commit	a075d67222832c234296ffd605f19e33023e6060	[log] [tgz]
author	Wei Wang <apollo.mobility@gmail.com>	Fri Nov 19 13:14:41 2021 -0800
committer	Wei Wang <apollo.mobility@gmail.com>	Fri Nov 19 13:22:07 2021 -0800
tree	ed7cf4103640261287654470c5d24429b35bf395
parent	491efa7f31cbc39fe0aaad261a840cc5311346fb [diff]

[Sema] fix nondeterminism in ASTContext::getDeducedTemplateSpecializationType

`DeducedTemplateSpecializationTypes` is a `llvm::FoldingSet<DeducedTemplateSpecializationType>` [1],
where `FoldingSetNodeID` is based on the values: {`TemplateName`, `QualType`, `IsDeducedAsDependent`},
those values are also used as `DeducedTemplateSpecializationType` constructor arguments.

A `FoldingSetNodeID` created by the static `DeducedTemplateSpecializationType::Profile` may not be equal
to`FoldingSetNodeID` created by a member `DeducedTemplateSpecializationType::Profile` of an instance
created with the same {`TemplateName`, `QualType`, `IsDeducedAsDependent`}, which makes
`DeducedTemplateSpecializationTypes` lookups nondeterministic.

Specifically, while `IsDeducedAsDependent` value is passes to the constructor, `IsDependent()` method on
the created instance may return a different value, because `IsDependent` is not saved as is:
```name=clang/include/clang/AST/Type.h
  DeducedTemplateSpecializationType(TemplateName Template,  QualType DeducedAsType, bool IsDeducedAsDependent)
      : DeducedType(DeducedTemplateSpecialization, DeducedAsType,
                    toTypeDependence(Template.getDependence()) | // <~  also considers `TemplateName` parameter
                        (IsDeducedAsDependent ? TypeDependence::DependentInstantiation : TypeDependence::None)),
```
For example, if an instance A with key `FoldingSetNodeID {A, B, false}` is inserted. Then a key
`FoldingSetNodeID {A, B, true}` is probed:
If it happens to correspond to the same bucket in `FoldingSet` as the first key, and `A.Profile()` returns
`FoldingSetNodeID {A, B, true}`, then it's a hit.
If the bucket for the second key is different from the first key, instance A is not considered at all, and it's
a no hit, even if `A.Profile()` returns  `FoldingSetNodeID {A, B, true}`.

Since `TemplateName`, `QualType` parameter values involve memory pointers, the lookup result depend on allocator,
and may differ from run to run. When this is used as part of modules compilation, it may result in "module out of date"
errors, if imported modules are built on different machines.

This makes `ASTContext::getDeducedTemplateSpecializationType` consider `Template.isDependent()` similar
`DeducedTemplateSpecializationType` constructor.

Tested on a very big codebase, by running modules compilations from directories with varied path length
(seem to affect allocator seed).

1. https://llvm.org/docs/ProgrammersManual.html#llvm-adt-foldingset-h

Patch by Wei Wang and Igor Sugak!

Reviewed By: bruno

Differential Revision: https://reviews.llvm.org/D112481

2 files changed

tree: ed7cf4103640261287654470c5d24429b35bf395

README.md

The LLVM Compiler Infrastructure

This directory and its sub-directories contain source code for LLVM, a toolkit for the construction of highly optimized compilers, optimizers, and run-time 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 convert them 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 work-flow and configuration to get and build the LLVM source:

Checkout LLVM (including related sub-projects 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
- cmake -S llvm -B build -G <generator> [options]
  Some common build system 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 sub-projects you'd like to additionally build. Can include any of: clang, clang-tools-extra, compiler-rt,cross-project-tests, flang, libc, libclc, libcxx, libcxxabi, libunwind, lld, lldb, mlir, openmp, polly, or pstl.
    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 path name 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).
- cmake --build build [-- [options] <target>] or your build system specified above directly.
  - 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 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 the option -j NNN, where NNN is the number of parallel jobs, e.g. the 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.