Adding TAU profiling calls via LLVM pass

Building

This pass (in lib/Instrument.cpp) should be built against the install of LLVM you want to use it in. The master branch was tested against 6.x, 7.x, 8.x, 9.x, 10.x, 11.x, 12.x and the current master branch (13.x).

Independently

Starting at the project root,

mkdir -p build
cd build
cmake .. -DLLVM_ROOT=$LLVM_DIR
cmake --build .

Don't forget to set the $LLVM_DIR, $CC and $CXX environment variables. The configuration of TAU that will be used is set by the environment variable TAU_MAKEFILE. The compilers ($CC and $CXX) must be the same as the ones used to build the LLVM installation you are building against.

Usage

The plugin accepts some optional command line arguments, that permit the user to specify:

• -tau-start-func
  The function to call before each instrumented function call-site. By default this is Tau_start
• -tau-stop-func
  The function to call after each instrumented function call-site. By default this is Tau_stop
• -tau-input-file
  A file containing the names of functions to instrument. This has no default, but failing to specify such a file will result in no instrumentation.
• -tau-regex
  A case-sensitive ECMAScript Regular Expression to test against function names. All functions matching the expression will be instrumented
• -tau-iregex
  A case-insensitive ECMAScript Regular Expression to test against function names. All functions matching the expression will be instrumented

They can be set using clang, clang++, or opt with LLVM bitcode files. Only usage with Clang frontends is detailed here.

To use the plugin with the default start and stop functions, you must know the path to the TAU shared library. To use alternative functions, you'll need the path to the appropriate libraries for them.

At the moment, there are three source files and a sample file containing function names in the sandbox directory that can be used for simple tests.

• rtlib.c defines two functions that could be used as alternatives to Tau_start and Tau_stop.
• example.c is a Hello World C program to test the pass on
• example.cc is a Hello World C++ program with some OO features to see what kinds of calls are visible after lowering to LLVM IR.

An example using several source files is also given in the sandbox/mm directory, and a more complex program is given in the sandbox/hh directory.

The following instructions assume TAU_Profiling.so and Tau_Profiling_CXX.so have been built against the system installed LLVM. If this is not the case, replace invocations of clang and clang++ with the appropriate versions.

If used, the runtime library must be compiled first (producing a shared library is also OK):

clang -c rtlib.c
# produces rtlib.o

To compile and link the example C program with the plugin and TAU profiling:

clang -fplugin=/path/to/TAU_Profiling.so              \
  -mllvm -tau-input-file=./functions_C.txt            \
  -ldl -L/path/to/TAU/and/archi/$TAU_MAKEFILE -l TAU  \
  -Wl,-rpath,/path/to/TAU/and/archi/$TAU_MAKEFILE     \
  -o example example.c

Linking against `libdl` is required for TAU. Specifying the path for dynamic linking also appears to be necessary.

The process is similar for the example C++ program:

clang++ -fplugin=/path/to/TAU_Profiling_CXX.so        \
  -mllvm -tau-input-file=./functions_CXX.txt        \
  -ldl -L/path/to/TAU/and/archi/$TAU_MAKEFILE -lTAU \
  -Wl,-rpath,/path/to/TAU/and/archi/$TAU_MAKEFILE   \
  -o example example.cc

In the sandbox/mm directory, an example of a C++ program using several source files is given and can be compiled using:

clang++ -O3 -g -fplugin=/path/to/TAU_Profiling_CXX.so   \ 
  -mllvm -tau-input-file=./functions_CXX_mm.txt -ldl  \
  -L/path/to/TAU/and/archi/lib/$TAU_MAKEFILE -lTAU    \
  -Wl,-rpath,/path/to/TAU/and/archi/lib/$TAU_MAKEFILE \
  matmult.cpp matmult_initialize.cpp -o mm_cpp

Running the resulting executable in either case should produce a profile.* file.

LLVM 13

A new pass manager was introduced in LLVM 13. For the moment, both pass managers are available in LLVM 13. This pass uses the "old" one, which must be enabled with -flegacy-pass-manager.

Therefore, if you are using LLVM 13, you need to compile with, for instance:

clang++ -flegacy-pass-manager -fplugin=/path/to/TAU_Profiling_CXX.so        \
  -mllvm -tau-input-file=./functions_CXX.txt        \
  -ldl -L/path/to/TAU/and/archi/$TAU_MAKEFILE -lTAU \
  -Wl,-rpath,/path/to/TAU/and/archi/$TAU_MAKEFILE   \
  -o example example.cc

Input file syntax

The input file contains function names, one on each line. For C programs, use the function name; for C++ programs, use the whole prototype.

