[profile] Add a mode to continuously sync counter updates to a file

Add support for continuously syncing profile counter updates to a file.

The motivation for this is that programs do not always exit cleanly. On
iOS, for example, programs are usually killed via a signal from the OS.
Running atexit() handlers after catching a signal is unreliable, so some
method for progressively writing out profile data is necessary.

The approach taken here is to mmap() the `__llvm_prf_cnts` section onto
a raw profile. To do this, the linker must page-align the counter and
data sections, and the runtime must ensure that counters are mapped to a
page-aligned offset within a raw profile.

Continuous mode is (for the moment) incompatible with the online merging
mode. This limitation is lifted in https://reviews.llvm.org/D69586.

Continuous mode is also (for the moment) incompatible with value
profiling, as I'm not sure whether there is interest in this and the
implementation may be tricky.

As I have not been able to test extensively on non-Darwin platforms,
only Darwin support is included for the moment. However, continuous mode
may "just work" without modification on Linux and some UNIX-likes. AIUI
the default value for the GNU linker's `--section-alignment` flag is set
to the page size on many systems. This appears to be true for LLD as
well, as its `no_nmagic` option is on by default. Continuous mode will
not "just work" on Fuchsia or Windows, as it's not possible to mmap() a
section on these platforms. There is a proposal to add a layer of
indirection to the profile instrumentation to support these platforms.

rdar://54210980

Differential Revision: https://reviews.llvm.org/D68351

1 files changed, 10 insertions, 0 deletions

diff --git a/compiler-rt/lib/profile/InstrProfilingPort.h b/compiler-rt/lib/profile/InstrProfilingPort.h
index da5b5c0f8bb..9462cf1a240 100644
--- a/compiler-rt/lib/profile/InstrProfilingPort.h
+++ b/compiler-rt/lib/profile/InstrProfilingPort.h
@@ -99,6 +99,16 @@
   (((ch) == DIR_SEPARATOR) || ((ch) == DIR_SEPARATOR_2))
 #endif /* DIR_SEPARATOR_2 */
 
+#if defined(_WIN32)
+static inline size_t getpagesize() {
+  SYSTEM_INFO S;
+  GetNativeSystemInfo(&S);
+  return S.dwPageSize;
+}
+#else /* defined(_WIN32) */
+#include <unistd.h>
+#endif /* defined(_WIN32) */
+
 #define PROF_ERR(Format, ...)                                                  \
   fprintf(stderr, "LLVM Profile Error: " Format, __VA_ARGS__);