docs/FAQ.rst - fp2-dev/platform/external/llvm - Gitiles

 .. _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.

 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.


 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:: 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
    }
	.. _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.

	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.


	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:: 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
	}