clang plugin undefined reference

as https://patchwork.freedesktop.org/patch/219763/ suggestes, reorder the library including sequence

diff --git a/src/gallium/targets/opencl/Makefile.am b/src/gallium/targets/opencl/Makefile.am
index de68a93ad5..f0e1de7797 100644
--- a/src/gallium/targets/opencl/Makefile.am
+++ b/src/gallium/targets/opencl/Makefile.am
@@ -23,11 +23,10 @@  lib@OPENCL_LIBNAME@_la_LIBADD = \
  $(LIBELF_LIBS) \
  $(DLOPEN_LIBS) \
  -lclangCodeGen \
- -lclangFrontendTool \
  -lclangFrontend \
+ -lclangFrontendTool \
  -lclangDriver \
  -lclangSerialization \
- -lclangCodeGen \
  -lclangParse \
  -lclangSema \
  -lclangAnalysis \

llvm opt no effect?

in clang 5.0, it added a function call O0-optnone, which make your modification to llvm ir in passes not effective

compile you code with -Xclang -disable-O0-optnone then it will be effective again