[profile] Support counter relocation at runtime

This is an alternative to the continous mode that was implemented in
D68351. This mode relies on padding and the ability to mmap a file over
the existing mapping which is generally only available on POSIX systems
and isn't suitable for other platforms.

This change instead introduces the ability to relocate counters at
runtime using a level of indirection. On every counter access, we add a
bias to the counter address. This bias is stored in a symbol that's
provided by the profile runtime and is initially set to zero, meaning no
relocation. The runtime can mmap the profile into memory at abitrary
location, and set bias to the offset between the original and the new
counter location, at which point every subsequent counter access will be
to the new location, which allows updating profile directly akin to the
continous mode.

The advantage of this implementation is that doesn't require any special
OS support. The disadvantage is the extra overhead due to additional
instructions required for each counter access (overhead both in terms of
binary size and performance) plus duplication of counters (i.e. one copy
in the binary itself and another copy that's mmapped).

Differential Revision: https://reviews.llvm.org/D69740

1 files changed, 30 insertions, 3 deletions

diff --git a/clang/docs/SourceBasedCodeCoverage.rst b/clang/docs/SourceBasedCodeCoverage.rst
index 7e711819be34..1575e4faaa01 100644
--- a/clang/docs/SourceBasedCodeCoverage.rst
+++ b/clang/docs/SourceBasedCodeCoverage.rst
@@ -92,15 +92,42 @@ directory structure will be created.  Additionally, the following special
   instrumented program crashes, or is killed by a signal, perfect coverage
   information can still be recovered. Continuous mode does not support value
   profiling for PGO, and is only supported on Darwin at the moment. Support for
-  Linux may be mostly complete but requires testing, and support for
-  Fuchsia/Windows may require more extensive changes: please get involved if
-  you are interested in porting this feature.
+  Linux may be mostly complete but requires testing, and support for Windows
+  may require more extensive changes: please get involved if you are interested
+  in porting this feature.
 
 .. code-block:: console
 
     # Step 2: Run the program.
     % LLVM_PROFILE_FILE="foo.profraw" ./foo
 
+Note that continuous mode is also used on Fuchsia where it's the only supported
+mode, but the implementation is different. The Darwin and Linux implementation
+relies on padding and the ability to map a file over the existing memory
+mapping which is generally only available on POSIX systems and isn't suitable
+for other platforms.
+
+On Fuchsia, we rely on the the ability to relocate counters at runtime using a
+level of indirection. On every counter access, we add a bias to the counter
+address. This bias is stored in ``__llvm_profile_counter_bias`` symbol that's
+provided by the profile runtime and is initially set to zero, meaning no
+relocation. The runtime can map the profile into memory at abitrary location,
+and set bias to the offset between the original and the new counter location,
+at which point every subsequent counter access will be to the new location,
+which allows updating profile directly akin to the continous mode.
+
+The advantage of this approach is that doesn't require any special OS support.
+The disadvantage is the extra overhead due to additional instructions required
+for each counter access (overhead both in terms of binary size and performance)
+plus duplication of counters (i.e. one copy in the binary itself and another
+copy that's mapped into memory). This implementation can be also enabled for
+other platforms by passing the ``-runtime-counter-relocation`` option to the
+backend during compilation.
+
+.. code-block:: console
+
+    % clang++ -fprofile-instr-generate -fcoverage-mapping -mllvm -runtime-counter-relocation foo.cc -o foo
+
 Creating coverage reports
 =========================