[profile] Mark lprofCurFilename as COMPILER_RT_WEAK

[vsk]

Summary

This makes __llvm_profile_set_filename() work across dylib boundaries on
Darwin.

This functionality was originally meant to work on all platforms, but
was moved to a Linux-only directory with r272404. The root cause of the
test failure on Darwin was that lprofCurFilename was not marked weak.
Each dylib maintained its own copy of the variable due to the two-level
namespace.

Tested with check-profile (on Darwin). I don't expect this to regress
other platforms.

[davidxl]

lgtm

lib/profile/InstrProfilingFile.c

typedef struct lprofFilename {
    * is not enabled, its value should be 0. This parameter is specified
    * by the %[0-9]m specifier. For instance %2m enables merging using
    * 2 profile data files. %1m is equivalent to %m. Also %m specifier
    * can only appear once at the end of the name pattern. */
   unsigned MergePoolSize;
   ProfileNameSpecifier PNS;
 } lprofFilename;
 
-lprofFilename lprofCurFilename = {0, 0, 0, {0}, {0}, 0, 0, 0, PNS_unknown};
+COMPILER_RT_WEAK lprofFilename lprofCurFilename = {0, 0, 0, {0}, {0},
+                                                   0, 0, 0, PNS_unknown};
 
 int getpid(void);
 static int getCurFilenameLength();
 static const char *getCurFilename(char *FilenameBuf);
 static unsigned doMerging() { return lprofCurFilename.MergePoolSize; }
 
 /* Return 1 if there is an error, otherwise return  0.  */
 static uint32_t fileWriter(ProfDataIOVec *IOVecs, uint32_t NumIOVecs,

test/profile/instrprof-set-filename-shared.test

 # Test that __llvm_profile_set_filename is honored by shared libary too.
 RUN: mkdir -p %t.d
-RUN: %clang_profgen=%t.shared.profraw -fPIC -shared -o %t.d/t.shared %S/../Inputs/instrprof-dlopen-func.c
-RUN: %clang_profgen -DCALL_SHARED -o %t.m -O3 -rpath %t.d %t.d/t.shared %S/../instrprof-set-filename.c 
+RUN: %clang_profgen=%t.shared.profraw -fPIC -shared -o %t.d/t.shared %S/Inputs/instrprof-dlopen-func.c
+RUN: %clang_profgen -DCALL_SHARED -o %t.m -O3 -rpath %t.d %t.d/t.shared %S/instrprof-set-filename.c
 RUN: %run %t.m %t.main.profraw
 RUN: llvm-profdata show %t.main.profraw | FileCheck --check-prefix=SHARED %s
 
 # SHARED: Total functions: 2