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16 LLVM: Frequently Asked Questions
20 <li><a href="#license">License</a>
22 <li>Why are the LLVM source code and the front-end distributed under
23 different licenses?</li>
25 <li>Does the University of Illinois Open Source License really qualify as an
26 "open source" license?</li>
28 <li>Can I modify LLVM source code and redistribute the modified source?</li>
30 <li>Can I modify LLVM source code and redistribute binaries or other tools
31 based on it, without redistributing the source?</li>
34 <li><a href="#source">Source code</a>
36 <li>In what language is LLVM written?</li>
38 <li>How portable is the LLVM source code?</li>
41 <li><a href="#build">Build Problems</a>
43 <li>When I run configure, it finds the wrong C compiler.</li>
45 <li>The <tt>configure</tt> script finds the right C compiler, but it uses
46 the LLVM linker from a previous build. What do I do?</li>
48 <li>When creating a dynamic library, I get a strange GLIBC error.</li>
50 <li>I've updated my source tree from Subversion, and now my build is trying
51 to use a file/directory that doesn't exist.</li>
53 <li>I've modified a Makefile in my source tree, but my build tree keeps
54 using the old version. What do I do?</li>
56 <li>I've upgraded to a new version of LLVM, and I get strange build
59 <li>I've built LLVM and am testing it, but the tests freeze.</li>
61 <li>Why do test results differ when I perform different types of
64 <li>Compiling LLVM with GCC 3.3.2 fails, what should I do?</li>
66 <li>Compiling LLVM with GCC succeeds, but the resulting tools do not work,
67 what can be wrong?</li>
69 <li>When I use the test suite, all of the C Backend tests fail. What is
72 <li>After Subversion update, rebuilding gives the error "No rule to make
75 <li><a href="#srcdir-objdir">When I compile LLVM-GCC with srcdir == objdir,
76 it fails. Why?</a></li>
79 <li><a href="#felangs">Source Languages</a>
81 <li><a href="#langs">What source languages are supported?</a></li>
83 <li><a href="#langirgen">I'd like to write a self-hosting LLVM compiler. How
84 should I interface with the LLVM middle-end optimizers and back-end code
87 <li><a href="#langhlsupp">What support is there for higher level source
88 language constructs for building a compiler?</a></li>
90 <li><a href="GetElementPtr.html">I don't understand the GetElementPtr
91 instruction. Help!</a></li>
94 <li><a href="#cfe">Using the GCC Front End</a>
96 <li>When I compile software that uses a configure script, the configure
97 script thinks my system has all of the header files and libraries it is
98 testing for. How do I get configure to work correctly?</li>
100 <li>When I compile code using the LLVM GCC front end, it complains that it
101 cannot find libcrtend.a?</li>
103 <li>How can I disable all optimizations when compiling code using the LLVM
106 <li><a href="#translatecxx">Can I use LLVM to convert C++ code to C
109 <li><a href="#platformindependent">Can I compile C or C++ code to
110 platform-independent LLVM bitcode?</a></li>
114 <li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
116 <li><a href="#iosinit">What is this <tt>llvm.global_ctors</tt> and
117 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
118 #include <iostream>?</a></li>
120 <li><a href="#codedce">Where did all of my code go??</a></li>
122 <li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in
125 <li><a href="#callconvwrong">Why does instcombine + simplifycfg turn
126 a call to a function with a mismatched calling convention into "unreachable"?
127 Why not make the verifier reject it?</a></li>
132 <div class="doc_author">
133 <p>Written by <a href="http://llvm.org/">The LLVM Team</a></p>
137 <!-- *********************************************************************** -->
139 <a name="license">License</a>
141 <!-- *********************************************************************** -->
145 <div class="question">
146 <p>Why are the LLVM source code and the front-end distributed under different
151 <p>The C/C++ front-ends are based on GCC and must be distributed under the GPL.
