[win] Add a flag to control the Control Flow Guard mechanism on Windows (#176276) Windows Control Flow Guard (CFG) has two different "mechanisms" or "patterns": * Dispatch: the caller calls into the CFG function, which both checks the target callee and then calls it. * Check: the caller calls the CFG function which only checks the target callee and then must separately call the callee. LLVM has followed MSVC's pattern for selecting the mechanism based on the target architecture. These defaults in MSVC are based on tests for performance: Dispatch produces a smaller code size, whereas Check is more friendly to branch predictors. It is possible, however, for a given workload, call pattern or target CPU that someone may want to select a different mechanism to use for their code. This change adds a new Clang and CC1 flag to force a CFG mechanism: `-fwin-cfg-mechanism`. This can be set to `automatic` (lets LLVM choose a mechanism), `force-dispatch` or `force-check`. Also adds the support for the equivalent MSVC flag `/d2guardcfgdispatch`. NOTE: Arm64EC only supports the check mechanism. It should be noted that MSVC emits the "dispatch" name for the call checker (for legacy reasons) but uses the check mechanism.
Welcome to the LLVM project!
This repository contains the source code for LLVM, a toolkit for the construction of highly optimized compilers, optimizers, and run-time environments.
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 convert them into object files. Tools include an assembler, disassembler, bitcode analyzer, and bitcode optimizer.
C-like languages use the Clang frontend. 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.
Consult the Getting Started with LLVM page for information on building and running LLVM.
For information on how to contribute to the LLVM project, please take a look at the Contributing to LLVM guide.
Join the LLVM Discourse forums, Discord chat, LLVM Office Hours or Regular sync-ups.
The LLVM project has adopted a code of conduct for participants to all modes of communication within the project.