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Reality-check the FAQ entry for "Can I use LLVM to convert C++ to C?"

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Dan Gohman 2009-01-25 16:04:50 +00:00
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@ -564,9 +564,8 @@ code that you desire.
Note that the generated C code will be very low level (all loops are lowered
to gotos, etc) and not very pretty (comments are stripped, original source
formatting is totally lost, variables are renamed, expressions are regrouped),
so this may not be what you're looking for. However, this is a good way to add
C++ support for a processor that does not otherwise have a C++ compiler.
</p>
so this may not be what you're looking for. Also, there are several
limitations noted below.<p>
<p>Use commands like this:</p>
@ -603,19 +602,30 @@ C++ support for a processor that does not otherwise have a C++ compiler.
</ol>
<p>Using LLVM does not eliminate the need for C++ library support.
If you use the llvm-g++ front-end, the generated code will depend on
g++'s C++ support libraries in the same way that code generated from
g++ would. If you use another C++ front-end, the generated code will
depend on whatever library that front-end would normally require.</p>
<p>If you are working on a platform that does not provide any C++
libraries, you may be able to manually compile libstdc++ to LLVM
bitcode, statically link it into your program, then use the commands above to
convert the whole result into C code. Alternatively, you might compile the
libraries and your application into two different chunks of C code and link
them.</p>
<p>Note that, by default, the C back end does not support exception handling. If
you want/need it for a certain program, you can enable it by passing
"-enable-correct-eh-support" to the llc program. The resultant code will use
setjmp/longjmp to implement exception support that is correct but relatively
slow.</p>
setjmp/longjmp to implement exception support that is relatively slow, and
not C++-ABI-conforming on most platforms, but otherwise correct.</p>
<p>Also note: this specific sequence of commands won't work if you use a
function defined in the C++ runtime library (or any other C++ library). To
access an external C++ library, you must manually compile libstdc++ to LLVM
bitcode, statically link it into your program, then use the commands above to
convert the whole result into C code. Alternatively, you can compile the
libraries and your application into two different chunks of C code and link
them.</p>
<p>Also, there are a number of other limitations of the C backend that
cause it to produce code that does not fully conform to the C++ ABI on
most platforms. Some of the C++ programs in LLVM's test suite are known
to fail when compiled with the C back end because of ABI incompatiblities
with standard C++ libraries.</p>
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