152 Our aim is to distribute LLVM source code under a <em>much less
153 restrictive</em> license, in particular one that does not compel users who
154 distribute tools based on modifying the source to redistribute the modified
155 source code as well.</p>
158 <div class="question">
159 <p>Does the University of Illinois Open Source License really qualify as an
160 "open source" license?</p>
165 is <a href="http://www.opensource.org/licenses/UoI-NCSA.php">certified</a> by
166 the Open Source Initiative (OSI).</p>
169 <div class="question">
170 <p>Can I modify LLVM source code and redistribute the modified source?</p>
174 <p>Yes. The modified source distribution must retain the copyright notice and
175 follow the three bulletted conditions listed in
176 the <a href="http://llvm.org/svn/llvm-project/llvm/trunk/LICENSE.TXT">LLVM
180 <div class="question">
181 <p>Can I modify LLVM source code and redistribute binaries or other tools based
182 on it, without redistributing the source?</p>
186 <p>Yes. This is why we distribute LLVM under a less restrictive license than
187 GPL, as explained in the first question above.</p>
192 <!-- *********************************************************************** -->
194 <a name="source">Source Code</a>
196 <!-- *********************************************************************** -->
200 <div class="question">
201 <p>In what language is LLVM written?</p>
205 <p>All of the LLVM tools and libraries are written in C++ with extensive use of
209 <div class="question">
210 <p>How portable is the LLVM source code?</p>
214 <p>The LLVM source code should be portable to most modern UNIX-like operating
215 systems. Most of the code is written in standard C++ with operating system
216 services abstracted to a support library. The tools required to build and test
217 LLVM have been ported to a plethora of platforms.</p>
219 <p>Some porting problems may exist in the following areas:</p>
222 <li>The GCC front end code is not as portable as the LLVM suite, so it may not
223 compile as well on unsupported platforms.</li>
225 <li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
226 Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9)
227 will require more effort.</li>
234 <!-- *********************************************************************** -->
236 <a name="build">Build Problems</a>
238 <!-- *********************************************************************** -->
242 <div class="question">
243 <p>When I run configure, it finds the wrong C compiler.</p>
247 <p>The <tt>configure</tt> script attempts to locate first <tt>gcc</tt> and then
248 <tt>cc</tt>, unless it finds compiler paths set in <tt>CC</tt>
249 and <tt>CXX</tt> for the C and C++ compiler, respectively.</p>
251 <p>If <tt>configure</tt> finds the wrong compiler, either adjust your
252 <tt>PATH</tt> environment variable or set <tt>CC</tt> and <tt>CXX</tt>
257 <div class="question">
258 <p>The <tt>configure</tt> script finds the right C compiler, but it uses the
259 LLVM linker from a previous build. What do I do?</p>
263 <p>The <tt>configure</tt> script uses the <tt>PATH</tt> to find executables, so
264 if it's grabbing the wrong linker/assembler/etc, there are two ways to fix
268 <li><p>Adjust your <tt>PATH</tt> environment variable so that the correct
269 program appears first in the <tt>PATH</tt>. This may work, but may not be
270 convenient when you want them <i>first</i> in your path for other
273 <li><p>Run <tt>configure</tt> with an alternative <tt>PATH</tt> that is
274 correct. In a Borne compatible shell, the syntax would be:</p>
276 <pre class="doc_code">
277 % PATH=[the path without the bad program] ./configure ...
280 <p>This is still somewhat inconvenient, but it allows <tt>configure</tt>
281 to do its work without having to adjust your <tt>PATH</tt>
282 permanently.</p></li>
286 <div class="question">
287 <p>When creating a dynamic library, I get a strange GLIBC error.</p>
291 <p>Under some operating systems (i.e. Linux), libtool does not work correctly if
292 GCC was compiled with the --disable-shared option. To work around this,
293 install your own version of GCC that has shared libraries enabled by
297 <div class="question">
298 <p>I've updated my source tree from Subversion, and now my build is trying to
299 use a file/directory that doesn't exist.</p>
303 <p>You need to re-run configure in your object directory. When new Makefiles
304 are added to the source tree, they have to be copied over to the object tree
305 in order to be used by the build.</p>
308 <div class="question">
309 <p>I've modified a Makefile in my source tree, but my build tree keeps using the
310 old version. What do I do?</p>
314 <p>If the Makefile already exists in your object tree, you can just run the
315 following command in the top level directory of your object tree:</p>
317 <pre class="doc_code">
318 % ./config.status <relative path to Makefile>
321 <p>If the Makefile is new, you will have to modify the configure script to copy
325 <div class="question">
326 <p>I've upgraded to a new version of LLVM, and I get strange build errors.</p>
331 <p>Sometimes, changes to the LLVM source code alters how the build system works.
