docs/FAQ.html - platform/external/llvm - Gitiles

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 <h1>
 <center>
 LLVM: Frequently Asked Questions
 </center>
 </h1>

 <hr>

 <!--=====================================================================-->
 <h2>
 <a name="license">Licenses</a>
 </h2>
 <!--=====================================================================-->

 <dl compact>
 	<dt> <b>Why are the LLVM source code and the front-end distributed
 	under different licenses?</b>
 	<dd>
 	The C/C++ front-ends are based on GCC and must be distributed under
 	the GPL.  Our aim is to distribute LLVM source code under a <em>much
 	less restrictive</em> license, in particular one that does not
 	compel users who distribute tools based on modifying the source to
 	redistribute the modified source code as well.
 	<p>
 	<dt><b>Does the University of Illinois Open Source License really qualify
 	  as an "open source" license?</b>
 	<dd>Yes, the license is
 	<a href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a>
 	by the Open Source Initiative (OSI).
 	<p>
 	<dt> <b>Can I modify LLVM source code and redistribute the modified
 	  source?</b>
 	<dd>
 	Yes.  The modified source distribution must retain the
 	copyright notice and follow the three bulletted conditions listed in
 	the <a href="http://llvm.cs.uiuc.edu/releases/1.0/LICENSE.TXT">LLVM license</a>.
 	<p>
 	<dt> <b>Can I modify LLVM source code and redistribute binaries or
 	  other tools based on it, without redistributing the source?</b>
 	<dd>
 	Yes, this is why we distribute LLVM under a less restrictive license
 	than GPL, as explained in the first question above.
 	<p>
 </dl>
 <hr>

 <!--=====================================================================-->
 <h2>
 <a name="source">Source Code</a>
 </h2>
 <!--=====================================================================-->

 <dl compact>
 	<dt> <b>In what language is LLVM written?</b>
 	<dd>
 	All of the LLVM tools and libraries are written in C++ with extensive use
 	of the STL.
 	<p>

 	<dt><b>How portable is the LLVM source code?</b>
 	<dd>
 	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.
 	<p>
 	Some porting problems may exist in the following areas:
 	<ul>
 		<li>The GCC front end code is not as portable as the LLVM suite, so it
 		may not compile as well on unsupported platforms.

 		<p>

 		<li>The Python test classes are more UNIX-centric than they should be,
 		so porting to non-UNIX like platforms (i.e. Windows, MacOS 9) will
 		require some effort.
 		<p>

 		<li>The LLVM 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.
 	</ul>
 </dl>

 <hr>

 <!--=====================================================================-->
 <h2>
 <a name="build">Build Problems</a>
 </h2>
 <!--=====================================================================-->

 <dl compact>
 	<dt><b>When I run configure, it finds the wrong C compiler.</b>
 	<dd>
 	The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and
 	then <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and
 	<tt>CXX</tt> for the C and C++ compiler, respectively.

 	If <tt>configure</tt> finds the wrong compiler, either adjust your
 	<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
 	explicitly.
 	<p>

 	<dt><b>I compile the code, and I get some error about /localhome</b>.
 	<dd>
 	There are several possible causes for this.  The first is that you
 	didn't set a pathname properly when using <tt>configure</tt>, and it
 	defaulted to a pathname that we use on our research machines.
 	<p>
 	Another possibility is that we hardcoded a path in our Makefiles.  If
 	you see this, please email the LLVM bug mailing list with the name of
 	the offending Makefile and a description of what is wrong with it.

