[PGO]: Eliminate __llvm_profile_register calls for Linux (compiler_rt)

[davidxl]

Summary

This is patch-3.

[davidxl]

Conditionally define start/end symbols

[davidxl]

Fixed linker script bug that broke gold linker

[davidxl]

There is another solution that is similar to Darwin, so no linker script is needed. I will update the patch to go that route.

From Ian Taylor:

"When a section name is a valid C identifier, the linker automatically provides start and stop symbols set to the addresses of the beginning and end of the section. The symbols are start_SECNAME and stop_SECNAME where SECNAME is the name of the section.

This is not defined by the ELF ABI, but it is completely reliable on
GNU/Linux. In fact it is reliable on any ELF system except possibly
Solaris, and it probably works on Solaris too. This is definitely
what you should do instead of using a linker script. It's more
reliable and more efficient, and it's what everybody else does"

[davidxl]

This is the latest version that does not rely on linker script.

[davidxl]

Make new change strictly limited to linux platform.

[davidxl]

The revision has been LGTM ed. Clang-formatted and committed upstream at r250200.

lib/profile/CMakeLists.txt

   InstrProfilingBuffer.c
   InstrProfilingFile.c
   InstrProfilingPlatformDarwin.c
+  InstrProfilingPlatformLinux.c
   InstrProfilingPlatformOther.c
   InstrProfilingRuntime.cc
   InstrProfilingUtil.c)

lib/profile/InstrProfilingPlatformLinux.c

+/*===- InstrProfilingPlatformLinux.c - Profile data Linux platform ------===*\
+|*
+|*                     The LLVM Compiler Infrastructure
+|*
+|* This file is distributed under the University of Illinois Open Source
+|* License. See LICENSE.TXT for details.
+|*
+\*===----------------------------------------------------------------------===*/
+
+#include "InstrProfiling.h"
+
+#if defined(__linux__)
+#include <stdlib.h>
+
+extern __llvm_profile_data __start___llvm_prf_data
+    __attribute__((visibility("hidden")));
+extern __llvm_profile_data __stop___llvm_prf_data
+    __attribute__((visibility("hidden")));
+extern uint64_t __start___llvm_prf_cnts __attribute__((visibility("hidden")));
+extern uint64_t __stop___llvm_prf_cnts __attribute__((visibility("hidden")));
+extern char __start___llvm_prf_names __attribute__((visibility("hidden")));
+extern char __stop___llvm_prf_names __attribute__((visibility("hidden")));
+
+__attribute__((visibility("hidden"))) const __llvm_profile_data *
+__llvm_profile_begin_data(void) {
+  return &__start___llvm_prf_data;
+}
+__attribute__((visibility("hidden"))) const __llvm_profile_data *
+__llvm_profile_end_data(void) {
+  return &__stop___llvm_prf_data;
+}
+__attribute__((visibility("hidden"))) const char *__llvm_profile_begin_names(
+    void) {
+  return &__start___llvm_prf_names;
+}
+__attribute__((visibility("hidden"))) const char *__llvm_profile_end_names(
+    void) {
+  return &__stop___llvm_prf_names;
+}
+__attribute__((visibility("hidden"))) uint64_t *__llvm_profile_begin_counters(
+    void) {
+  return &__start___llvm_prf_cnts;
+}
+__attribute__((visibility("hidden"))) uint64_t *__llvm_profile_end_counters(
+    void) {
+  return &__stop___llvm_prf_cnts;
+}
+#endif

lib/profile/InstrProfilingPlatformOther.c

 
 #include "InstrProfiling.h"
 
-#if !defined(__APPLE__)
+#if !defined(__APPLE__) && !defined(__linux__)
 #include <stdlib.h>
 
 static const __llvm_profile_data *DataFirst = NULL;