llvm-6502/docs/FAQ.rst

================================
Frequently Asked Questions (FAQ)
================================

.. contents::
   :local:


License
=======

Does the University of Illinois Open Source License really qualify as an "open source" license?
-----------------------------------------------------------------------------------------------
Yes, the license is `certified
<http://www.opensource.org/licenses/UoI-NCSA.php>`_ by the Open Source
Initiative (OSI).


Can I modify LLVM source code and redistribute the modified source?
-------------------------------------------------------------------
Yes.  The modified source distribution must retain the copyright notice and
follow the three bulletted conditions listed in the `LLVM license
<http://llvm.org/svn/llvm-project/llvm/trunk/LICENSE.TXT>`_.


Can I modify the LLVM source code and redistribute binaries or other tools based on it, without redistributing the source?
--------------------------------------------------------------------------------------------------------------------------
Yes. This is why we distribute LLVM under a less restrictive license than GPL,
as explained in the first question above.


Source Code
===========

In what language is LLVM written?
---------------------------------
All of the LLVM tools and libraries are written in C++ with extensive use of
the STL.


How portable is the LLVM source code?
-------------------------------------
The LLVM source code should be portable to most modern Unix-like operating
systems.  Most of the code is written in standard C++ with operating system
services abstracted to a support library.  The tools required to build and
test LLVM have been ported to a plethora of platforms.

Some porting problems may exist in the following areas:

* The autoconf/makefile build system relies heavily on UNIX shell tools,
  like the Bourne Shell and sed.  Porting to systems without these tools
  (MacOS 9, Plan 9) will require more effort.

What API do I use to store a value to one of the virtual registers in LLVM IR's SSA representation?
---------------------------------------------------------------------------------------------------

In short: you can't. It's actually kind of a silly question once you grok
what's going on. Basically, in code like:

.. code-block:: llvm

    %result = add i32 %foo, %bar

, ``%result`` is just a name given to the ``Value`` of the ``add``
instruction. In other words, ``%result`` *is* the add instruction. The
"assignment" doesn't explicitly "store" anything to any "virtual register";
the "``=``" is more like the mathematical sense of equality.

Longer explanation: In order to generate a textual representation of the
IR, some kind of name has to be given to each instruction so that other
instructions can textually reference it. However, the isomorphic in-memory
representation that you manipulate from C++ has no such restriction since
instructions can simply keep pointers to any other ``Value``'s that they
reference. In fact, the names of dummy numbered temporaries like ``%1`` are
not explicitly represented in the in-memory representation at all (see
``Value::getName()``).

Build Problems
==============

When I run configure, it finds the wrong C compiler.
----------------------------------------------------
The ``configure`` script attempts to locate first ``gcc`` and then ``cc``,
unless it finds compiler paths set in ``CC`` and ``CXX`` for the C and C++
compiler, respectively.

If ``configure`` finds the wrong compiler, either adjust your ``PATH``
environment variable or set ``CC`` and ``CXX`` explicitly.


The ``configure`` script finds the right C compiler, but it uses the LLVM tools from a previous build.  What do I do?
---------------------------------------------------------------------------------------------------------------------
The ``configure`` script uses the ``PATH`` to find executables, so if it's
grabbing the wrong linker/assembler/etc, there are two ways to fix it:

#. Adjust your ``PATH`` environment variable so that the correct program
   appears first in the ``PATH``.  This may work, but may not be convenient
   when you want them *first* in your path for other work.

#. Run ``configure`` with an alternative ``PATH`` that is correct. In a
   Bourne compatible shell, the syntax would be:

.. code-block:: console

   % PATH=[the path without the bad program] ./configure ...

This is still somewhat inconvenient, but it allows ``configure`` to do its
work without having to adjust your ``PATH`` permanently.


When creating a dynamic library, I get a strange GLIBC error.
-------------------------------------------------------------
Under some operating systems (i.e. Linux), libtool does not work correctly if
GCC was compiled with the ``--disable-shared option``.  To work around this,
install your own version of GCC that has shared libraries enabled by default.


I've updated my source tree from Subversion, and now my build is trying to use a file/directory that doesn't exist.
-------------------------------------------------------------------------------------------------------------------
You need to re-run configure in your object directory.  When new Makefiles
are added to the source tree, they have to be copied over to the object tree
in order to be used by the build.


I've modified a Makefile in my source tree, but my build tree keeps using the old version.  What do I do?
---------------------------------------------------------------------------------------------------------
If the Makefile already exists in your object tree, you can just run the
following command in the top level directory of your object tree:

.. code-block:: console

   % ./config.status <relative path to Makefile>;

If the Makefile is new, you will have to modify the configure script to copy
it over.


I've upgraded to a new version of LLVM, and I get strange build errors.
-----------------------------------------------------------------------
Sometimes, changes to the LLVM source code alters how the build system works.
Changes in ``libtool``, ``autoconf``, or header file dependencies are
especially prone to this sort of problem.

The best thing to try is to remove the old files and re-build.  In most cases,
this takes care of the problem.  To do this, just type ``make clean`` and then
``make`` in the directory that fails to build.


