Setting up the build environment
--------------------------------
-.. FIXME: Why does this recommend to build in-tree?
-
-First, configure and build LLVM. This needs to be done directly inside the
-LLVM source tree rather than in a separate objects directory. Next, you need
-to create a new directory somewhere in the LLVM source base. For this example,
-we'll assume that you made ``lib/Transforms/Hello``. Finally, you must set up
-a build script (``Makefile``) that will compile the source code for the new
-pass. To do this, copy the following into ``Makefile``:
+First, configure and build LLVM. Next, you need to create a new directory
+somewhere in the LLVM source base. For this example, we'll assume that you
+made ``lib/Transforms/Hello``. Finally, you must set up a build script
+(``Makefile``) that will compile the source code for the new pass. To do this,
+copy the following into ``Makefile``:
.. code-block:: make
struct Hello : public FunctionPass {
-This declares a "``Hello``" class that is a subclass of `FunctionPass
+This declares a "``Hello``" class that is a subclass of :ref:`FunctionPass
<writing-an-llvm-pass-FunctionPass>`. The different builtin pass subclasses
are described in detail :ref:`later <writing-an-llvm-pass-pass-classes>`, but
for now, know that ``FunctionPass`` operates on a function at a time.
.. code-block:: c++
- virtual bool runOnFunction(Function &F) {
+ bool runOnFunction(Function &F) override {
errs() << "Hello: ";
errs().write_escaped(F.getName()) << "\n";
return false;
static char ID;
Hello() : FunctionPass(ID) {}
- virtual bool runOnFunction(Function &F) {
+ bool runOnFunction(Function &F) override {
errs() << "Hello: ";
errs().write_escaped(F.getName()) << '\n';
return false;
static RegisterPass<Hello> X("hello", "Hello World Pass", false, false);
Now that it's all together, compile the file with a simple "``gmake``" command
-in the local directory and you should get a new file
-"``Debug+Asserts/lib/Hello.so``" under the top level directory of the LLVM
-source tree (not in the local directory). Note that everything in this file is
+from the top level of your build directory and you should get a new file
+"``Debug+Asserts/lib/Hello.so``". Note that everything in this file is
contained in an anonymous namespace --- this reflects the fact that passes
are self contained units that do not need external interfaces (although they
can have them) to be useful.
.. code-block:: console
- $ opt -load ../../../Debug+Asserts/lib/Hello.so -hello < hello.bc > /dev/null
+ $ opt -load ../../Debug+Asserts/lib/Hello.so -hello < hello.bc > /dev/null
Hello: __main
Hello: puts
Hello: main
.. code-block:: console
- $ opt -load ../../../Debug+Asserts/lib/Hello.so -help
+ $ opt -load ../../Debug+Asserts/lib/Hello.so -help
OVERVIEW: llvm .bc -> .bc modular optimizer
USAGE: opt [options] <input bitcode>
-hello - Hello World Pass
-indvars - Induction Variable Simplification
-inline - Function Integration/Inlining
- -insert-edge-profiling - Insert instrumentation for edge profiling
...
The pass name gets added as the information string for your pass, giving some
.. code-block:: console
- $ opt -load ../../../Debug+Asserts/lib/Hello.so -hello -time-passes < hello.bc > /dev/null
+ $ opt -load ../../Debug+Asserts/lib/Hello.so -hello -time-passes < hello.bc > /dev/null
Hello: __main
Hello: puts
Hello: main
virtual bool doFinalization(CallGraph &CG);
The ``doFinalization`` method is an infrequently used method that is called
-when the pass framework has finished calling :ref:`runOnFunction
-<writing-an-llvm-pass-runOnFunction>` for every function in the program being
-compiled.
+when the pass framework has finished calling :ref:`runOnSCC
+<writing-an-llvm-pass-runOnSCC>` for every SCC in the program being compiled.
.. _writing-an-llvm-pass-FunctionPass:
#. Inspect or modify a ``Function`` other than the one currently being processed.
#. Add or remove ``Function``\ s from the current ``Module``.
#. Add or remove global variables from the current ``Module``.
-#. Maintain state across invocations of:ref:`runOnFunction
+#. Maintain state across invocations of :ref:`runOnFunction
<writing-an-llvm-pass-runOnFunction>` (including global data).
Implementing a ``FunctionPass`` is usually straightforward (See the :ref:`Hello
// This example modifies the program, but does not modify the CFG
void LICM::getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
- AU.addRequired<LoopInfo>();
+ AU.addRequired<LoopInfoWrapperPass>();
}
.. _writing-an-llvm-pass-getAnalysis:
.. code-block:: c++
bool LICM::runOnFunction(Function &F) {
- LoopInfo &LI = getAnalysis<LoopInfo>();
+ LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
//...
}
.. code-block:: console
- $ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -licm --debug-pass=Structure < hello.bc > /dev/null
+ $ opt -load ../../Debug+Asserts/lib/Hello.so -gcse -licm --debug-pass=Structure < hello.bc > /dev/null
Module Pass Manager
Function Pass Manager
Dominator Set Construction
.. code-block:: console
- $ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
+ $ opt -load ../../Debug+Asserts/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
Module Pass Manager
Function Pass Manager
Dominator Set Construction
.. code-block:: c++
// We don't modify the program, so we preserve all analyses
- virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+ void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesAll();
}
.. code-block:: console
- $ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
+ $ opt -load ../../Debug+Asserts/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
Pass Arguments: -gcse -hello -licm
Module Pass Manager
Function Pass Manager
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