Functions to instrument

Examples are given in sandbox directory.

$ cat sandbox/functions_C.txt 
BEGIN_INCLUDE_LIST
hw
END_INCLUDE_LIST
$ cat sandbox/functions_CXX.txt 
BEGIN_INCLUDE_LIST
main()
A::foo()
END_INCLUDE_LIST

The input file for C++ programs must provide the arguments' datatypes.

$ cat sandbox/mm/functions_CXX_mm.txt 
BEGIN_INCLUDE_LIST
initialize(double**, int, int)
compute_interchange(double**, double**, double**, int, int, int)
END_INCLUDE_LIST

Templates work with the type or non-type parameter specified:

void householder<double>(int, int, double**, double**, double**)
void householder<float>(int, int, float**, float**, float**)

Regular expressions

The module provides two ways of passing function names as regular expressions.

As command line arguments

The options -tau-regex and -tau-iregex can be used to pass case-sensitive and case-insensitive ECMAScript Regular Expressions on the command line. For example:

clang -fplugin=/path/to/TAU_Profiling.so -mllvm -tau-regex="apply*"  \
  -ldl -L/path/to/TAU/and/archi/$TAU_MAKEFILE -lTAU                  \
  -Wl,-rpath,/path/to/TAU/and/archi/$TAU_MAKEFILE                    \
  -O3 -g ./householder.c -o householder -lm

In the input file

In the input file, regular expressions use the Kleene star (#) as the wildcard, since the star (*) already means something in function names. For instance, using the example code given in sandbox/hh, we can instrument all the functions starting with apply (ie, applyQ and applyR) using:

apply#

Exclude functions from the instrumentation

Functions can be explicitely excluded from the instrumentation. The function names are given between the tag BEGIN_EXCLUDE_LIST and the tag END_EXCLUDE_LIST. For instance, using the example code given in sandbox/hh, we can exclude the function check using:

BEGIN_EXCLUDE_LIST
check
END_EXCLUDE_LIST

Regular expressions work on excluded functions too. For instance, we can exclude all the function whose names start with check using:

BEGIN_EXCLUDE_LIST
check#
END_EXCLUDE_LIST

Wildcards also work for template types:

void applyQ<#>(int, #**, #*, #, int)

Include or exclude files from the instrumentation

Similarly, some files can be included or excluded from the instrumentation. Regular expressions can also be used, using '*' to match a sequence of characters and '?' to match a given character. The syntax is:

BEGIN_FILE_INCLUDE_LIST
file1.c
bar*.h
END_FILE_INCLUDE_LIST
BEGIN_FILE_EXCLUDE_LIST
file4.c
foo?.h
END_FILE_EXCLUDE_LIST

An example for this is given in sandbox/hh2. The input file requests the instrumentation of householder, matmul and all the functions starting with apply (apply#). There is an applyR function in R.c and an applyQ function in Q.c. However, we are excluding Q.c from the instrumentation. We should see householder, matmul and applyR in the output and not applyQ using the following compilation command:

clang -fplugin=/path/to/TAU_Profiling.so              \
  -mllvm -tau-input-file=functions_hh.txt             \
  -ldl -L/path/to/TAU/and/archi/$TAU_MAKEFILE -l TAU  \
  -Wl,-rpath,/path/to/TAU/and/archi/$TAU_MAKEFILE     \
  -o householder3 householder3.c matmul.c Q.c R.c -lm

Functions in header files

Functions defined in header files can be included or excluded by file name using the -g option in the compilation command. Otherwise, they will appear to be defined in the .c or .cpp file(s) that include(s) them.

You can find an example in the sandbox/header directory.

TODO

• Write something to spit out the names of known called functions, demangled if possible/necessary. This will help the user know exactly what name of the function to use to make sure it's instrumented.
• Look into regexes, maybe? Having to write the fully-qualified name of all the functions requiring instrumentation sounds tedious and error-prone.
• Give better output about what is being instrumented.

TOTHINK

Where to insert calls

Profiling function calls are currently inserted around call sites. But they could be inserted at function entry and exit (or it could be a plugin parameter).

1. Entry/Exit Pros

   • If I were doing it manually, that's what I'd do.
   • Presumably less noise in the IR, if ever inspected.
   • Can produce an instrumented library that just needs to be linked properly. This would be particularly useful for profiling across several apps using the same library.

2. Entry/Exit Cons

   • Can't profile library calls (I think?) if all I have is the .so or .a, which may be a more realistic use-case.
   • Without better knowledge of IR function structure, it's not clear whether preserving semantics (esp. at function exit) is difficult.

References

• Writing an LLVM Pass
• Adrian Sampson's LLVM pass guide (2015)