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335 lines
13 KiB
ReStructuredText
================================
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Frequently Asked Questions (FAQ)
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================================
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.. contents::
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:local:
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License
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=======
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Can I modify LLVM source code and redistribute the modified source?
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-------------------------------------------------------------------
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Yes. The modified source distribution must retain the copyright notice and
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follow the conditions listed in the `Apache License v2.0 with LLVM Exceptions
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<https://github.com/llvm/llvm-project/blob/master/llvm/LICENSE.TXT>`_.
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Can I modify the LLVM source code and redistribute binaries or other tools based on it, without redistributing the source?
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--------------------------------------------------------------------------------------------------------------------------
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Yes. This is why we distribute LLVM under a less restrictive license than GPL,
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as explained in the first question above.
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Source Code
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===========
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In what language is LLVM written?
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---------------------------------
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All of the LLVM tools and libraries are written in C++ with extensive use of
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the STL.
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How portable is the LLVM source code?
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-------------------------------------
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The LLVM source code should be portable to most modern Unix-like operating
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systems. LLVM also has excellent support on Windows systems.
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Most of the code is written in standard C++ with operating system
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services abstracted to a support library. The tools required to build and
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test LLVM have been ported to a plethora of platforms.
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What API do I use to store a value to one of the virtual registers in LLVM IR's SSA representation?
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---------------------------------------------------------------------------------------------------
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In short: you can't. It's actually kind of a silly question once you grok
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what's going on. Basically, in code like:
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.. code-block:: llvm
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%result = add i32 %foo, %bar
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, ``%result`` is just a name given to the ``Value`` of the ``add``
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instruction. In other words, ``%result`` *is* the add instruction. The
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"assignment" doesn't explicitly "store" anything to any "virtual register";
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the "``=``" is more like the mathematical sense of equality.
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Longer explanation: In order to generate a textual representation of the
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IR, some kind of name has to be given to each instruction so that other
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instructions can textually reference it. However, the isomorphic in-memory
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representation that you manipulate from C++ has no such restriction since
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instructions can simply keep pointers to any other ``Value``'s that they
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reference. In fact, the names of dummy numbered temporaries like ``%1`` are
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not explicitly represented in the in-memory representation at all (see
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``Value::getName()``).
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Source Languages
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================
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What source languages are supported?
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------------------------------------
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LLVM currently has full support for C and C++ source languages through
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`Clang <https://clang.llvm.org/>`_. Many other language frontends have
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been written using LLVM, and an incomplete list is available at
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`projects with LLVM <https://llvm.org/ProjectsWithLLVM/>`_.
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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?
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----------------------------------------------------------------------------------------------------------------------------------------
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Your compiler front-end will communicate with LLVM by creating a module in the
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LLVM intermediate representation (IR) format. Assuming you want to write your
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language's compiler in the language itself (rather than C++), there are 3
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major ways to tackle generating LLVM IR from a front-end:
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1. **Call into the LLVM libraries code using your language's FFI (foreign
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function interface).**
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* *for:* best tracks changes to the LLVM IR, .ll syntax, and .bc format
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* *for:* enables running LLVM optimization passes without a emit/parse
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overhead
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* *for:* adapts well to a JIT context
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* *against:* lots of ugly glue code to write
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2. **Emit LLVM assembly from your compiler's native language.**
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* *for:* very straightforward to get started
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* *against:* the .ll parser is slower than the bitcode reader when
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interfacing to the middle end
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* *against:* it may be harder to track changes to the IR
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3. **Emit LLVM bitcode from your compiler's native language.**
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* *for:* can use the more-efficient bitcode reader when interfacing to the
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middle end
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* *against:* you'll have to re-engineer the LLVM IR object model and bitcode
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writer in your language
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* *against:* it may be harder to track changes to the IR
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If you go with the first option, the C bindings in include/llvm-c should help
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a lot, since most languages have strong support for interfacing with C. The
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most common hurdle with calling C from managed code is interfacing with the
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garbage collector. The C interface was designed to require very little memory
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management, and so is straightforward in this regard.
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What support is there for a higher level source language constructs for building a compiler?
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--------------------------------------------------------------------------------------------
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Currently, there isn't much. LLVM supports an intermediate representation
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which is useful for code representation but will not support the high level
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(abstract syntax tree) representation needed by most compilers. There are no
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facilities for lexical nor semantic analysis.
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I don't understand the ``GetElementPtr`` instruction. Help!
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-----------------------------------------------------------
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See `The Often Misunderstood GEP Instruction <GetElementPtr.html>`_.
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Using the C and C++ Front Ends
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==============================
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Can I compile C or C++ code to platform-independent LLVM bitcode?
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-----------------------------------------------------------------
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No. C and C++ are inherently platform-dependent languages. The most obvious
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example of this is the preprocessor. A very common way that C code is made
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portable is by using the preprocessor to include platform-specific code. In
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practice, information about other platforms is lost after preprocessing, so
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the result is inherently dependent on the platform that the preprocessing was
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targeting.
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Another example is ``sizeof``. It's common for ``sizeof(long)`` to vary
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between platforms. In most C front-ends, ``sizeof`` is expanded to a
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constant immediately, thus hard-wiring a platform-specific detail.
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Also, since many platforms define their ABIs in terms of C, and since LLVM is
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lower-level than C, front-ends currently must emit platform-specific IR in
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order to have the result conform to the platform ABI.