332 Changes in libtool, autoconf, or header file dependencies are especially
333 prone to this sort of problem.</p>
335 <p>The best thing to try is to remove the old files and re-build. In most
336 cases, this takes care of the problem. To do this, just type <tt>make
337 clean</tt> and then <tt>make</tt> in the directory that fails to build.</p>
340 <div class="question">
341 <p>I've built LLVM and am testing it, but the tests freeze.</p>
345 <p>This is most likely occurring because you built a profile or release
346 (optimized) build of LLVM and have not specified the same information on the
347 <tt>gmake</tt> command line.</p>
349 <p>For example, if you built LLVM with the command:</p>
351 <pre class="doc_code">
352 % gmake ENABLE_PROFILING=1
355 <p>...then you must run the tests with the following commands:</p>
357 <pre class="doc_code">
359 % gmake ENABLE_PROFILING=1
363 <div class="question">
364 <p>Why do test results differ when I perform different types of builds?</p>
368 <p>The LLVM test suite is dependent upon several features of the LLVM tools and
371 <p>First, the debugging assertions in code are not enabled in optimized or
372 profiling builds. Hence, tests that used to fail may pass.</p>
374 <p>Second, some tests may rely upon debugging options or behavior that is only
375 available in the debug build. These tests will fail in an optimized or
379 <div class="question">
380 <p>Compiling LLVM with GCC 3.3.2 fails, what should I do?</p>
384 <p>This is <a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13392">a bug in
385 GCC</a>, and affects projects other than LLVM. Try upgrading or downgrading
389 <div class="question">
390 <p>Compiling LLVM with GCC succeeds, but the resulting tools do not work, what
395 <p>Several versions of GCC have shown a weakness in miscompiling the LLVM
396 codebase. Please consult your compiler version (<tt>gcc --version</tt>) to
397 find out whether it is <a href="GettingStarted.html#brokengcc">broken</a>.
398 If so, your only option is to upgrade GCC to a known good version.</p>
401 <div class="question">
402 <p>After Subversion update, rebuilding gives the error "No rule to make
407 <p>If the error is of the form:</p>
409 <pre class="doc_code">
410 gmake[2]: *** No rule to make target `/path/to/somefile', needed by
411 `/path/to/another/file.d'.<br>
415 <p>This may occur anytime files are moved within the Subversion repository or
416 removed entirely. In this case, the best solution is to erase all
417 <tt>.d</tt> files, which list dependencies for source files, and rebuild:</p>
419 <pre class="doc_code">
421 % rm -f `find . -name \*\.d`
425 <p>In other cases, it may be necessary to run <tt>make clean</tt> before
429 <div class="question">
430 <p><a name="srcdir-objdir">When I compile LLVM-GCC with srcdir == objdir, it
435 <p>The <tt>GNUmakefile</tt> in the top-level directory of LLVM-GCC is a special
436 <tt>Makefile</tt> used by Apple to invoke the <tt>build_gcc</tt> script after
437 setting up a special environment. This has the unfortunate side-effect that
438 trying to build LLVM-GCC with srcdir == objdir in a "non-Apple way" invokes
439 the <tt>GNUmakefile</tt> instead of <tt>Makefile</tt>. Because the
440 environment isn't set up correctly to do this, the build fails.</p>
442 <p>People not building LLVM-GCC the "Apple way" need to build LLVM-GCC with
443 srcdir != objdir, or simply remove the GNUmakefile entirely.</p>
445 <p>We regret the inconvenience.</p>
450 <!-- *********************************************************************** -->
452 <a name="felangs">Source Languages</a>
457 <div class="question">
458 <p><a name="langs">What source languages are supported?</a></p>
462 <p>LLVM currently has full support for C and C++ source languages. These are
463 available through a special version of GCC that LLVM calls the
464 <a href="#cfe">C Front End</a></p>
466 <p>There is an incomplete version of a Java front end available in the
467 <tt>java</tt> module. There is no documentation on this yet so you'll need to
468 download the code, compile it, and try it.</p>
470 <p>The PyPy developers are working on integrating LLVM into the PyPy backend so
471 that PyPy language can translate to LLVM.</p>
474 <div class="question">
475 <p><a name="langirgen">I'd like to write a self-hosting LLVM compiler. How
476 should I interface with the LLVM middle-end optimizers and back-end code
481 <p>Your compiler front-end will communicate with LLVM by creating a module in
482 the LLVM intermediate representation (IR) format. Assuming you want to write
