[profile] Make atexit hook a no-op on Fuchsia

[phosek]

Summary

On Fuchsia, we always use the continuous mode with runtime counter
relocation, so there's no need for atexit hook or support for dumping
the profile manually.

[phosek]

compiler-rt/lib/profile/InstrProfilingPlatformFuchsia.c

186

To remove this function altogether, we would need to wrap its invocation in https://github.com/llvm/llvm-project/blob/master/compiler-rt/lib/profile/InstrProfilingRuntime.cpp#L24 in #if !defined(__Fuchsia__), I'm not sure if that's better than defining an empty function?

[mcgrathr]

compiler-rt/lib/profile/InstrProfilingPlatformFuchsia.c

186

Some other runtimes (e.g. hwasan) have an OS-specific InstallAtExitHandler() called by the common initialization function, so we can define that as a no-op.

[mcgrathr]

compiler-rt/lib/profile/InstrProfilingFile.c

39 ↗

(On Diff #251873)

Can we make this a function defined in File.c to return a static variable kept there and in Fuchsia.c to return constant 1?
Then at least in an LTO future it will fold away for Fuchsia, and not add a data word.

compiler-rt/lib/profile/InstrProfilingPlatformFuchsia.c

99

This function is responsible for initializing before return and the link-time initializer in BiasVar.c is -1 so this will always be true at startup.

[phosek]

compiler-rt/lib/profile/InstrProfilingPlatformFuchsia.c

99

It's defined as -1 in https://github.com/llvm/llvm-project/blob/master/compiler-rt/lib/profile/InstrProfilingBiasVar.c#L15 which is a weak definition, but compiler will generate a strong definition with the default value 0 when runtime counter relocation is enabled (default on Fuchsia), so this should only kick in when someone accidentally disables runtime counter relocation on Fuchsia? I'm happy to omit this check if you think it's unnecessary.

[mcgrathr]

lgtm with comment and nits

compiler-rt/lib/profile/InstrProfilingInternal.h

188 ↗

(On Diff #252448)

C functions need (void).

compiler-rt/lib/profile/InstrProfilingPlatformFuchsia.c

37

C functions need (void).

99

I see. That makes sense but there should be a comment here explaining the compiler's semantics for this symbol.

compiler-rt/lib/profile/InstrProfilingPlatformFuchsia.c

 #include <zircon/sanitizer.h>
 #include <zircon/status.h>
 #include <zircon/syscalls.h>
 
 #include "InstrProfiling.h"
 #include "InstrProfilingInternal.h"
 #include "InstrProfilingUtil.h"
 
-/* VMO that contains the coverage data shared across all modules. */
+/* VMO that contains the profile data for this module. */
 static zx_handle_t __llvm_profile_vmo;
 /* Current offset within the VMO where data should be written next. */
 static uint64_t __llvm_profile_offset;
 
 static const char ProfileSinkName[] = "llvm-profile";
 
 static inline void lprofWrite(const char *fmt, ...) {
   char s[256];
static uint32_t lprofVMOWriter(ProfDataWriter *This, ProfDataIOVec *IOVecs,
   return 0;
 }
 
 static void initVMOWriter(ProfDataWriter *This) {
   This->Write = lprofVMOWriter;
   This->WriterCtx = NULL;
 }
 
-static int dump(void) {
-  if (lprofProfileDumped()) {
-    lprofWrite("LLVM Profile: data not published: already written.\n");
-    return 0;
-  }
-
-  /* Check if there is llvm/runtime version mismatch. */
-  if (GET_VERSION(__llvm_profile_get_version()) != INSTR_PROF_RAW_VERSION) {
-    lprofWrite("LLVM Profile: runtime and instrumentation version mismatch: "
-               "expected %d, but got %d\n",
-               INSTR_PROF_RAW_VERSION,
-               (int)GET_VERSION(__llvm_profile_get_version()));
-    return -1;
-  }
-
-  /* Write the profile data into the mapped region. */
-  ProfDataWriter VMOWriter;
-  initVMOWriter(&VMOWriter);
-  if (lprofWriteData(&VMOWriter, lprofGetVPDataReader(), 0) != 0)
-    return -1;
-
-  return 0;
-}
-
+/* This method is invoked by the runtime initialization hook
+ * InstrProfilingRuntime.o if it is linked in.
+ */
 COMPILER_RT_VISIBILITY
-int __llvm_profile_dump(void) {
-  int rc = dump();
-  lprofSetProfileDumped();
-  return rc;
-}
-
-static void dumpWithoutReturn(void) { dump(); }
-
-static void createVMO(void) {
+void __llvm_profile_initialize_file(void) {
   /* Don't create VMO if it has been alread created. */
   if (__llvm_profile_vmo != ZX_HANDLE_INVALID)
     return;
 
   const __llvm_profile_data *DataBegin = __llvm_profile_begin_data();
   const __llvm_profile_data *DataEnd = __llvm_profile_end_data();
   const uint64_t DataSize = __llvm_profile_get_data_size(DataBegin, DataEnd);
   const uint64_t CountersOffset = sizeof(__llvm_profile_header) +
if (Status != ZX_OK) {
     return;
   }
 
   /* Update the profile fields based on the current mapping. */
   __llvm_profile_counter_bias = (intptr_t)Mapping -
       (uintptr_t)__llvm_profile_begin_counters() + CountersOffset;
 }
 
-/* This method is invoked by the runtime initialization hook
- * InstrProfilingRuntime.o if it is linked in.
- */
-COMPILER_RT_VISIBILITY
-void __llvm_profile_initialize_file(void) { createVMO(); }
-
 COMPILER_RT_VISIBILITY
 int __llvm_profile_register_write_file_atexit(void) {
-  static bool HasBeenRegistered = false;
-
-  if (HasBeenRegistered)
-    return 0;
-
-  lprofSetupValueProfiler();
-
-  HasBeenRegistered = true;
-  return atexit(dumpWithoutReturn);
 }
 
 #endif