8c387cbea7
This change introduces three new builtins (which work on both pointers and integers) that can be used instead of common bitwise arithmetic: __builtin_align_up(x, alignment), __builtin_align_down(x, alignment) and __builtin_is_aligned(x, alignment). I originally added these builtins to the CHERI fork of LLVM a few years ago to handle the slightly different C semantics that we use for CHERI [1]. Until recently these builtins (or sequences of other builtins) were required to generate correct code. I have since made changes to the default C semantics so that they are no longer strictly necessary (but using them does generate slightly more efficient code). However, based on our experience using them in various projects over the past few years, I believe that adding these builtins to clang would be useful. These builtins have the following benefit over bit-manipulation and casts via uintptr_t: - The named builtins clearly convey the semantics of the operation. While checking alignment using __builtin_is_aligned(x, 16) versus ((x & 15) == 0) is probably not a huge win in readably, I personally find __builtin_align_up(x, N) a lot easier to read than (x+(N-1))&~(N-1). - They preserve the type of the argument (including const qualifiers). When using casts via uintptr_t, it is easy to cast to the wrong type or strip qualifiers such as const. - If the alignment argument is a constant value, clang can check that it is a power-of-two and within the range of the type. Since the semantics of these builtins is well defined compared to arbitrary bit-manipulation, it is possible to add a UBSAN checker that the run-time value is a valid power-of-two. I intend to add this as a follow-up to this change. - The builtins avoids int-to-pointer casts both in C and LLVM IR. In the future (i.e. once most optimizations handle it), we could use the new llvm.ptrmask intrinsic to avoid the ptrtoint instruction that would normally be generated. - They can be used to round up/down to the next aligned value for both integers and pointers without requiring two separate macros. - In many projects the alignment operations are already wrapped in macros (e.g. roundup2 and rounddown2 in FreeBSD), so by replacing the macro implementation with a builtin call, we get improved diagnostics for many call-sites while only having to change a few lines. - Finally, the builtins also emit assume_aligned metadata when used on pointers. This can improve code generation compared to the uintptr_t casts. [1] In our CHERI compiler we have compilation mode where all pointers are implemented as capabilities (essentially unforgeable 128-bit fat pointers). In our original model, casts from uintptr_t (which is a 128-bit capability) to an integer value returned the "offset" of the capability (i.e. the difference between the virtual address and the base of the allocation). This causes problems for cases such as checking the alignment: for example, the expression `if ((uintptr_t)ptr & 63) == 0` is generally used to check if the pointer is aligned to a multiple of 64 bytes. The problem with offsets is that any pointer to the beginning of an allocation will have an offset of zero, so this check always succeeds in that case (even if the address is not correctly aligned). The same issues also exist when aligning up or down. Using the alignment builtins ensures that the address is used instead of the offset. While I have since changed the default C semantics to return the address instead of the offset when casting, this offset compilation mode can still be used by passing a command-line flag. Reviewers: rsmith, aaron.ballman, theraven, fhahn, lebedev.ri, nlopes, aqjune Reviewed By: aaron.ballman, lebedev.ri Differential Revision: https://reviews.llvm.org/D71499 |
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| clang | ||
| clang-tools-extra | ||
| compiler-rt | ||
| debuginfo-tests | ||
| libc | ||
| libclc | ||
| libcxx | ||
| libcxxabi | ||
| libunwind | ||
| lld | ||
| lldb | ||
| llgo | ||
| llvm | ||
| mlir | ||
| openmp | ||
| parallel-libs | ||
| polly | ||
| pstl | ||
| .arcconfig | ||
| .clang-format | ||
| .clang-tidy | ||
| .git-blame-ignore-revs | ||
| .gitignore | ||
| CONTRIBUTING.md | ||
| README.md | ||
README.md
The LLVM Compiler Infrastructure
This directory and its subdirectories contain source code for LLVM, a toolkit for the construction of highly optimized compilers, optimizers, and runtime environments.
The README briefly describes how to get started with building LLVM. For more information on how to contribute to the LLVM project, please take a look at the Contributing to LLVM guide.
Getting Started with the LLVM System
Taken from https://llvm.org/docs/GettingStarted.html.
Overview
Welcome to the LLVM project!
The LLVM project has multiple components. The core of the project is itself called "LLVM". This contains all of the tools, libraries, and header files needed to process intermediate representations and converts it into object files. Tools include an assembler, disassembler, bitcode analyzer, and bitcode optimizer. It also contains basic regression tests.
C-like languages use the Clang front end. This component compiles C, C++, Objective C, and Objective C++ code into LLVM bitcode -- and from there into object files, using LLVM.
Other components include: the libc++ C++ standard library, the LLD linker, and more.
Getting the Source Code and Building LLVM
The LLVM Getting Started documentation may be out of date. The Clang Getting Started page might have more accurate information.
This is an example workflow and configuration to get and build the LLVM source:
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Checkout LLVM (including related subprojects like Clang):
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git clone https://github.com/llvm/llvm-project.git -
Or, on windows,
git clone --config core.autocrlf=false https://github.com/llvm/llvm-project.git
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Configure and build LLVM and Clang:
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cd llvm-project -
mkdir build -
cd build -
cmake -G <generator> [options] ../llvmSome common generators are:
Ninja--- for generating Ninja build files. Most llvm developers use Ninja.Unix Makefiles--- for generating make-compatible parallel makefiles.Visual Studio--- for generating Visual Studio projects and solutions.Xcode--- for generating Xcode projects.
Some Common options:
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-DLLVM_ENABLE_PROJECTS='...'--- semicolon-separated list of the LLVM subprojects you'd like to additionally build. Can include any of: clang, clang-tools-extra, libcxx, libcxxabi, libunwind, lldb, compiler-rt, lld, polly, or debuginfo-tests.For example, to build LLVM, Clang, libcxx, and libcxxabi, use
-DLLVM_ENABLE_PROJECTS="clang;libcxx;libcxxabi". -
-DCMAKE_INSTALL_PREFIX=directory--- Specify for directory the full pathname of where you want the LLVM tools and libraries to be installed (default/usr/local). -
-DCMAKE_BUILD_TYPE=type--- Valid options for type are Debug, Release, RelWithDebInfo, and MinSizeRel. Default is Debug. -
-DLLVM_ENABLE_ASSERTIONS=On--- Compile with assertion checks enabled (default is Yes for Debug builds, No for all other build types).
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Run your build tool of choice!
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The default target (i.e.
ninjaormake) will build all of LLVM. -
The
check-alltarget (i.e.ninja check-all) will run the regression tests to ensure everything is in working order. -
CMake will generate build targets for each tool and library, and most LLVM sub-projects generate their own
check-<project>target. -
Running a serial build will be slow. To improve speed, try running a parallel build. That's done by default in Ninja; for
make, usemake -j NNN(NNN is the number of parallel jobs, use e.g. number of CPUs you have.)
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For more information see CMake
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Consult the Getting Started with LLVM page for detailed information on configuring and compiling LLVM. You can visit Directory Layout to learn about the layout of the source code tree.