Travis: stop testing with LLVM4, start testing with LLVM6

Also:
- make permanent a change that was "LLVM4 or newer"
- add "no longer tested" warning when LLVM4 is detected

.travis.yml

@@ -18,9 +18,9 @@ env:
     #
     # Test a mix of llvm versions, a mix of build systems, and a mix of shared vs static library
     # Don't build as a static library with cmake. It risks exceeding the travis memory limit.
-    - LLVM_VERSION=4.0.1 BUILD_SYSTEM=MAKE CXX_=g++-4.8 CC_=gcc-4.8
+    - LLVM_VERSION=6.0.0 BUILD_SYSTEM=MAKE CXX_=g++-4.8 CC_=gcc-4.8
     - LLVM_VERSION=5.0.0 BUILD_SYSTEM=MAKE CXX_=g++-4.8 CC_=gcc-4.8
-    - LLVM_VERSION=4.0.1 BUILD_SYSTEM=CMAKE CXX_=g++-4.8 CC_=gcc-4.8 HALIDE_SHARED_LIBRARY=1
+    - LLVM_VERSION=6.0.0 BUILD_SYSTEM=CMAKE CXX_=g++-4.8 CC_=gcc-4.8 HALIDE_SHARED_LIBRARY=1
 cache: apt ccache
 dist: trusty
 # Note the commands below are written assuming Ubuntu 12.04LTS

CMakeLists.txt

@@ -31,6 +31,10 @@ else()
   endif()
 endif()
 
+if("${LLVM_VERSION_MAJOR}${LLVM_VERSION_MINOR}" STREQUAL "40")
+  message(WARNING "*** Warning: LLVM 4.x is no longer actively tested with Halide. ***")
+endif()
+
 # Notify the user what paths and LLVM version we are using
 message(STATUS "Found LLVM ${LLVM_PACKAGE_VERSION}")
 message(STATUS "Using LLVMConfig.cmake in: ${LLVM_DIR}")

Makefile

@@ -148,12 +148,7 @@ CXX_FLAGS += -funwind-tables
 print-%:
 	@echo '$*=$($*)'
 
-# Append the --link-static flag to llvm-config if it exists. We can
-# make this unconditional once llvm 4.0 is the minimum version we
-# support.
-LLVM_LINK_STATIC_FLAG = $(shell $(LLVM_CONFIG) --link-static 2>/dev/null && echo " --link-static")
-
-LLVM_STATIC_LIBS = -L $(LLVM_LIBDIR) $(shell $(LLVM_CONFIG) $(LLVM_LINK_STATIC_FLAG) --libs bitwriter bitreader linker ipo mcjit $(X86_LLVM_CONFIG_LIB) $(ARM_LLVM_CONFIG_LIB) $(OPENCL_LLVM_CONFIG_LIB) $(METAL_LLVM_CONFIG_LIB) $(PTX_LLVM_CONFIG_LIB) $(AARCH64_LLVM_CONFIG_LIB) $(MIPS_LLVM_CONFIG_LIB) $(POWERPC_LLVM_CONFIG_LIB) $(HEXAGON_LLVM_CONFIG_LIB) $(AMDGPU_LLVM_CONFIG_LIB))
+LLVM_STATIC_LIBS = -L $(LLVM_LIBDIR) $(shell $(LLVM_CONFIG) --link-static --libs bitwriter bitreader linker ipo mcjit $(X86_LLVM_CONFIG_LIB) $(ARM_LLVM_CONFIG_LIB) $(OPENCL_LLVM_CONFIG_LIB) $(METAL_LLVM_CONFIG_LIB) $(PTX_LLVM_CONFIG_LIB) $(AARCH64_LLVM_CONFIG_LIB) $(MIPS_LLVM_CONFIG_LIB) $(POWERPC_LLVM_CONFIG_LIB) $(HEXAGON_LLVM_CONFIG_LIB) $(AMDGPU_LLVM_CONFIG_LIB))
 
 # Add a rpath to the llvm used for linking, in case multiple llvms are
 # installed. Bakes a path on the build system into the .so, so don't
@@ -1635,6 +1630,11 @@ endif
 ifneq ($(LLVM_OK), )
 $(BUILD_DIR)/llvm_ok: $(BUILD_DIR)/rtti_ok
 	@echo "Found a new enough version of llvm"
+ifeq ($(LLVM_VERSION_TIMES_10), 40)
+	@echo
+	@echo "*** Warning: LLVM 4.x is no longer actively tested with Halide. ***"
+	@echo
+endif
 	mkdir -p $(BUILD_DIR)
 	touch $(BUILD_DIR)/llvm_ok
 else