[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, 32 insertions, 0 deletions

diff --git a/compiler-rt/test/profile/ContinuousSyncMode/set-file-object.c b/compiler-rt/test/profile/ContinuousSyncMode/set-file-object.c
new file mode 100644
index 00000000000..ac3be3b5237
--- /dev/null
+++ b/compiler-rt/test/profile/ContinuousSyncMode/set-file-object.c
@@ -0,0 +1,32 @@
+// RUN: %clang_pgogen -o %t.exe %s
+// RUN: env LLVM_PROFILE_FILE="%c%t.profraw" %run %t.exe %t.bad 2>&1 | FileCheck %s
+
+// CHECK: __llvm_profile_set_file_object(fd={{[0-9]+}}) not supported
+// CHECK: Profile data not written to file: already written.
+
+#include <stdio.h>
+
+extern int __llvm_profile_is_continuous_mode_enabled(void);
+extern void __llvm_profile_set_file_object(FILE *, int);
+extern int __llvm_profile_write_file(void);
+
+int main(int argc, char **argv) {
+  if (!__llvm_profile_is_continuous_mode_enabled())
+    return 1;
+
+  FILE *f = fopen(argv[1], "a+b");
+  if (!f)
+    return 1;
+
+  __llvm_profile_set_file_object(f, 0); // Try to set the file to "%t.bad".
+
+  if (__llvm_profile_write_file() != 0)
+    return 1;
+
+  f = fopen(argv[1], "r");
+  if (!f)
+    return 1;
+
+  fseek(f, 0, SEEK_END);
+  return ftell(f); // Check that the "%t.bad" is empty.
+}