[profile] Add back zero-sized dummy sections

[pirama]

Summary

These were removed in dc93b1127c5e0621e672745d26bce5a11f68d943 (D97648).
In addition to adding them back, mark them as "retained".

This change is needed for allowing continuous-mode profile creation for
Linux (and Android in particular).

• We're relying on 'OVERWRITE_SECTIONS' linker feature to set alignment

of llvm_prf_data and llvm_prf_cnts. If an OVERWRITE_SECTIONS is
present, lld adds the start/stop symbols for these sections even if
the eventual binary doesn't have these sections leading to
llvm-objcopy/llvm-strip complaining about malformed binaries. Adding
these dummy sections ensures that the start/stop symbols point to a
valid, albeit zero-sized section.

• We also need to mark these symbols with attribute "retain" to ensure

these symbols and sections are not removed with --gc-sections.

compiler-rt/lib/profile/InstrProfilingPlatformLinux.c

 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_RETAINED;
+uint64_t __prof_cnts_sect_data[0] COMPILER_RT_SECTION(INSTR_PROF_CNTS_SECT_NAME)
+    COMPILER_RT_RETAINED;
+uint32_t __prof_orderfile_sect_data[0] COMPILER_RT_SECTION(
+    INSTR_PROF_ORDERFILE_SECT_NAME) COMPILER_RT_RETAINED;
+const char __prof_nms_sect_data[0] COMPILER_RT_SECTION(
+    INSTR_PROF_NAME_SECT_NAME) COMPILER_RT_RETAINED;
+ValueProfNode __prof_vnodes_sect_data[0] COMPILER_RT_SECTION(
+    INSTR_PROF_VNODES_SECT_NAME) COMPILER_RT_RETAINED;
+
 COMPILER_RT_VISIBILITY const __llvm_profile_data *
 __llvm_profile_begin_data(void) {
   return &PROF_DATA_START;
 }
 COMPILER_RT_VISIBILITY const __llvm_profile_data *
 __llvm_profile_end_data(void) {
   return &PROF_DATA_STOP;
 }

compiler-rt/lib/profile/InstrProfilingPort.h

 #define COMPILER_RT_WEAK __declspec(selectany)
 /* Need to include <windows.h> */
 #define COMPILER_RT_ALLOCA _alloca
 /* Need to include <stdio.h> and <io.h> */
 #define COMPILER_RT_FTRUNCATE(f,l) _chsize(_fileno(f),l)
 #define COMPILER_RT_ALWAYS_INLINE __forceinline
 #define COMPILER_RT_CLEANUP(x)
 #define COMPILER_RT_USED
+#define COMPILER_RT_RETAINED
 #elif __GNUC__
 #ifdef _WIN32
 #define COMPILER_RT_FTRUNCATE(f, l) _chsize(fileno(f), l)
 #define COMPILER_RT_VISIBILITY
 #define COMPILER_RT_WEAK __attribute__((selectany))
 #else
 #define COMPILER_RT_FTRUNCATE(f, l) ftruncate(fileno(f), l)
 #define COMPILER_RT_VISIBILITY __attribute__((visibility("hidden")))
 #define COMPILER_RT_WEAK __attribute__((weak))
 #endif
 #define COMPILER_RT_ALIGNAS(x) __attribute__((aligned(x)))
 #define COMPILER_RT_ALLOCA __builtin_alloca
 #define COMPILER_RT_ALWAYS_INLINE inline __attribute((always_inline))
 #define COMPILER_RT_CLEANUP(x) __attribute__((cleanup(x)))
 #define COMPILER_RT_USED __attribute__((used))
+#define COMPILER_RT_RETAINED __attribute__((retain))
 #endif
 
 #if defined(__APPLE__)
 #define COMPILER_RT_SEG "__DATA,"
 #else
 #define COMPILER_RT_SEG ""
 #endif