https://github.com/david-xl updated
https://github.com/llvm/llvm-project/pull/73845
>From b2c9081a0c3d5a982c2a23857bf986ec80c83cb5 Mon Sep 17 00:00:00 2001
From: David Li
Date: Mon, 27 Nov 2023 13:49:25 -0800
Subject: [PATCH 1/2] Fix stale comment
---
llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp
b/llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp
index 601903c29f799a2..73a7116f74e1180 100644
--- a/llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp
+++ b/llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp
@@ -6,7 +6,7 @@
//
//===--===//
//
-// This pass lowers instrprof_* intrinsics emitted by a frontend for profiling.
+// This pass lowers instrprof_* intrinsics emitted by an instrumentor.
// It also builds the data structures and initialization code needed for
// updating execution counts and emitting the profile at runtime.
//
>From dfd0ae0197b8e7425af1fd02636dda876b54d5bc Mon Sep 17 00:00:00 2001
From: David Li
Date: Wed, 29 Nov 2023 11:56:31 -0800
Subject: [PATCH 2/2] Fix PGO documentation in user manual
---
clang/docs/UsersManual.rst | 45 ++
1 file changed, 31 insertions(+), 14 deletions(-)
diff --git a/clang/docs/UsersManual.rst b/clang/docs/UsersManual.rst
index 2e658557b0e310c..51b9cc246ea8b55 100644
--- a/clang/docs/UsersManual.rst
+++ b/clang/docs/UsersManual.rst
@@ -2348,9 +2348,10 @@ differences between the two:
1. Profile data generated with one cannot be used by the other, and there is no
conversion tool that can convert one to the other. So, a profile generated
- via ``-fprofile-instr-generate`` must be used with ``-fprofile-instr-use``.
- Similarly, sampling profiles generated by external profilers must be
- converted and used with ``-fprofile-sample-use``.
+ via ``-fprofile-generate`` or ``-fprofile-instr-generate`` must be used with
+ ``-fprofile-use`` or ``-fprofile-instr-use``. Similarly, sampling profiles
+ generated by external profilers must be converted and used with
``-fprofile-sample-use``
+ or ``-fauto-profile``.
2. Instrumentation profile data can be used for code coverage analysis and
optimization.
@@ -2607,11 +2608,25 @@ overhead during the profiling, but it provides more
detailed results than a
sampling profiler. It also provides reproducible results, at least to the
extent that the code behaves consistently across runs.
+Clang supports two types of instrumentation: frontend-based and IR-based.
+Frontend-based instrumentation can be enabled with the option
``-fprofile-instr-generate``,
+and IR-based instrumentation can be enabled with the option
``-fprofile-generate``.
+For best performance with PGO, IR-based instrumentation should be used. It has
+the benefits of lower instrumentation overhead, smaller raw profile size, and
+better runtime performance. Frontend-based instrumentation, on the other hand,
+has better source correlation, so it should be used with source line-based
+coverage testing.
+
+The flag ``-fcs-profile-generate`` also instruments programs using the same
+instrumentation method as ``-fprofile-generate``. However, it performs a
+post-inline late instrumentation and can produce context-sensitive profiles.
+
+
Here are the steps for using profile guided optimization with
instrumentation:
1. Build an instrumented version of the code by compiling and linking with the
- ``-fprofile-instr-generate`` option.
+ ``-fprofile-generate`` or ``-fprofile-instr-generate`` option.
.. code-block:: console
@@ -2674,8 +2689,8 @@ instrumentation:
Note that this step is necessary even when there is only one "raw" profile,
since the merge operation also changes the file format.
-4. Build the code again using the ``-fprofile-instr-use`` option to specify the
- collected profile data.
+4. Build the code again using the ``-fprofile-use`` or ``-fprofile-instr-use``
+ option to specify the collected profile data.
.. code-block:: console
@@ -2685,13 +2700,10 @@ instrumentation:
profile. As you make changes to your code, clang may no longer be able to
use the profile data. It will warn you when this happens.
-Profile generation using an alternative instrumentation method can be
-controlled by the GCC-compatible flags ``-fprofile-generate`` and
-``-fprofile-use``. Although these flags are semantically equivalent to
-their GCC counterparts, they *do not* handle GCC-compatible profiles.
-They are only meant to implement GCC's semantics with respect to
-profile creation and use. Flag ``-fcs-profile-generate`` also instruments
-programs using the same instrumentation method as ``-fprofile-generate``.
+Note that ``-fprofile-use`` option is semantically equivalent to
+its GCC counterpart, it *does not* handle profile formats produced by GCC.
