[PATCH v4 02/18] perf build: Support dynamic linking clang libraries

[Wang Nan]

Statical linking result a very large perf executable. This patch makes
perf link clang libraries dynamically by using '-lclangBasic' style
linking option. If dynamic clang libraries are detected, gcc will use
them by default.

 Test result:

 (Build clang/llvm dynamically by setting -DBUILD_SHARED_LIBS=ON
  in its cmake configuration.)

 $ size ~/perf
 text      data     bss         dec        hex      filename
 4223234   754544   23956048    28933826   1b97ec2  /home/wn/perf

 $ strip ~/perf
 $ ls -sh ~/perf
 4.8M /home/wn/perf

 Compare with statical linking:
 $ ls -sh ~/perf
 969M /home/wn/perf
 $ strip ~/perf
 $ ls -sh ~/perf
 52M /home/wn/perf

Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxx>
Cc: He Kuang <hekuang@xxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Zefan Li <lizefan@xxxxxxxxxx>
Cc: pi3orama@xxxxxxx
---
 tools/perf/Makefile.perf | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
index 10495c9..192f2d6 100644
--- a/tools/perf/Makefile.perf
+++ b/tools/perf/Makefile.perf
@@ -329,9 +329,25 @@ endif
 LIBS = -Wl,--whole-archive $(PERFLIBS) -Wl,--no-whole-archive -Wl,--start-group $(EXTLIBS) -Wl,--end-group
 
 ifeq ($(USE_CLANG), 1)
+  # Only partial of the required clang libraries are listed.
+  #
+  # In case of dynamical linking, if clang is built and installed
+  # correctly, ld can infer the full list automatically. However,
+  # I observed a potential bug in 3.9.0 that four libraries
+  # (ASTMatchers, Format Rewrite and ToolingCore) are lost.
+  #
+  # In case of statical linking, providing the full list causes
+  # linking time dramatically increases because of --{start,end}-group.
+  #
+  # If linking error, replace CLANGLIBS_LIST with the full list
+  # and try again.
+  #
+  # The full list should be:
+  #
+  # Basic CodeGen Frontend Tooling AST Lex Driver Edit Parse Sema
+  # Serialization ASTMatchers Format Rewrite ToolingCore Analysis
   CLANGLIBS_LIST = AST Basic CodeGen Driver Frontend Lex Tooling Edit Sema Analysis Parse Serialization
-  LIBCLANG = $(foreach l,$(CLANGLIBS_LIST),$(wildcard $(shell $(LLVM_CONFIG) --libdir)/libclang$(l).a))
-  LIBS += -Wl,--start-group $(LIBCLANG) -Wl,--end-group
+  LIBS += -L$(shell $(LLVM_CONFIG) --libdir) -Wl,--start-group $(foreach l,$(CLANGLIBS_LIST),-lclang$(l)) -Wl,--end-group
 endif
 
 ifeq ($(USE_LLVM), 1)
-- 
2.10.1