X-Git-Url: http://demsky.eecs.uci.edu/git/?a=blobdiff_plain;f=docs%2FFAQ.html;h=d94c368d0c7b0f3532010f5a4680bb2e971dd504;hb=ca70533d3daeda66f6a0f19faf6691c20b34d086;hp=ea2f802d1dd17f2bac314d4fd1e7d588777196cd;hpb=a631a87ce1d390c87c542f8cf7d2c224b60221fb;p=oota-llvm.git diff --git a/docs/FAQ.html b/docs/FAQ.html index ea2f802d1dd..d94c368d0c7 100644 --- a/docs/FAQ.html +++ b/docs/FAQ.html @@ -59,6 +59,9 @@
  • Source Languages
    1. What source languages are supported?
      2. +
    2. 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?
    3. What support is there for higher level source language constructs for building a compiler?
    4. I don't understand the GetElementPtr @@ -82,7 +85,9 @@ How can I disable all optimizations when compiling code using the LLVM GCC front end?
      5. -
    5. Can I use LLVM to convert C++ code to C code?
      6. +
    6. Can I use LLVM to convert C++ code to C code?
      7. + +
    7. Can I compile C or C++ code to platform-independent LLVM bitcode?
    • @@ -139,7 +144,7 @@ Source Initiative (OSI).

    Yes. The modified source distribution must retain the copyright notice and follow the three bulletted conditions listed in the LLVM license.

    +href="http://llvm.org/svn/llvm-project/llvm/trunk/LICENSE.TXT">LLVM license.

    @@ -231,11 +236,9 @@ it:

  • Run configure with an alternative PATH that is correct. In a Borne compatible shell, the syntax would be:

    -
    -
    +
     % 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 @@ -275,9 +278,9 @@ old version. What do I do?

    can just run the following command in the top level directory of your object tree:

    -
    -
    % ./config.status <relative path to Makefile>
    -
    +
    +% ./config.status <relative path to Makefile>
+

    If the Makefile is new, you will have to modify the configure script to copy it over.

    @@ -312,18 +315,16 @@ clean and then make in the directory that fails to build.

    For example, if you built LLVM with the command:

    -
    -
    % gmake ENABLE_PROFILING=1
    -
    +
    +% gmake ENABLE_PROFILING=1
+

    ...then you must run the tests with the following commands:

    -
    -
    +
     % cd llvm/test
 % gmake ENABLE_PROFILING=1
 
    
-
    • @@ -362,25 +363,21 @@ target".

    If the error is of the form:

    -
    -
    +
     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:

    -
    -
    +
     % cd $LLVM_OBJ_DIR
 % rm -f `find . -name \*\.d` 
 % gmake 
 
    
-

    In other cases, it may be necessary to run make clean before rebuilding.

    @@ -408,11 +405,61 @@ using llvm-gcc instead.

    There is an incomplete version of a Java front end available in the java module. There is no documentation on this yet so you'll need to download the code, compile it, and try it.

    -

    In the stacker module is a compiler and runtime - library for the Stacker language, a "toy" language loosely based on Forth.

    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:

    + +

    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?

    @@ -427,7 +474,7 @@ using llvm-gcc instead.

    of running optimizations, linking, and executable generation.

    -

    +

    I don't understand the GetElementPtr instruction. Help!

    @@ -487,13 +534,11 @@ find libcrtend.a. The only way this can happen is if you haven't installed the runtime library. To correct this, do:

    -
    -
    +
     % cd llvm/runtime
 % make clean ; make install-bytecode
    -

    @@ -512,7 +557,7 @@ code that you desire.

    -Can I use LLVM to convert C++ code to C code? +Can I use LLVM to convert C++ code to C code?

    @@ -521,30 +566,25 @@ 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. -

    +so this may not be what you're looking for. Also, there are several +limitations noted below.

    Use commands like this:

    1. Compile your program as normal with llvm-g++:

      -
      -
      +
       % llvm-g++ x.cpp -o program
 
      
-

      or:

      -
      -
      +
       % llvm-g++ a.cpp -c
 % llvm-g++ b.cpp -c
 % llvm-g++ a.o b.o -o program
 
      
-

      With llvm-gcc3, this will generate program and program.bc. The .bc file is the LLVM version of the program all linked together.

      2. @@ -552,36 +592,68 @@ C++ support for a processor that does not otherwise have a C++ compiler.

    2. Convert the LLVM code to C code, using the LLC tool with the C backend:

      -
      -
      +
       % llc -march=c program.bc -o program.c
-
      
-
      3. +

    3. Finally, compile the C file:

      -
      -
      +
       % cc x.c
-
      
-
      4. +
    -

    Note that, by default, the C backend 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.

    +

    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.

    -

    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 +

    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 can compile the +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.

    +

    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 relatively slow, and +not C++-ABI-conforming on most platforms, but otherwise correct.

    + +

    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.

    + +
    + +
    +

    +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 targetting.

    + +

    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 hardwaring 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.

    +
    @@ -655,11 +727,9 @@ you can read from and assign to volatile global variables. 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:

    -
    -
    +
     int X() { int i; return i; }
 
    
-

    Is compiled to "ret i32 undef" because "i" never has a value specified for it.

    @@ -670,9 +740,9 @@ a value specified for it.

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