Add support for building subzero with LLVM

tools/Makefile

 ##===- tools/Makefile --------------------------------------*- Makefile -*-===##
 #
 #                     The LLVM Compiler Infrastructure
 #
 # This file is distributed under the University of Illinois Open Source
 # License. See LICENSE.TXT for details.
 #
 ##===----------------------------------------------------------------------===##
 
 LEVEL := ..
 
 include $(LEVEL)/Makefile.config
 
 # Build clang if present.
 
 ifneq ($(CLANG_SRC_ROOT),)
   OPTIONAL_PARALLEL_DIRS := $(CLANG_SRC_ROOT)
 else
   OPTIONAL_PARALLEL_DIRS := clang
 endif
-
+#ifneq ($(SUBZERO_SRC_ROOT),)

[jvoung (off chromium), 2014/05/22 19:37:58]

Can remove commented out check?

Seems like the point of the variable (and the check for the similar
CLANG_SRC_ROOT variable) is if you want to configure a different directory name.
E.g., configure --with-clang-srcdir=<other>:

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20111010/129983.html

Otherwise "OPTIONAL_PARALLEL_DIRS += subzero" could work if you just want to
assume the directory is always called "subzero", and you can remove the variable
too?

[Derek Schuff, 2014/05/22 23:21:56]

Removed.
Yes, SUBZERO_SRC_ROOT does not actually exist in llvm/tools but is external. so
it's an absolute path and not just 'subzero'
The Makefile.config.in bit substitutes llvm/tools/subzero with the real path
passed in to Make.

+OPTIONAL_PARALLEL_DIRS += $(SUBZERO_SRC_ROOT)
 # Build LLDB if present. Note LLDB must be built last as it depends on the
 # wider LLVM infrastructure (including Clang).
 OPTIONAL_DIRS := lldb
 
 # NOTE: The tools are organized into five groups of four consisting of one
 # large and three small executables. This is done to minimize memory load
 # in parallel builds.  Please retain this ordering.
 DIRS := llvm-config
 PARALLEL_DIRS := opt llvm-as llvm-dis llc llvm-ar llvm-nm llvm-link \
                  lli llvm-extract llvm-mc bugpoint llvm-bcanalyzer llvm-diff \
@@ skipping 36 matching lines @@
 
 # On Win32, loadable modules can be built with ENABLE_SHARED.
 ifneq ($(ENABLE_SHARED),1)
   ifneq (,$(filter $(HOST_OS), Cygwin MingW))
     PARALLEL_DIRS := $(filter-out bugpoint-passes, \
                         $(PARALLEL_DIRS))
   endif
 endif
 
 include $(LEVEL)/Makefile.common