I've built LLVM and am testing it, but the tests freeze.
--------------------------------------------------------
This is most likely occurring because you built a profile or release
(optimized) build of LLVM and have not specified the same information on the
``gmake`` command line.

For example, if you built LLVM with the command:

.. code-block:: console

   % gmake ENABLE_PROFILING=1

...then you must run the tests with the following commands:

.. code-block:: console

   % cd llvm/test
   % gmake ENABLE_PROFILING=1

Why do test results differ when I perform different types of builds?
--------------------------------------------------------------------
The LLVM test suite is dependent upon several features of the LLVM tools and
libraries.

First, the debugging assertions in code are not enabled in optimized or
profiling builds.  Hence, tests that used to fail may pass.

Second, some tests may rely upon debugging options or behavior that is only
available in the debug build.  These tests will fail in an optimized or
profile build.


Compiling LLVM with GCC 3.3.2 fails, what should I do?
------------------------------------------------------
This is `a bug in GCC <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13392>`_,
and affects projects other than LLVM.  Try upgrading or downgrading your GCC.


After Subversion update, rebuilding gives the error "No rule to make target".
-----------------------------------------------------------------------------
If the error is of the form:

.. code-block:: console

   gmake[2]: *** No rule to make target `/path/to/somefile',
                 needed by `/path/to/another/file.d'.
   Stop.

This may occur anytime files are moved within the Subversion repository or
removed entirely.  In this case, the best solution is to erase all ``.d``
files, which list dependencies for source files, and rebuild:

.. code-block:: console

   % cd $LLVM_OBJ_DIR
   % rm -f `find . -name \*\.d`
   % gmake

In other cases, it may be necessary to run ``make clean`` before rebuilding.


Source Languages
================

What source languages are supported?
------------------------------------
LLVM currently has full support for C and C++ source languages. These are
available through both `Clang <http://clang.llvm.org/>`_ and `DragonEgg
<http://dragonegg.llvm.org/>`_.

The PyPy developers are working on integrating LLVM into the PyPy backend so
that PyPy language can translate to LLVM.


I'd like to write a self-hosting LLVM compiler. How should I interface with the LLVM middle-end optimizers and back-end code generators?
----------------------------------------------------------------------------------------------------------------------------------------
Your compiler front-end will communicate with LLVM by creating a module in the
LLVM intermediate representation (IR) format. Assuming you want to write your
language's compiler in the language itself (rather than C++), there are 3
major ways to tackle generating LLVM IR from a front-end:

1. **Call into the LLVM libraries code using your language's FFI (foreign
   function interface).**

  * *for:* best tracks changes to the LLVM IR, .ll syntax, and .bc format

  * *for:* enables running LLVM optimization passes without a emit/parse
    overhead

  * *for:* adapts well to a JIT context

  * *against:* lots of ugly glue code to write

2. **Emit LLVM assembly from your compiler's native language.**

  * *for:* very straightforward to get started

  * *against:* the .ll parser is slower than the bitcode reader when
    interfacing to the middle end

  * *against:* it may be harder to track changes to the IR

3. **Emit LLVM bitcode from your compiler's native language.**

  * *for:* can use the more-efficient bitcode reader when interfacing to the
    middle end

  * *against:* you'll have to re-engineer the LLVM IR object model and bitcode
    writer in your language

  * *against:* it may be harder to track changes to the IR

If you go with the first option, the C bindings in include/llvm-c should help
a lot, since most languages have strong support for interfacing with C. The
most common hurdle with calling C from managed code is interfacing with the
garbage collector. The C interface was designed to require very little memory
management, and so is straightforward in this regard.

What support is there for a higher level source language constructs for building a compiler?
--------------------------------------------------------------------------------------------
Currently, there isn't much. LLVM supports an intermediate representation
which is useful for code representation but will not support the high level
(abstract syntax tree) representation needed by most compilers. There are no
facilities for lexical nor semantic analysis.


I don't understand the ``GetElementPtr`` instruction. Help!
-----------------------------------------------------------
See `The Often Misunderstood GEP Instruction <GetElementPtr.html>`_.


Using the C and C++ Front Ends
==============================

Can I compile C or C++ code to platform-independent LLVM bitcode?
-----------------------------------------------------------------
No. C and C++ are inherently platform-dependent languages. The most obvious
example of this is the preprocessor. A very common way that C code is made
portable is by using the preprocessor to include platform-specific code. In
practice, information about other platforms is lost after preprocessing, so
the result is inherently dependent on the platform that the preprocessing was
targeting.

Another example is ``sizeof``. It's common for ``sizeof(long)`` to vary
between platforms. In most C front-ends, ``sizeof`` is expanded to a
constant immediately, thus hard-wiring a platform-specific detail.

Also, since many platforms define their ABIs in terms of C, and since LLVM is
lower-level than C, front-ends currently must emit platform-specific IR in
order to have the result conform to the platform ABI.


