[lldb] Explicitly set libcxx paths when USE_SYSTEM_LIBCXX is provided

[fdeazeve]

Summary

For tests marked as "USE_SYSTEM_LIBCXX", the expectation is that the
system's standard library should be used. However, the implementation of
this flag is such that we simply don't pass _any_ libcxxx-related flags
to Clang; in turn, Clang will use its defaults.

For a Clang/Libcxx pair compiled together, Clang defaults to:

1. The headers of the sibling libcxx.
2. The libraries of the system.

This mismatch is actually a bug in the driver; once fixed, however, (2)
would point to the sibling libcxx as well, which is _not_ what test
authors intended with the USE_SYSTEM_LIBCXX flag.

As such, this patch explicitly sets a path to the system's libraries.
This change is done only in Apple platforms so that we can test this
works in this case first.

[Michael137]

Makes sense to me, thanks!

[Michael137]

lldb/packages/Python/lldbsuite/test/make/Makefile.rules

434

Should we add a hard error if we get here with an empty SDKROOT?

[Michael137]

Typo in commit message and description:
USE_SYSTEM_LIBCXX should be USE_SYSTEM_STDLIB

[fdeazeve]

lldb/packages/Python/lldbsuite/test/make/Makefile.rules

434

I thought about this, but at the start of this file we have this:

# Handle SDKROOT on Darwin
#----------------------------------------------------------------------

ifeq "$(OS)" "Darwin"
    ifeq "$(SDKROOT)" ""
	# We haven't otherwise set the SDKROOT, so set it now to macosx
	SDKROOT := $(shell xcrun --sdk macosx --show-sdk-path)
    endif
endif

So it would be a bit redundant...
But I think you're right that we could have an error in case someone, someday, changes the logic above.

[Michael137]

lldb/packages/Python/lldbsuite/test/make/Makefile.rules

434

Yea don't feel too strongly about it, since as you say, the check is at the top of the file already

[fdeazeve]

Fix typos in commit message.
Add hard error in case SDKROOT is not set.

[JDevlieghere]

LGTM

lldb/packages/Python/lldbsuite/test/make/Makefile.rules

434

[fdeazeve]

Improve error message

[fdeazeve]

Fix typo in error msg

lldb/packages/Python/lldbsuite/test/make/Makefile.rules

else
     ifneq (,$(filter $(OS), FreeBSD Linux NetBSD))
         ifneq (,$(LLVM_LIBS_DIR))
         LDFLAGS += -Wl,-rpath,$(LLVM_LIBS_DIR)
         endif
     endif
   endif
 endif
 
+ifeq (1, $(USE_SYSTEM_STDLIB))
+    ifeq "$(OS)" "Darwin"
+        ifeq "$(SDKROOT)" ""
+             $(error "SDKROOT must be set on Darwin to use the system libcxx")
+        endif
+        CXXFLAGS += -nostdlib++ -nostdinc++ -cxx-isystem $(SDKROOT)/usr/include/c++/v1
+        LDFLAGS += -L$(SDKROOT)/usr/lib -Wl,-rpath,$(SDKROOT)/usr/lib -lc++
+    endif
+endif
+
 # If no explicit request was made, but we have paths to a custom libcxx, use
 # them.
 ifeq ($(or $(USE_LIBSTDCPP), $(USE_LIBCPP), $(USE_SYSTEM_STDLIB)),)
   ifneq ($(and $(LIBCPP_INCLUDE_DIR), $(LIBCPP_LIBRARY_DIR)),)
     CXXFLAGS += -nostdlib++ -nostdinc++ -cxx-isystem $(LIBCPP_INCLUDE_DIR)
     ifneq "$(LIBCPP_INCLUDE_TARGET_DIR)" ""
         CXXFLAGS += -cxx-isystem $(LIBCPP_INCLUDE_TARGET_DIR)
     endif