Yonghong Song
4db1878158
In BPF Instruction Selection DAGToDAG transformation phase,
BPF backend had an optimization to turn load from readonly data
section to direct load of the values. This phase is implemented
before libbpf has readonly section support and before alu32
is supported.
This phase however may generate incorrect type when alu32 is
enabled. The following is an example,
-bash-4.4$ cat ~/tmp2/t.c
struct t {
unsigned char a;
unsigned char b;
unsigned char c;
};
extern void foo(void *);
int test() {
struct t v = {
.b = 2,
};
foo(&v);
return 0;
}
The compiler will turn local variable "v" into a readonly section.
During instruction selection phase, the compiler generates two
loads from readonly section, one 2 byte load or 1 byte load, e.g., for 2 loads,
t8: i32,ch = load<(dereferenceable load 2 from `i8* getelementptr inbounds
(%struct.t, %struct.t* @__const.test.v, i64 0, i32 0)`, align 1),
anyext from i16> t3, GlobalAddress:i64<%struct.t* @__const.test.v> 0, undef:i64
t9: ch = store<(store 2 into %ir.v1.sub1), trunc to i16> t3, t8,
FrameIndex:i64<0>, undef:i64
BPF backend changed t8 to i64 = Constant<2> and eventually the generated machine IR:
t10: i64 = MOV_ri TargetConstant:i64<2>
t40: i32 = SLL_ri_32 t10, TargetConstant:i32<8>
t41: i32 = OR_ri_32 t40, TargetConstant:i64<0>
t9: ch = STH32<Mem:(store 2 into %ir.v1.sub1)> t41, TargetFrameIndex:i64<0>,
TargetConstant:i64<0>, t3
Note that t10 in the above is not correct. The type should be i32 and instruction
should be MOV_ri_32. The reason for incorrect insn selection is BPF insn selection
generated an i64 constant instead of an i32 constant as specified in the original
load instruction. Such incorrect insn sequence eventually caused the following
fatal error when a COPY insn tries to copy a 64bit register to a 32bit subregister.
Impossible reg-to-reg copy
UNREACHABLE executed at ../lib/Target/BPF/BPFInstrInfo.cpp:42!
This patch fixed the issue by using the load result type instead of always i64
when doing readonly load optimization.
Differential Revision: https://reviews.llvm.org/D81630
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 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 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 -
mkdir build -
cd build -
cmake -G <generator> [options] ../llvmSome 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, 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 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 . [-- [options] <target>]or your build system specified above directly.-
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 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, whereNNNis 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.