From 0ed5cf4fc1f6d7947687114c9d0cbe0d1ba1d883 Mon Sep 17 00:00:00 2001 From: "Michael J. Spencer" Date: Mon, 18 Jun 2012 20:21:38 +0000 Subject: [docs] Port FAQ over to Sphinx. Patch by Mikael Lyngvig! git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158677 91177308-0d34-0410-b5e6-96231b3b80d8 --- docs/FAQ.html | 758 ---------------------------------------------------- docs/FAQ.rst | 464 ++++++++++++++++++++++++++++++++ docs/userguides.rst | 3 +- 3 files changed, 466 insertions(+), 759 deletions(-) delete mode 100644 docs/FAQ.html create mode 100644 docs/FAQ.rst diff --git a/docs/FAQ.html b/docs/FAQ.html deleted file mode 100644 index 60192a99ee..0000000000 --- a/docs/FAQ.html +++ /dev/null @@ -1,758 +0,0 @@ - - - - - LLVM: Frequently Asked Questions - - - - -

- LLVM: Frequently Asked Questions -

- -

License -
1. Does the University of Illinois Open Source License really qualify as an - "open source" license?
2. Can I modify LLVM source code and redistribute the modified source?
3. Can I modify LLVM source code and redistribute binaries or other tools - based on it, without redistributing the source?
Source code -
1. In what language is LLVM written?
2. How portable is the LLVM source code?
Build Problems -
1. When I run configure, it finds the wrong C compiler.
2. The configure script finds the right C compiler, but it uses - the LLVM linker from a previous build. What do I do?
3. When creating a dynamic library, I get a strange GLIBC error.
4. I've updated my source tree from Subversion, and now my build is trying - to use a file/directory that doesn't exist.
5. I've modified a Makefile in my source tree, but my build tree keeps - using the old version. What do I do?
6. I've upgraded to a new version of LLVM, and I get strange build - errors.
7. I've built LLVM and am testing it, but the tests freeze.
8. Why do test results differ when I perform different types of - builds?
9. Compiling LLVM with GCC 3.3.2 fails, what should I do?
10. Compiling LLVM with GCC succeeds, but the resulting tools do not work, - what can be wrong?
11. After Subversion update, rebuilding gives the error "No rule to make - target".
Source Languages -
- -
Using the C and C++ Front Ends -
1. Can I compile C or C++ code to - platform-independent LLVM bitcode?
-
Questions about code generated by the demo page -
-

- -

Written by The LLVM Team

- - - -

- License -

- - -

- -

Does the University of Illinois Open Source License really qualify as an - "open source" license?

- -

Yes, the license - is certified 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.

- -

Can I modify 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.

- -

- - -

- 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:
- -
```
-% 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:

- -

-% ./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:

- -

-% gmake ENABLE_PROFILING=1
-

- -

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

- -

-% 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, 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. - 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:

- -

-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.

- -

- - -

- Source Languages -

- -

What source languages are supported?

- -

LLVM currently has full support for C and C++ source languages. These are - available through both Clang and - DragonEgg.

- -

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:

- -

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
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: you'll have to re-engineer the LLVM IR object model - and asm writer in your language
- against: it may be harder to track changes to the IR
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.

- -

- - -

- 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 iostreams 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:

- -

-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:

- -

-define fastcc void @foo() {
-        ret void
-}
-define void @bar() {
-        call void @foo()
-        ret void
-}
-

- -

Is optimized to:

- -

-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:

- -

-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): -

- -

-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: -

- -

-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:

- -

-define fastcc void @foo() {
-	ret void
-}
-define void @test(i1 %X) {
-F.i:
-	call fastcc void @foo()
-	ret void
-}
-

- -

- - - -

- - LLVM Compiler Infrastructure
- Last modified: $Date$ -

- - - diff --git a/docs/FAQ.rst b/docs/FAQ.rst new file mode 100644 index 0000000000..b0e3ca0456 --- /dev/null +++ b/docs/FAQ.rst @@ -0,0 +1,464 @@ +.. _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 +`_ 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 +`_. + + +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. + + +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:: bash + + % 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:: bash + + % ./config.status ; + +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:: bash + + % gmake ENABLE_PROFILING=1 + +...then you must run the tests with the following commands: + +.. code-block:: bash + + % 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 `_, +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 `_. 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:: bash + + 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:: bash + + % 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 `_ and `DragonEgg +`_. + +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 `_. + + +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 ``? +------------------------------------------------------------------------------------------------------------- +If you ``#include`` the ```` 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 ````. 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 + } diff --git a/docs/userguides.rst b/docs/userguides.rst index eb70028e7b..d59ef7f519 100644 --- a/docs/userguides.rst +++ b/docs/userguides.rst @@ -7,6 +7,7 @@ User Guides :hidden: CommandGuide/index + FAQ \ @@ -45,7 +46,7 @@ User Guides A list of optimizations and analyses implemented in LLVM. - * `Frequently Asked Questions `_ + * :ref:`faq` A list of common questions and problems and their solutions. -- cgit v1.2.3