[profile] Make {__start_,__stop_}__llvm_prf_* symbols undefined weak

[MaskRay]

Summary

To make a kind of metadata section usage work, we may drop the
__start_/__stop_ references retain C identifier name sections rule from LLD (see D96914).

If an application has no __llvm_prf_data surviving --gc-sections,
LLD will error for {__start_,__stop_}__llvm_prf_* references from libclang_rt.profile-*.
Other __llvm_prf_* sections have similar issues.

Making the references weak can address the problem.
This probably enables the opportunity to drop zero size dummy sections in InstrProfilingPlatformLinux.c.

[MaskRay]

@davidxl In D14468, you mentioned:

Making start_../stop_.. weak extern symbols solve the problem when building executable, but PC relative dynamic reloc is not allowed so shared library case will still fail.

do you remember in what cases that can fail? That could mean an old compiler bug. I think we may be able to delete the dummy sections if we use undefined weak symbols.

[davidxl]

Does it work for other linkers with this change?

[MaskRay]

In D96936#2570675, @davidxl wrote:

Does it work for other linkers with this change?

Yes. Checked with:

clang++ -fprofile-generate -fuse-ld={bfd,gold,lld} -Wl,--gc-sections a.cc
clang++ -fprofile-generate -fuse-ld={bfd,gold,lld} -Wl,--gc-sections a.cc -fpic -shared

[davidxl]

lgtm

[MaskRay]

Thanks! I'll do that separately after this change sticks.

compiler-rt/lib/profile/InstrProfilingPlatformLinux.c

 #define PROF_CNTS_STOP INSTR_PROF_SECT_STOP(INSTR_PROF_CNTS_COMMON)
 #define PROF_ORDERFILE_START INSTR_PROF_SECT_START(INSTR_PROF_ORDERFILE_COMMON)
 #define PROF_VNODES_START INSTR_PROF_SECT_START(INSTR_PROF_VNODES_COMMON)
 #define PROF_VNODES_STOP INSTR_PROF_SECT_STOP(INSTR_PROF_VNODES_COMMON)
 
 /* Declare section start and stop symbols for various sections
  * generated by compiler instrumentation.
  */
-extern __llvm_profile_data PROF_DATA_START COMPILER_RT_VISIBILITY;
-extern __llvm_profile_data PROF_DATA_STOP COMPILER_RT_VISIBILITY;
-extern uint64_t PROF_CNTS_START COMPILER_RT_VISIBILITY;
-extern uint64_t PROF_CNTS_STOP COMPILER_RT_VISIBILITY;
-extern uint32_t PROF_ORDERFILE_START COMPILER_RT_VISIBILITY;
-extern char PROF_NAME_START COMPILER_RT_VISIBILITY;
-extern char PROF_NAME_STOP COMPILER_RT_VISIBILITY;
-extern ValueProfNode PROF_VNODES_START COMPILER_RT_VISIBILITY;
-extern ValueProfNode PROF_VNODES_STOP COMPILER_RT_VISIBILITY;
+extern __llvm_profile_data PROF_DATA_START COMPILER_RT_VISIBILITY
+    COMPILER_RT_WEAK;
+extern __llvm_profile_data PROF_DATA_STOP COMPILER_RT_VISIBILITY
+    COMPILER_RT_WEAK;
+extern uint64_t PROF_CNTS_START COMPILER_RT_VISIBILITY COMPILER_RT_WEAK;
+extern uint64_t PROF_CNTS_STOP COMPILER_RT_VISIBILITY COMPILER_RT_WEAK;
+extern uint32_t PROF_ORDERFILE_START COMPILER_RT_VISIBILITY COMPILER_RT_WEAK;
+extern char PROF_NAME_START COMPILER_RT_VISIBILITY COMPILER_RT_WEAK;
+extern char PROF_NAME_STOP COMPILER_RT_VISIBILITY COMPILER_RT_WEAK;
+extern ValueProfNode PROF_VNODES_START COMPILER_RT_VISIBILITY COMPILER_RT_WEAK;
+extern ValueProfNode PROF_VNODES_STOP COMPILER_RT_VISIBILITY COMPILER_RT_WEAK;
 
 /* Add dummy data to ensure the section is always created. Add used attribute so
  * that they are linker GC roots on supported ELF platforms.
  */
 __llvm_profile_data __prof_data_sect_data[0] COMPILER_RT_SECTION(
     INSTR_PROF_DATA_SECT_NAME) COMPILER_RT_USED;
 uint64_t __prof_cnts_sect_data[0] COMPILER_RT_SECTION(INSTR_PROF_CNTS_SECT_NAME)
     COMPILER_RT_USED;