[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/basic.c b/compiler-rt/test/profile/ContinuousSyncMode/basic.c
new file mode 100644
index 00000000000..9e29a0b0477
--- /dev/null
+++ b/compiler-rt/test/profile/ContinuousSyncMode/basic.c
@@ -0,0 +1,32 @@
+// RUN: %clang -fprofile-instr-generate -fcoverage-mapping -o %t.exe %s
+// RUN: echo "garbage" > %t.profraw
+// RUN: env LLVM_PROFILE_FILE="%c%t.profraw" %run %t.exe
+// RUN: llvm-profdata show --counts --all-functions %t.profraw | FileCheck %s -check-prefix=CHECK-COUNTS
+// RUN: llvm-profdata merge -o %t.profdata %t.profraw
+// RUN: llvm-cov report %t.exe -instr-profile %t.profdata | FileCheck %s -check-prefix=CHECK-COVERAGE
+
+// CHECK-COUNTS: Counters:
+// CHECK-COUNTS-NEXT:   main:
+// CHECK-COUNTS-NEXT:     Hash: 0x{{.*}}
+// CHECK-COUNTS-NEXT:     Counters: 2
+// CHECK-COUNTS-NEXT:     Function count: 1
+// CHECK-COUNTS-NEXT:     Block counts: [1]
+// CHECK-COUNTS-NEXT: Instrumentation level: Front-end
+// CHECK-COUNTS-NEXT: Functions shown: 1
+// CHECK-COUNTS-NEXT: Total functions: 1
+// CHECK-COUNTS-NEXT: Maximum function count: 1
+// CHECK-COUNTS-NEXT: Maximum internal block count: 1
+
+// CHECK-COVERAGE: Filename    Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover
+// CHECK-COVERAGE-NEXT: ---
+// CHECK-COVERAGE-NEXT: basic.c      4                 1    75.00%           1                 0   100.00%           5                 2    60.00%
+// CHECK-COVERAGE-NEXT: ---
+// CHECK-COVERAGE-NEXT: TOTAL        4                 1    75.00%           1                 0   100.00%           5                 2    60.00%
+
+extern int __llvm_profile_is_continuous_mode_enabled(void);
+
+int main() {
+  if (__llvm_profile_is_continuous_mode_enabled())
+    return 0;
+  return 1;
+}