483 your language's compiler in the language itself (rather than C++), there are
484 3 major ways to tackle generating LLVM IR from a front-end:</p>
487 <li><strong>Call into the LLVM libraries code using your language's FFI
488 (foreign function interface).</strong>
491 <li><em>for:</em> best tracks changes to the LLVM IR, .ll syntax, and .bc
494 <li><em>for:</em> enables running LLVM optimization passes without a
495 emit/parse overhead</li>
497 <li><em>for:</em> adapts well to a JIT context</li>
499 <li><em>against:</em> lots of ugly glue code to write</li>
502 <li> <strong>Emit LLVM assembly from your compiler's native language.</strong>
504 <li><em>for:</em> very straightforward to get started</li>
506 <li><em>against:</em> the .ll parser is slower than the bitcode reader
507 when interfacing to the middle end</li>
509 <li><em>against:</em> you'll have to re-engineer the LLVM IR object model
510 and asm writer in your language</li>
512 <li><em>against:</em> it may be harder to track changes to the IR</li>
515 <li><strong>Emit LLVM bitcode from your compiler's native language.</strong>
518 <li><em>for:</em> can use the more-efficient bitcode reader when
519 interfacing to the middle end</li>
521 <li><em>against:</em> you'll have to re-engineer the LLVM IR object
522 model and bitcode writer in your language</li>
524 <li><em>against:</em> it may be harder to track changes to the IR</li>
528 <p>If you go with the first option, the C bindings in include/llvm-c should help
529 a lot, since most languages have strong support for interfacing with C. The
530 most common hurdle with calling C from managed code is interfacing with the
531 garbage collector. The C interface was designed to require very little memory
532 management, and so is straightforward in this regard.</p>
535 <div class="question">
536 <p><a name="langhlsupp">What support is there for a higher level source language
537 constructs for building a compiler?</a></p>
541 <p>Currently, there isn't much. LLVM supports an intermediate representation
542 which is useful for code representation but will not support the high level
543 (abstract syntax tree) representation needed by most compilers. There are no
544 facilities for lexical nor semantic analysis.</p>
547 <div class="question">
548 <p><a name="getelementptr">I don't understand the GetElementPtr
549 instruction. Help!</a></p>
553 <p>See <a href="GetElementPtr.html">The Often Misunderstood GEP
559 <!-- *********************************************************************** -->
561 <a name="cfe">Using the GCC Front End</a>
566 <div class="question">
567 <p>When I compile software that uses a configure script, the configure script
568 thinks my system has all of the header files and libraries it is testing for.
569 How do I get configure to work correctly?</p>
573 <p>The configure script is getting things wrong because the LLVM linker allows
574 symbols to be undefined at link time (so that they can be resolved during JIT
575 or translation to the C back end). That is why configure thinks your system
576 "has everything."</p>
578 <p>To work around this, perform the following steps:</p>
581 <li>Make sure the CC and CXX environment variables contains the full path to
582 the LLVM GCC front end.</li>
584 <li>Make sure that the regular C compiler is first in your PATH. </li>
586 <li>Add the string "-Wl,-native" to your CFLAGS environment variable.</li>
589 <p>This will allow the <tt>llvm-ld</tt> linker to create a native code
590 executable instead of shell script that runs the JIT. Creating native code
591 requires standard linkage, which in turn will allow the configure script to
592 find out if code is not linking on your system because the feature isn't
593 available on your system.</p>
596 <div class="question">
597 <p>When I compile code using the LLVM GCC front end, it complains that it cannot
603 <p>The only way this can happen is if you haven't installed the runtime
604 library. To correct this, do:</p>
606 <pre class="doc_code">
608 % make clean ; make install-bytecode
612 <div class="question">
613 <p>How can I disable all optimizations when compiling code using the LLVM GCC
618 <p>Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
619 optimizations done at the llvm level, leaving you with the truly horrible
620 code that you desire.</p>
624 <div class="question">
625 <p><a name="translatecxx">Can I use LLVM to convert C++ code to C code?</a></p>
629 <p>Yes, you can use LLVM to convert code from any language LLVM supports to C.