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Questions about code generated by the demo page
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===============================================
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What is this ``llvm.global_ctors`` and ``_GLOBAL__I_a...`` stuff that happens when I ``#include <iostream>``?
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-------------------------------------------------------------------------------------------------------------
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If you ``#include`` the ``<iostream>`` header into a C++ translation unit,
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the file will probably use the ``std::cin``/``std::cout``/... global objects.
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However, C++ does not guarantee an order of initialization between static
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objects in different translation units, so if a static ctor/dtor in your .cpp
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file used ``std::cout``, for example, the object would not necessarily be
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automatically initialized before your use.
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To make ``std::cout`` and friends work correctly in these scenarios, the STL
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that we use declares a static object that gets created in every translation
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unit that includes ``<iostream>``. This object has a static constructor
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and destructor that initializes and destroys the global iostream objects
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before they could possibly be used in the file. The code that you see in the
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``.ll`` file corresponds to the constructor and destructor registration code.
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If you would like to make it easier to *understand* the LLVM code generated
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by the compiler in the demo page, consider using ``printf()`` instead of
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``iostream``\s to print values.
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Where did all of my code go??
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-----------------------------
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If you are using the LLVM demo page, you may often wonder what happened to
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all of the code that you typed in. Remember that the demo script is running
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the code through the LLVM optimizers, so if your code doesn't actually do
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anything useful, it might all be deleted.
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To prevent this, make sure that the code is actually needed. For example, if
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you are computing some expression, return the value from the function instead
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of leaving it in a local variable. If you really want to constrain the
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optimizer, you can read from and assign to ``volatile`` global variables.
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What is this "``undef``" thing that shows up in my code?
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--------------------------------------------------------
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``undef`` is the LLVM way of representing a value that is not defined. You
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can get these if you do not initialize a variable before you use it. For
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example, the C function:
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.. code-block:: c
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int X() { int i; return i; }
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Is compiled to "``ret i32 undef``" because "``i``" never has a value specified
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for it.
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Why does instcombine + simplifycfg turn a call to a function with a mismatched calling convention into "unreachable"? Why not make the verifier reject it?
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----------------------------------------------------------------------------------------------------------------------------------------------------------
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This is a common problem run into by authors of front-ends that are using
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custom calling conventions: you need to make sure to set the right calling
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convention on both the function and on each call to the function. For
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example, this code:
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.. code-block:: llvm
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define fastcc void @foo() {
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ret void
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}
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define void @bar() {
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call void @foo()
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ret void
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}
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Is optimized to:
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.. code-block:: llvm
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define fastcc void @foo() {
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ret void
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}
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define void @bar() {
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unreachable
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}
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... with "``opt -instcombine -simplifycfg``". This often bites people because
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"all their code disappears". Setting the calling convention on the caller and
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callee is required for indirect calls to work, so people often ask why not
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make the verifier reject this sort of thing.
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The answer is that this code has undefined behavior, but it is not illegal.
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If we made it illegal, then every transformation that could potentially create
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this would have to ensure that it doesn't, and there is valid code that can
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create this sort of construct (in dead code). The sorts of things that can
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cause this to happen are fairly contrived, but we still need to accept them.
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Here's an example:
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.. code-block:: llvm
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define fastcc void @foo() {
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ret void
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}
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define internal void @bar(void()* %FP, i1 %cond) {
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br i1 %cond, label %T, label %F
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T:
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call void %FP()
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ret void
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F:
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call fastcc void %FP()
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ret void
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}
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define void @test() {
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%X = or i1 false, false
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call void @bar(void()* @foo, i1 %X)
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ret void
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}
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In this example, "test" always passes ``@foo``/``false`` into ``bar``, which
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ensures that it is dynamically called with the right calling conv (thus, the
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code is perfectly well defined). If you run this through the inliner, you
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get this (the explicit "or" is there so that the inliner doesn't dead code
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eliminate a bunch of stuff):
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.. code-block:: llvm
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define fastcc void @foo() {
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ret void
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}
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define void @test() {
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%X = or i1 false, false
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br i1 %X, label %T.i, label %F.i
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T.i:
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call void @foo()
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br label %bar.exit
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F.i:
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call fastcc void @foo()
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br label %bar.exit
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bar.exit:
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ret void
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}
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Here you can see that the inlining pass made an undefined call to ``@foo``
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with the wrong calling convention. We really don't want to make the inliner
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have to know about this sort of thing, so it needs to be valid code. In this
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case, dead code elimination can trivially remove the undefined code. However,
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if ``%X`` was an input argument to ``@test``, the inliner would produce this:
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.. code-block:: llvm
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define fastcc void @foo() {
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ret void
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}
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define void @test(i1 %X) {
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br i1 %X, label %T.i, label %F.i
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T.i:
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call void @foo()
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br label %bar.exit
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F.i:
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call fastcc void @foo()
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br label %bar.exit
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bar.exit:
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ret void
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}
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The interesting thing about this is that ``%X`` *must* be false for the
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code to be well-defined, but no amount of dead code elimination will be able
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to delete the broken call as unreachable. However, since
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``instcombine``/``simplifycfg`` turns the undefined call into unreachable, we
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end up with a branch on a condition that goes to unreachable: a branch to
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unreachable can never happen, so "``-inline -instcombine -simplifycfg``" is
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able to produce:
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.. code-block:: llvm
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define fastcc void @foo() {
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ret void
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}
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define void @test(i1 %X) {
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F.i:
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call fastcc void @foo()
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ret void
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}
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