 	<dt><b>The <tt>configure</tt> script finds the right C compiler, but it
 	uses the LLVM linker from a previous build.  What do I do?</b>
 	<dd>
 	The <tt>configure</tt> script uses the <tt>PATH</tt> to find
 	executables, so if it's grabbing the wrong linker/assembler/etc, there
 	are two ways to fix it:
 	<ol>
 		<li>Adjust your <tt>PATH</tt> environment variable so that the
 		correct program appears first in the <tt>PATH</tt>.  This may work,
 		but may not be convenient when you want them <i>first</i> in your
 		path for other work.
 		<p>

 		<li>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that
 		is correct.  In a Borne compatible shell, the syntax would be:
 		<p>
 		<tt>PATH=<the path without the bad program> ./configure ...</tt>
 		<p>
 		This is still somewhat inconvenient, but it allows
 		<tt>configure</tt> to do its work without having to adjust your
 		<tt>PATH</tt> permanently.
 	</ol>

 	<dt><b>When creating a dynamic library, I get a strange GLIBC error.</b>
 	<dd>
 	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.
 	<p>

 	<dt><b>I've updated my source tree from CVS, and now my build is trying to
 	use a file/directory that doesn't exist.</b>
 	<dd>
 	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.
 	<p>

 	<dt><b>I've modified a Makefile in my source tree, but my build tree keeps
 	using the old version.  What do I do?</b>
 	<dd>
 	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:
 	<p>
 	<tt>./config.status &lt;relative path to Makefile&gt;</tt>
 	<p>
 	If the Makefile is new, you will have to modify the configure script to copy
 	it over.
 	<p>

 	<dt><b>I've upgraded to a new version of LLVM, and I get strange build
 	errors.</b>
 	<dd>
 	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.
 	<p>
 	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 <tt>make
 	clean</tt> and then <tt>make</tt> in the directory that fails to build.
 	<p>

 	<dt><b>I've built LLVM and am testing it, but the tests freeze.</b>
 	<dd>
 	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 <tt>gmake</tt> command line.
 	<p>
 	For example, if you built LLVM with the command:
 	<p>
 	<tt>gmake ENABLE_PROFILING=1</tt>
 	<p>
 	...then you must run the tests with the following commands:
 	<p>
 	<tt>cd llvm/test<br>gmake  ENABLE_PROFILING=1</tt>
 	<p>

 	<dt><b>Why do test results differ when I perform different types of
 	builds?</b>
 	<dd>
 	The LLVM test suite is dependent upon several features of the LLVM tools
 	and libraries.
 	<p>
 	First, the debugging assertions in code are not enabled in optimized or
 	profiling builds.  Hence, tests that used to fail may pass.
 	<p>
 	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.
 </dl>
 <hr>

 <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
 <br>

 </body>
 </html>
	<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

	<h1>
	<center>
	LLVM: Frequently Asked Questions
	</center>
	</h1>

	<hr>

	<!--=====================================================================-->
	<h2>
	<a name="license">Licenses</a>
	</h2>
	<!--=====================================================================-->

	<dl compact>
	<dt> <b>Why are the LLVM source code and the front-end distributed
	under different licenses?</b>
	<dd>
	The C/C++ front-ends are based on GCC and must be distributed under
	the GPL. Our aim is to distribute LLVM source code under a <em>much
	less restrictive</em> license, in particular one that does not
	compel users who distribute tools based on modifying the source to
	redistribute the modified source code as well.
	<p>
	<dt><b>Does the University of Illinois Open Source License really qualify
	as an "open source" license?</b>
	<dd>Yes, the license is
	<a href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a>
	by the Open Source Initiative (OSI).
	<p>
	<dt> <b>Can I modify LLVM source code and redistribute the modified
	source?</b>
	<dd>
	Yes. The modified source distribution must retain the
	copyright notice and follow the three bulletted conditions listed in
	the <a href="http://llvm.cs.uiuc.edu/releases/1.0/LICENSE.TXT">LLVM license</a>.
	<p>
	<dt> <b>Can I modify LLVM source code and redistribute binaries or
	other tools based on it, without redistributing the source?</b>
	<dd>
	Yes, this is why we distribute LLVM under a less restrictive license
	than GPL, as explained in the first question above.
	<p>
	</dl>
	<hr>

	<!--=====================================================================-->
	<h2>
	<a name="source">Source Code</a>
	</h2>
	<!--=====================================================================-->

	<dl compact>
	<dt> <b>In what language is LLVM written?</b>
	<dd>
	All of the LLVM tools and libraries are written in C++ with extensive use
	of the STL.
	<p>

	<dt><b>How portable is the LLVM source code?</b>
	<dd>
	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.
	<p>
	Some porting problems may exist in the following areas:
	<ul>
	<li>The GCC front end code is not as portable as the LLVM suite, so it
	may not compile as well on unsupported platforms.

