Nick Desaulniers (Google)
[video], [video (raw)], [slides]
This is a refinement of a presentation on the overall architecture of LLVM, and how to debug certain pieces of it. From this talk, beginners should learn how to think through where a bug in the compilation pipeline may be, how to find it, and how to start debugging it. There are some intermediate tips, too. There’s no in-depth focus on any one particular pass, more so general advice that’s widely applicable to most of the code base. No UML diagrams, but high quality SVG diagrams of the high level view and corresponding source files in the tree for certain concepts. Demos include opt, llc, and test case reduction.
The previous version (to be refined) can be viewed: https://clangbuiltlinux.github.io/CBL-meetup-2020-slides/nick/debugging_llvm.html
$ git clone git@github.com:llvm/llvm-project.git
$ mkdir llvm-project/llvm/build
$ cd $!
$ cmake .. \
-DCMAKE_BUILD_TYPE=Release \
-G Ninja \
-DCMAKE_C_COMPILER=/android1/android-master/prebuilts/clang/host/
-DCMAKE_CXX_COMPILER=/android1/android-master/prebuilts/clang/hos
-DLLVM_ENABLE_LLD=ON \
-DLLVM_ENABLE_PROJECTS="clang;lld;compiler-rt" \
-DLLVM_TARGETS_TO_BUILD="AArch64;ARM;X64" \
-DLLVM_ENABLE_ASSERTIONS=OFF
$ ninja
-DCMAKE_BUILD_TYPE=Release - Release builds run faster than debug.
Debug needed for debug symbols and some debugging features.-G Ninja - Ninja builds faster than GNU Make.-DCMAKE_C_COMPILER= / -DCMAKE_CXX_COMPILER= - Build Clang with
Clang.-DLLVM_ENABLE_LLD=ON - Linking Clang with LLD is way faster than
BFD.-DLLVM_ENABLE_PROJECTS="clang;lld;compiler-rt-DLLVM_TARGETS_TO_BUILD="AArch64;ARM;X86 - Anti-pattern; Clang is
distributed with all non-experimental backends enabled. I turn off
backends I don’t need to speed up builds of Clang. llc --version
shows backends it was configured with.-DLLVM_ENABLE_ASSERTIONS=OFF - LLVM is full of assertions. Turning
off assertions is general faster than a debug build.ninja clang can rebuild just Clang.-E: stop before compiling, after preprocessing, produces .i-S: stop before assembling, produces .s-c: stop before linking, produces .o-v: print commands executed and run-###: print commands executed, don’t run-o: print to stdout rather than output to file$ llvm-readelf --string-dump=.comment vmlinux
String dump of section '.comment':
[ 0] Linker: LLD 11.0.0
[ 64] clang version 11.0.0
$ opt -O2 -print-after-all -S foo.ll
# print-after-all doesn't print a pass if it did no optimization
$ opt -O2 -print-before-all -S foo.ll
#include <llvm/Support/raw_ostream.h>
llvm::outs() << object;
llvm::errs() << object;
object->print(llvm::errs());
object->dump();
// Per-pass debug info
#include <llvm/Support/Debug.h>
#define DEBUG_TYPE "foo"
LLVM_DEBUG(llvm::dbgs() << object);
// $ opt -licm -debug-only=licm foo.ll -S -o .
Recursively pull out function definition and function definitions that it depends on.
$ llvm-extract --func=a -o a.ll quarantine.ll