630 Note that the generated C code will be very low level (all loops are lowered
631 to gotos, etc) and not very pretty (comments are stripped, original source
632 formatting is totally lost, variables are renamed, expressions are
633 regrouped), so this may not be what you're looking for. Also, there are
634 several limitations noted below.<p>
636 <p>Use commands like this:</p>
639 <li><p>Compile your program with llvm-g++:</p>
641 <pre class="doc_code">
642 % llvm-g++ -emit-llvm x.cpp -o program.bc -c
647 <pre class="doc_code">
648 % llvm-g++ a.cpp -c -emit-llvm
649 % llvm-g++ b.cpp -c -emit-llvm
650 % llvm-ld a.o b.o -o program
653 <p>This will generate program and program.bc. The .bc
654 file is the LLVM version of the program all linked together.</p></li>
656 <li><p>Convert the LLVM code to C code, using the LLC tool with the C
659 <pre class="doc_code">
660 % llc -march=c program.bc -o program.c
663 <li><p>Finally, compile the C file:</p>
665 <pre class="doc_code">
671 <p>Using LLVM does not eliminate the need for C++ library support. If you use
672 the llvm-g++ front-end, the generated code will depend on g++'s C++ support
673 libraries in the same way that code generated from g++ would. If you use
674 another C++ front-end, the generated code will depend on whatever library
675 that front-end would normally require.</p>
677 <p>If you are working on a platform that does not provide any C++ libraries, you
678 may be able to manually compile libstdc++ to LLVM bitcode, statically link it
679 into your program, then use the commands above to convert the whole result
680 into C code. Alternatively, you might compile the libraries and your
681 application into two different chunks of C code and link them.</p>
683 <p>Note that, by default, the C back end does not support exception handling.
684 If you want/need it for a certain program, you can enable it by passing
685 "-enable-correct-eh-support" to the llc program. The resultant code will use
686 setjmp/longjmp to implement exception support that is relatively slow, and
687 not C++-ABI-conforming on most platforms, but otherwise correct.</p>
689 <p>Also, there are a number of other limitations of the C backend that cause it
690 to produce code that does not fully conform to the C++ ABI on most
691 platforms. Some of the C++ programs in LLVM's test suite are known to fail
692 when compiled with the C back end because of ABI incompatibilities with
693 standard C++ libraries.</p>
696 <div class="question">
697 <p><a name="platformindependent">Can I compile C or C++ code to
698 platform-independent LLVM bitcode?</a></p>
702 <p>No. C and C++ are inherently platform-dependent languages. The most obvious
703 example of this is the preprocessor. A very common way that C code is made
704 portable is by using the preprocessor to include platform-specific code. In
705 practice, information about other platforms is lost after preprocessing, so
706 the result is inherently dependent on the platform that the preprocessing was
709 <p>Another example is <tt>sizeof</tt>. It's common for <tt>sizeof(long)</tt> to
710 vary between platforms. In most C front-ends, <tt>sizeof</tt> is expanded to
711 a constant immediately, thus hard-wiring a platform-specific detail.</p>
713 <p>Also, since many platforms define their ABIs in terms of C, and since LLVM is
714 lower-level than C, front-ends currently must emit platform-specific IR in
715 order to have the result conform to the platform ABI.</p>
720 <!-- *********************************************************************** -->
722 <a name="cfe_code">Questions about code generated by the GCC front-end</a>
727 <div class="question">
728 <p><a name="iosinit">What is this <tt>llvm.global_ctors</tt> and
729 <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I <tt>#include
730 <iostream></tt>?</a></p>
734 <p>If you <tt>#include</tt> the <tt><iostream></tt> header into a C++
735 translation unit, the file will probably use
736 the <tt>std::cin</tt>/<tt>std::cout</tt>/... global objects. However, C++
737 does not guarantee an order of initialization between static objects in
738 different translation units, so if a static ctor/dtor in your .cpp file
739 used <tt>std::cout</tt>, for example, the object would not necessarily be
740 automatically initialized before your use.</p>
742 <p>To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
743 STL that we use declares a static object that gets created in every
744 translation unit that includes <tt><iostream></tt>. This object has a
745 static constructor and destructor that initializes and destroys the global
746 iostream objects before they could possibly be used in the file. The code
747 that you see in the .ll file corresponds to the constructor and destructor
751 <p>If you would like to make it easier to <b>understand</b> the LLVM code