	<p>

	<li>The Python test classes are more UNIX-centric than they should be,
	so porting to non-UNIX like platforms (i.e. Windows, MacOS 9) will
	require some effort.
	<p>

	<li>The LLVM 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.
	</ul>
	</dl>

	<hr>

	<!--=====================================================================-->
	<h2>
	<a name="build">Build Problems</a>
	</h2>
	<!--=====================================================================-->

	<dl compact>
	<dt><b>When I run configure, it finds the wrong C compiler.</b>
	<dd>
	The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and
	then <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt> and
	<tt>CXX</tt> for the C and C++ compiler, respectively.

	If <tt>configure</tt> finds the wrong compiler, either adjust your
	<tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
	explicitly.
	<p>

	<dt><b>I compile the code, and I get some error about /localhome</b>.
	<dd>
	There are several possible causes for this. The first is that you
	didn't set a pathname properly when using <tt>configure</tt>, and it
	defaulted to a pathname that we use on our research machines.
	<p>
	Another possibility is that we hardcoded a path in our Makefiles. If
	you see this, please email the LLVM bug mailing list with the name of
	the offending Makefile and a description of what is wrong with it.

	<dt><b>The <tt>configure</tt> script finds the right C compiler, but it
	uses the LLVM linker from a previous build. What do I do?</b>
	<dd>
	The <tt>configure</tt> script uses the <tt>PATH</tt> to find
	executables, so if it's grabbing the wrong linker/assembler/etc, there
	are two ways to fix it:
	<ol>
	<li>Adjust your <tt>PATH</tt> environment variable so that the
	correct program appears first in the <tt>PATH</tt>. This may work,
	but may not be convenient when you want them <i>first</i> in your
	path for other work.
	<p>

	<li>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that
	is correct. In a Borne compatible shell, the syntax would be:
	<p>
	<tt>PATH=<the path without the bad program> ./configure ...</tt>
	<p>
	This is still somewhat inconvenient, but it allows
	<tt>configure</tt> to do its work without having to adjust your
	<tt>PATH</tt> permanently.
	</ol>

	<dt><b>When creating a dynamic library, I get a strange GLIBC error.</b>
	<dd>
	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.
	<p>

	<dt><b>I've updated my source tree from CVS, and now my build is trying to
	use a file/directory that doesn't exist.</b>
	<dd>
	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.
	<p>

	<dt><b>I've modified a Makefile in my source tree, but my build tree keeps
	using the old version. What do I do?</b>
	<dd>
	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:
	<p>
	<tt>./config.status <relative path to Makefile></tt>
	<p>
	If the Makefile is new, you will have to modify the configure script to copy
	it over.
	<p>

	<dt><b>I've upgraded to a new version of LLVM, and I get strange build
	errors.</b>
	<dd>
	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.
	<p>
	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 <tt>make
	clean</tt> and then <tt>make</tt> in the directory that fails to build.
	<p>

	<dt><b>I've built LLVM and am testing it, but the tests freeze.</b>
	<dd>
	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 <tt>gmake</tt> command line.
	<p>
	For example, if you built LLVM with the command:
	<p>
	<tt>gmake ENABLE_PROFILING=1</tt>
	<p>
	...then you must run the tests with the following commands:
	<p>
	<tt>cd llvm/test<br>gmake ENABLE_PROFILING=1</tt>
	<p>

	<dt><b>Why do test results differ when I perform different types of
	builds?</b>
	<dd>
	The LLVM test suite is dependent upon several features of the LLVM tools
	and libraries.
	<p>
	First, the debugging assertions in code are not enabled in optimized or
	profiling builds. Hence, tests that used to fail may pass.
	<p>
	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.
	</dl>
	<hr>

	<a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
	<br>

	</body>
	</html>