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Roman Lebedev b98a0c7f6c
[clang][CodeGen] Implicit Conversion Sanitizer: handle increment/decrement (PR44054)(take 2) 
Summary:
Implicit Conversion Sanitizer is *almost* feature complete.
There aren't *that* much unsanitized things left,
two major ones are increment/decrement (this patch) and bit fields.

As it was discussed in
[[ https://bugs.llvm.org/show_bug.cgi?id=39519 | PR39519 ]],
unlike `CompoundAssignOperator` (which is promoted internally),
or `BinaryOperator` (for which we always have promotion/demotion in AST)
or parts of `UnaryOperator` (we have promotion/demotion but only for
certain operations), for inc/dec, clang omits promotion/demotion
altogether, under as-if rule.

This is technically correct: https://rise4fun.com/Alive/zPgD
As it can be seen in `InstCombineCasts.cpp` `canEvaluateTruncated()`,
`add`/`sub`/`mul`/`and`/`or`/`xor` operators can all arbitrarily
be extended or truncated:
901cd3b3f6/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp (L1320-L1334)

But that has serious implications:
1. Since we no longer model implicit casts, do we pessimise
   their AST representation and everything that uses it?
2. There is no demotion, so lossy demotion sanitizer does not trigger :]

Now, i'm not going to argue about the first problem here,
but the second one **needs** to be addressed. As it was stated
in the report, this is done intentionally, so changing
this in all modes would be considered a penalization/regression.
Which means, the sanitization-less codegen must not be altered.

It was also suggested to not change the sanitized codegen
to the one with demotion, but i quite strongly believe
that will not be the wise choice here:
1. One will need to re-engineer the check that the inc/dec was lossy
   in terms of `@llvm.{u,s}{add,sub}.with.overflow` builtins
2. We will still need to compute the result we would lossily demote.
   (i.e. the result of wide `add`ition/`sub`traction)
3. I suspect it would need to be done right here, in sanitization.
   Which kinda defeats the point of
   using `@llvm.{u,s}{add,sub}.with.overflow` builtins:
   we'd have two `add`s with basically the same arguments,
   one of which is used for check+error-less codepath and other one
   for the error reporting. That seems worse than a single wide op+check.
4. OR, we would need to do that in the compiler-rt handler.
   Which means we'll need a whole new handler.
   But then what about the `CompoundAssignOperator`,
   it would also be applicable for it.
   So this also doesn't really seem like the right path to me.
5. At least X86 (but likely others) pessimizes all sub-`i32` operations
   (due to partial register stalls), so even if we avoid promotion+demotion,
   the computations will //likely// be performed in `i32` anyways.

So i'm not really seeing much benefit of
not doing the straight-forward thing.

While looking into this, i have noticed a few more LLVM middle-end
missed canonicalizations, and filed
[[ https://bugs.llvm.org/show_bug.cgi?id=44100 | PR44100 ]],
[[ https://bugs.llvm.org/show_bug.cgi?id=44102 | PR44102 ]].

Those are not specific to inc/dec, we also have them for
`CompoundAssignOperator`, and it can happen for normal arithmetics, too.
But if we take some other path in the patch, it will not be applicable
here, and we will have most likely played ourselves.

TLDR: front-end should emit canonical, easy-to-optimize yet
un-optimized code. It is middle-end's job to make it optimal.

I'm really hoping reviewers agree with my personal assessment
of the path this patch should take..

This originally landed in 9872ea4ed1
but got immediately reverted in cbfa237892
because the assertion was faulty. That fault ended up being caused
by the enum - while there will be promotion, both types are unsigned,
with same width. So we still don't need to sanitize non-signed cases.
So far. Maybe the assert will tell us this isn't so.

Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=44054 | PR44054 ]].
Refs. https://github.com/google/sanitizers/issues/940

Reviewers: rjmccall, erichkeane, rsmith, vsk

Reviewed By: erichkeane

Subscribers: mehdi_amini, dexonsmith, cfe-commits, #sanitizers, llvm-commits, aaron.ballman, t.p.northover, efriedma, regehr

Tags: #llvm, #clang, #sanitizers

Differential Revision: https://reviews.llvm.org/D70539
2019-11-27 21:52:41 +03:00
clang [clang][CodeGen] Implicit Conversion Sanitizer: handle increment/decrement (PR44054)(take 2) 2019-11-27 21:52:41 +03:00
clang-tools-extra [clangd] Handle the missing call expr in targetDecl. 2019-11-27 16:22:20 +01:00
compiler-rt [clang][CodeGen] Implicit Conversion Sanitizer: handle increment/decrement (PR44054)(take 2) 2019-11-27 21:52:41 +03:00
debuginfo-tests [debuginfo] Update test to account for missing __debug_macinfo 2019-11-11 10:40:47 -08:00
libc [libc] Add a TableGen based header generator. 2019-11-22 13:02:24 -08:00
libclc libclc: Drop the old python based build system 2019-11-08 09:59:40 -05:00
libcxx Optimize and fix basic_string move assignment operator. Reviewed as https://reviews.llvm.org/D68623. Thanks to mvels for the patch. 2019-11-27 07:13:32 -08:00
libcxxabi [libcxxabi] Prevent cmake from removing our explicit system C++ include paths 2019-11-12 10:08:40 -08:00
libunwind [libunwind] Adjust the signal_frame test for Arm 2019-11-19 09:58:46 +00:00
lld [ELF][ARM] Add getPCBias() 2019-11-27 09:09:46 -08:00
lldb [lldb][NFC] Move TypeSystem RTTI to static variable to remove swift reference 2019-11-27 10:28:02 +01:00
llgo IR: Support parsing numeric block ids, and emit them in textual output. 2019-03-22 18:27:13 +00:00
llvm [LegalizeTypes][FPEnv][X86] Add initial support for softening strict fp nodes 2019-11-27 10:50:10 -08:00
openmp [openmp] Fixed nonmonotonic schedule when #threads > #chunks in a loop. 2019-11-27 15:26:51 +03:00
parallel-libs
polly Add missing includes needed to prune LLVMContext.h include, NFC 2019-11-14 15:23:15 -08:00
pstl [pstl] Allow customizing whether per-TU insulation is provided 2019-08-13 12:49:00 +00:00
.arcconfig
.clang-format
.clang-tidy Disable tidy checks with too many hits 2019-02-01 11:20:13 +00:00
.git-blame-ignore-revs Add LLDB reformatting to .git-blame-ignore-revs 2019-09-04 09:31:55 +00:00
.gitignore Add a newline at the end of the file 2019-09-04 06:33:46 +00:00
README.md Add beginning of LLVM's GettingStarted to GitHub readme 2019-10-23 18:03:37 -07:00

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.

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:

  1. 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

  2. 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.