[docs] Port FAQ over to Sphinx.

Patch by Mikael Lyngvig!

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+.. _faq:
+
+================================
+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.
+
+
+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:: bash
+
+   % 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:: bash
+
+   % ./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:: bash
+
+   % gmake ENABLE_PROFILING=1
+
+...then you must run the tests with the following commands:
+
+.. code-block:: bash
+
+   % 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.
+
+
+Compiling LLVM with GCC succeeds, but the resulting tools do not work, what can be wrong?
+-----------------------------------------------------------------------------------------
+Several versions of GCC have shown a weakness in miscompiling the LLVM
+codebase.  Please consult your compiler version (``gcc --version``) to find
+out whether it is `broken <GettingStarted.html#brokengcc>`_.  If so, your only
+option is to upgrade GCC to a known good version.
+
+
+After Subversion update, rebuilding gives the error "No rule to make target".
+-----------------------------------------------------------------------------
+If the error is of the form:
+
+.. code-block:: bash
+
+   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:: bash
+
+   % 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
+   }