752 generated by the compiler in the demo page, consider using <tt>printf()</tt>
753 instead of <tt>iostream</tt>s to print values.</p>
756 <!--=========================================================================-->
758 <div class="question">
759 <p><a name="codedce">Where did all of my code go??</a></p>
763 <p>If you are using the LLVM demo page, you may often wonder what happened to
764 all of the code that you typed in. Remember that the demo script is running
765 the code through the LLVM optimizers, so if your code doesn't actually do
766 anything useful, it might all be deleted.</p>
768 <p>To prevent this, make sure that the code is actually needed. For example, if
769 you are computing some expression, return the value from the function instead
770 of leaving it in a local variable. If you really want to constrain the
771 optimizer, you can read from and assign to <tt>volatile</tt> global
775 <!--=========================================================================-->
777 <div class="question">
778 <p><a name="undef">What is this "<tt>undef</tt>" thing that shows up in my
783 <p><a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of
784 representing a value that is not defined. You can get these if you do not
785 initialize a variable before you use it. For example, the C function:</p>
787 <pre class="doc_code">
788 int X() { int i; return i; }
791 <p>Is compiled to "<tt>ret i32 undef</tt>" because "<tt>i</tt>" never has a
792 value specified for it.</p>
795 <!--=========================================================================-->
797 <div class="question">
798 <p><a name="callconvwrong">Why does instcombine + simplifycfg turn
799 a call to a function with a mismatched calling convention into "unreachable"?
800 Why not make the verifier reject it?</a></p>
804 <p>This is a common problem run into by authors of front-ends that are using
805 custom calling conventions: you need to make sure to set the right calling
806 convention on both the function and on each call to the function. For example,
809 <pre class="doc_code">
810 define fastcc void @foo() {
819 <p>Is optimized to:</p>
821 <pre class="doc_code">
822 define fastcc void @foo() {
830 <p>... with "opt -instcombine -simplifycfg". This often bites people because
831 "all their code disappears". Setting the calling convention on the caller and
832 callee is required for indirect calls to work, so people often ask why not make
833 the verifier reject this sort of thing.</p>
835 <p>The answer is that this code has undefined behavior, but it is not illegal.
836 If we made it illegal, then every transformation that could potentially create
837 this would have to ensure that it doesn't, and there is valid code that can
838 create this sort of construct (in dead code). The sorts of things that can
839 cause this to happen are fairly contrived, but we still need to accept them.
840 Here's an example:</p>
842 <pre class="doc_code">
843 define fastcc void @foo() {
846 define internal void @bar(void()* %FP, i1 %cond) {
847 br i1 %cond, label %T, label %F
852 call fastcc void %FP()
855 define void @test() {
856 %X = or i1 false, false
857 call void @bar(void()* @foo, i1 %X)
862 <p>In this example, "test" always passes @foo/false into bar, which ensures that
863 it is dynamically called with the right calling conv (thus, the code is
864 perfectly well defined). If you run this through the inliner, you get this
865 (the explicit "or" is there so that the inliner doesn't dead code eliminate
869 <pre class="doc_code">
870 define fastcc void @foo() {
873 define void @test() {
874 %X = or i1 false, false
875 br i1 %X, label %T.i, label %F.i
880 call fastcc void @foo()
887 <p>Here you can see that the inlining pass made an undefined call to @foo with
888 the wrong calling convention. We really don't want to make the inliner have
889 to know about this sort of thing, so it needs to be valid code. In this case,
890 dead code elimination can trivially remove the undefined code. However, if %X
891 was an input argument to @test, the inliner would produce this:
894 <pre class="doc_code">
895 define fastcc void @foo() {
899 define void @test(i1 %X) {
900 br i1 %X, label %T.i, label %F.i
905 call fastcc void @foo()
912 <p>The interesting thing about this is that %X <em>must</em> be false for the
913 code to be well-defined, but no amount of dead code elimination will be able to
914 delete the broken call as unreachable. However, since instcombine/simplifycfg
915 turns the undefined call into unreachable, we end up with a branch on a
916 condition that goes to unreachable: a branch to unreachable can never happen, so
917 "-inline -instcombine -simplifycfg" is able to produce:</p>
919 <pre class="doc_code">
920 define fastcc void @foo() {
923 define void @test(i1 %X) {
925 call fastcc void @foo()
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