${lib.getDev targetLlvmLibraries.libcxx.cxxabi}/include/c++/v1"
         ]
         ++ lib.optional (!stdenv.targetPlatform.isWasm) "--unwindlib=libunwind"
         ++ lib.optional
@@ -297,7 +308,12 @@ in let
       bintools = bintoolsNoLibc';
       extraPackages = [ ];
       extraBuildCommands = mkExtraBuildCommands0 cc;
-      nixSupport.cc-cflags = [ "-nostartfiles" ];
+      nixSupport.cc-cflags = [
+        "-nostartfiles"
+        # The compiler got stricter about this, so a number of downstream
+        # things seem to fail with it switched on.
+        "-Wno-implicit-function-declaration"
+      ];
     };
 
     clangNoCompilerRtWithLibc = wrapCCWith rec {
@@ -306,6 +322,13 @@ in let
       bintools = bintools';
       extraPackages = [ ];
       extraBuildCommands = mkExtraBuildCommands0 cc;
+      nixSupport.cc-cflags = [
+        # This appears to be necessary to build libcxxabi on x86_64.pkgsLLVM
+        # with llvmPackages_16.
+        "-nostdlib++"
+        "-nostartfiles"
+        "-nodefaultlibs"
+      ];
     };
 
   });

pkgs/development/compilers/llvm/16/libcxx/default.nix

@@ -75,8 +75,15 @@ stdenv.mkDerivation rec {
     "-DLIBCXX_CXX_ABI=${if headersOnly then "none" else libcxx_cxx_abi_opt}"
   ] ++ lib.optional (!headersOnly && cxxabi.libName == "c++abi") "-DLIBCXX_CXX_ABI_INCLUDE_PATHS=${cxxabi.dev}/include/c++/v1"
     ++ lib.optional (stdenv.hostPlatform.isMusl || stdenv.hostPlatform.isWasi) "-DLIBCXX_HAS_MUSL_LIBC=1"
-    ++ lib.optional (stdenv.hostPlatform.useLLVM or false) "-DLIBCXX_USE_COMPILER_RT=ON"
-    ++ lib.optionals stdenv.hostPlatform.isWasm [
+    ++ lib.optionals (stdenv.hostPlatform.useLLVM or false) [
+      "-DLIBCXX_USE_COMPILER_RT=ON"
+      # There's precedent for this in llvm-project/libcxx/cmake/caches.
+      # In a monorepo build you might do the following in the libcxxabi build:
+      #   -DLLVM_ENABLE_PROJECTS=libcxxabi;libunwinder
+      #   -DLIBCXXABI_STATICALLY_LINK_UNWINDER_IN_STATIC_LIBRARY=On
+      # libcxx appears to require unwind and doesn't pull it in via other means.
+      "-DLIBCXX_ADDITIONAL_LIBRARIES=unwind"
+    ] ++ lib.optionals stdenv.hostPlatform.isWasm [
       "-DLIBCXX_ENABLE_THREADS=OFF"
       "-DLIBCXX_ENABLE_FILESYSTEM=OFF"
       "-DLIBCXX_ENABLE_EXCEPTIONS=OFF"

pkgs/development/compilers/llvm/16/libcxxabi/default.nix

@@ -53,7 +53,7 @@ stdenv.mkDerivation rec {
   '';
 
   nativeBuildInputs = [ cmake ninja python3 ];
-  buildInputs = lib.optional (!stdenv.isDarwin && !stdenv.hostPlatform.isWasm) libunwind;
+  propagatedBuildInputs = lib.optional (!stdenv.isDarwin && !stdenv.hostPlatform.isWasm) libunwind;
 
   cmakeFlags = [
     "-DLLVM_ENABLE_RUNTIMES=libcxxabi"

pkgs/development/compilers/llvm/16/libunwind/default.nix

@@ -40,6 +40,11 @@ stdenv.mkDerivation rec {
     cd ../runtimes
   '';
 
+  postInstall = lib.optionalString (enableShared) ''
+    # libcxxabi wants to link to libunwind_shared.so (?).
+    ln -s $out/lib/libunwind.so $out/lib/libunwind_shared.so
+  '';
+
   outputs = [ "out" "dev" ];
 
   nativeBuildInputs = [ cmake ninja python3 ];