[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, 6 insertions, 2 deletions

diff --git a/llvm/lib/ProfileData/InstrProfReader.cpp b/llvm/lib/ProfileData/InstrProfReader.cpp
index 23d078a3dde..b904f983dce 100644
--- a/llvm/lib/ProfileData/InstrProfReader.cpp
+++ b/llvm/lib/ProfileData/InstrProfReader.cpp
@@ -362,7 +362,9 @@ Error RawInstrProfReader<IntPtrT>::readHeader(
   CountersDelta = swap(Header.CountersDelta);
   NamesDelta = swap(Header.NamesDelta);
   auto DataSize = swap(Header.DataSize);
+  auto PaddingBytesBeforeCounters = swap(Header.PaddingBytesBeforeCounters);
   auto CountersSize = swap(Header.CountersSize);
+  auto PaddingBytesAfterCounters = swap(Header.PaddingBytesAfterCounters);
   NamesSize = swap(Header.NamesSize);
   ValueKindLast = swap(Header.ValueKindLast);
 
@@ -370,8 +372,10 @@ Error RawInstrProfReader<IntPtrT>::readHeader(
   auto PaddingSize = getNumPaddingBytes(NamesSize);
 
   ptrdiff_t DataOffset = sizeof(RawInstrProf::Header);
-  ptrdiff_t CountersOffset = DataOffset + DataSizeInBytes;
-  ptrdiff_t NamesOffset = CountersOffset + sizeof(uint64_t) * CountersSize;
+  ptrdiff_t CountersOffset =
+      DataOffset + DataSizeInBytes + PaddingBytesBeforeCounters;
+  ptrdiff_t NamesOffset = CountersOffset + (sizeof(uint64_t) * CountersSize) +
+                          PaddingBytesAfterCounters;
   ptrdiff_t ValueDataOffset = NamesOffset + NamesSize + PaddingSize;
 
   auto *Start = reinterpret_cast<const char *>(&Header);