Questions about code generated by the demo page
===============================================

What is this ``llvm.global_ctors`` and ``_GLOBAL__I_a...`` stuff that happens when I ``#include <iostream>``?
-------------------------------------------------------------------------------------------------------------
If you ``#include`` the ``<iostream>`` header into a C++ translation unit,
the file will probably use the ``std::cin``/``std::cout``/... global objects.
However, C++ does not guarantee an order of initialization between static
objects in different translation units, so if a static ctor/dtor in your .cpp
file used ``std::cout``, for example, the object would not necessarily be
automatically initialized before your use.

To make ``std::cout`` and friends work correctly in these scenarios, the STL
that we use declares a static object that gets created in every translation
unit that includes ``<iostream>``.  This object has a static constructor
and destructor that initializes and destroys the global iostream objects
before they could possibly be used in the file.  The code that you see in the
``.ll`` file corresponds to the constructor and destructor registration code.

If you would like to make it easier to *understand* the LLVM code generated
by the compiler in the demo page, consider using ``printf()`` instead of
``iostream``\s to print values.


Where did all of my code go??
-----------------------------
If you are using the LLVM demo page, you may often wonder what happened to
all of the code that you typed in.  Remember that the demo script is running
the code through the LLVM optimizers, so if your code doesn't actually do
anything useful, it might all be deleted.

To prevent this, make sure that the code is actually needed.  For example, if
you are computing some expression, return the value from the function instead
of leaving it in a local variable.  If you really want to constrain the
optimizer, you can read from and assign to ``volatile`` global variables.


What is this "``undef``" thing that shows up in my code?
--------------------------------------------------------
``undef`` is the LLVM way of representing a value that is not defined.  You
can get these if you do not initialize a variable before you use it.  For
example, the C function:

.. code-block:: c

   int X() { int i; return i; }

Is compiled to "``ret i32 undef``" because "``i``" never has a value specified
for it.


Why does instcombine + simplifycfg turn a call to a function with a mismatched calling convention into "unreachable"? Why not make the verifier reject it?
----------------------------------------------------------------------------------------------------------------------------------------------------------
This is a common problem run into by authors of front-ends that are using
custom calling conventions: you need to make sure to set the right calling
convention on both the function and on each call to the function.  For
example, this code:

.. code-block:: llvm

   define fastcc void @foo() {
       ret void
   }
   define void @bar() {
       call void @foo()
       ret void
   }

Is optimized to:

.. code-block:: llvm

   define fastcc void @foo() {
       ret void
   }
   define void @bar() {
       unreachable
   }

... with "``opt -instcombine -simplifycfg``".  This often bites people because
"all their code disappears".  Setting the calling convention on the caller and
callee is required for indirect calls to work, so people often ask why not
make the verifier reject this sort of thing.

The answer is that this code has undefined behavior, but it is not illegal.
If we made it illegal, then every transformation that could potentially create
this would have to ensure that it doesn't, and there is valid code that can
create this sort of construct (in dead code).  The sorts of things that can
cause this to happen are fairly contrived, but we still need to accept them.
Here's an example:

.. code-block:: llvm

   define fastcc void @foo() {
       ret void
   }
   define internal void @bar(void()* %FP, i1 %cond) {
       br i1 %cond, label %T, label %F
   T:
       call void %FP()
       ret void
   F:
       call fastcc void %FP()
       ret void
   }
   define void @test() {
       %X = or i1 false, false
       call void @bar(void()* @foo, i1 %X)
       ret void
   }

In this example, "test" always passes ``@foo``/``false`` into ``bar``, which
ensures that it is dynamically called with the right calling conv (thus, the
code is perfectly well defined).  If you run this through the inliner, you
get this (the explicit "or" is there so that the inliner doesn't dead code
eliminate a bunch of stuff):

.. code-block:: llvm

   define fastcc void @foo() {
       ret void
   }
   define void @test() {
       %X = or i1 false, false
       br i1 %X, label %T.i, label %F.i
   T.i:
       call void @foo()
       br label %bar.exit
   F.i:
       call fastcc void @foo()
       br label %bar.exit
   bar.exit:
       ret void
   }

Here you can see that the inlining pass made an undefined call to ``@foo``
with the wrong calling convention.  We really don't want to make the inliner
have to know about this sort of thing, so it needs to be valid code.  In this
case, dead code elimination can trivially remove the undefined code.  However,
if ``%X`` was an input argument to ``@test``, the inliner would produce this:

.. code-block:: llvm

   define fastcc void @foo() {
       ret void
   }

   define void @test(i1 %X) {
       br i1 %X, label %T.i, label %F.i
   T.i:
       call void @foo()
       br label %bar.exit
   F.i:
       call fastcc void @foo()
       br label %bar.exit
   bar.exit:
       ret void
   }

The interesting thing about this is that ``%X`` *must* be false for the
code to be well-defined, but no amount of dead code elimination will be able
to delete the broken call as unreachable.  However, since
``instcombine``/``simplifycfg`` turns the undefined call into unreachable, we
end up with a branch on a condition that goes to unreachable: a branch to
unreachable can never happen, so "``-inline -instcombine -simplifycfg``" is
able to produce:

.. code-block:: llvm

   define fastcc void @foo() {
      ret void
   }
   define void @test(i1 %X) {
   F.i:
      call fastcc void @foo()
      ret void
   }