commit	5a1778057f72b8e0444a7932144a3fa441b641bc	[log] [tgz]
author	Alex Zinenko <zinenko@google.com>	Mon Feb 10 14:12:47 2020 +0100
committer	Alex Zinenko <zinenko@google.com>	Mon Feb 10 15:03:43 2020 +0100
tree	bf191c35edd4be72903319ec59108fba9500d9da
parent	1dc62d0358c89d3e5db970e62723fa8b0b0c56e3 [diff]

[mlir] use unpacked memref descriptors at function boundaries

The existing (default) calling convention for memrefs in standard-to-LLVM
conversion was motivated by interfacing with LLVM IR produced from C sources.
In particular, it passes a pointer to the memref descriptor structure when
calling the function. Therefore, the descriptor is allocated on stack before
the call. This convention leads to several problems. PR44644 indicates a
problem with stack exhaustion when calling functions with memref-typed
arguments in a loop. Allocating outside of the loop may lead to concurrent
access problems in case the loop is parallel. When targeting GPUs, the contents
of the stack-allocated memory for the descriptor (passed by pointer) needs to
be explicitly copied to the device. Using an aggregate type makes it impossible
to attach pointer-specific argument attributes pertaining to alignment and
aliasing in the LLVM dialect.

Change the default calling convention for memrefs in standard-to-LLVM
conversion to transform a memref into a list of arguments, each of primitive
type, that are comprised in the memref descriptor. This avoids stack allocation
for ranked memrefs (and thus stack exhaustion and potential concurrent access
problems) and simplifies the device function invocation on GPUs.

Provide an option in the standard-to-LLVM conversion to generate auxiliary
wrapper function with the same interface as the previous calling convention,
compatible with LLVM IR porduced from C sources. These auxiliary functions
pack the individual values into a descriptor structure or unpack it. They also
handle descriptor stack allocation if necessary, serving as an allocation
scope: the memory reserved by `alloca` will be freed on exiting the auxiliary
function.

The effect of this change on MLIR-generated only LLVM IR is minimal. When
interfacing MLIR-generated LLVM IR with C-generated LLVM IR, the integration
only needs to require auxiliary functions and change the function name to call
the wrapper function instead of the original function.

This also opens the door to forwarding aliasing and alignment information from
memrefs to LLVM IR pointers in the standrd-to-LLVM conversion.

25 files changed

tree: bf191c35edd4be72903319ec59108fba9500d9da

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:

Checkout LLVM (including related subprojects like Clang):
- 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
Configure and build LLVM and Clang:
- cd llvm-project
- mkdir build
- cd build
- cmake -G <generator> [options] ../llvm
  Some 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:
  - -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).
- Run your build tool of choice!
  - The default target (i.e. ninja or make) will build all of LLVM.
  - The check-all target (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, use make -j NNN (NNN is the number of parallel jobs, use e.g. number of CPUs you have.)
- For more information see CMake

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.