Update documentation and release notes for llvm-profgen COFF support (#84864)

This change:
- Updates the existing Clang User's Manual section on SPGO so that it
describes how to use llvm-profgen to perform SPGO on Windows. This is
new functionality implemented in #83972.
- Fixes a minor typo in the existing llvm-profgen invocation example.
- Adds an LLVM release note on this new functionality in llvm-profgen.

Author: tcreech-intel

clang/docs/UsersManual.rst

@@ -2441,20 +2441,39 @@ usual build cycle when using sample profilers for optimization:
 
 1. Build the code with source line table information. You can use all the
    usual build flags that you always build your application with. The only
-   requirement is that you add ``-gline-tables-only`` or ``-g`` to the
-   command line. This is important for the profiler to be able to map
-   instructions back to source line locations.
+   requirement is that DWARF debug info including source line information is
+   generated. This DWARF information is important for the profiler to be able
+   to map instructions back to source line locations.
+
+   On Linux, ``-g`` or just ``-gline-tables-only`` is sufficient:
 
    .. code-block:: console
 
      $ clang++ -O2 -gline-tables-only code.cc -o code
 
+   While MSVC-style targets default to CodeView debug information, DWARF debug
+   information is required to generate source-level LLVM profiles. Use
+   ``-gdwarf`` to include DWARF debug information:
+
+   .. code-block:: console
+
+     $ clang-cl -O2 -gdwarf -gline-tables-only coff-profile.cpp -fuse-ld=lld -link -debug:dwarf
+
 2. Run the executable under a sampling profiler. The specific profiler
    you use does not really matter, as long as its output can be converted
-   into the format that the LLVM optimizer understands. Currently, there
-   exists a conversion tool for the Linux Perf profiler
-   (https://perf.wiki.kernel.org/), so these examples assume that you
-   are using Linux Perf to profile your code.
+   into the format that the LLVM optimizer understands.
+
+   Two such profilers are the the Linux Perf profiler
+   (https://perf.wiki.kernel.org/) and Intel's Sampling Enabling Product (SEP),
+   available as part of `Intel VTune
+   <https://software.intel.com/content/www/us/en/develop/tools/oneapi/components/vtune-profiler.html>`_.
+   While Perf is Linux-specific, SEP can be used on Linux, Windows, and FreeBSD.
+
+   The LLVM tool ``llvm-profgen`` can convert output of either Perf or SEP. An
+   external project, `AutoFDO <https://github.com/google/autofdo>`_, also
+   provides a ``create_llvm_prof`` tool which supports Linux Perf output.
+
+   When using Perf:
 
    .. code-block:: console
 
@@ -2465,11 +2484,19 @@ usual build cycle when using sample profilers for optimization:
    it provides better call information, which improves the accuracy of
    the profile data.
 
-3. Convert the collected profile data to LLVM's sample profile format.
-   This is currently supported via the AutoFDO converter ``create_llvm_prof``.
-   It is available at https://github.com/google/autofdo. Once built and
-   installed, you can convert the ``perf.data`` file to LLVM using
-   the command:
+   When using SEP:
+
+   .. code-block:: console
+
+     $ sep -start -out code.tb7 -ec BR_INST_RETIRED.NEAR_TAKEN:precise=yes:pdir -lbr no_filter:usr -perf-script brstack -app ./code
+
+   This produces a ``code.perf.data.script`` output which can be used with
+   ``llvm-profgen``'s ``--perfscript`` input option.
+
+3. Convert the collected profile data to LLVM's sample profile format. This is
+   currently supported via the `AutoFDO <https://github.com/google/autofdo>`_
+   converter ``create_llvm_prof``. Once built and installed, you can convert
+   the ``perf.data`` file to LLVM using the command:
 
    .. code-block:: console
 
@@ -2485,7 +2512,14 @@ usual build cycle when using sample profilers for optimization:
 
    .. code-block:: console
 
-     $ llvm-profgen --binary=./code --output=code.prof--perfdata=perf.data
+     $ llvm-profgen --binary=./code --output=code.prof --perfdata=perf.data
+
+   When using SEP the output is in the textual format corresponding to
+   ``llvm-profgen --perfscript``. For example:
+
+   .. code-block:: console
+
+     $ llvm-profgen --binary=./code --output=code.prof --perfscript=code.perf.data.script
 
 
 4. Build the code again using the collected profile. This step feeds

llvm/docs/ReleaseNotes.rst

@@ -181,6 +181,11 @@ Changes to the LLVM tools
   for ELF input to skip the specified symbols when executing other options
   that can change a symbol's name, binding or visibility.
 
+* llvm-profgen now supports COFF+DWARF binaries. This enables Sample-based PGO
+  on Windows using Intel VTune's SEP. For details on usage, see the `end-user
+  documentation for SPGO
+  <https://clang.llvm.org/docs/UsersManual.html#using-sampling-profilers>`_.
+
 Changes to LLDB
 ---------------------------------