commit d99033e4b4081bc47510c451b618ef93fe2f3c62
author Mingming Liu <mingmingl@google.com> Wed Mar 19 22:10:57 2025 -0700
committer GitHub <noreply@github.com> Wed Mar 19 22:10:57 2025 -0700
tree 895787d8b778f8717ee54ecdbad60f97724a6758
parent d0d33d2b5ba3369d4a5586234181055935965e49

[LTO][WPD] Suppress WPD on a class if the LTO unit doesn't have the prevailing definition of this class (#131721)

Before this patch, whole program devirtualization is suppressed on a
class if any superclass is visible to regular object files, by recording
the class GUID in `VisibleToRegularObjSymbols`.

This patch suppresses whole program devirtualization on a class if the
LTO unit doesn't have the prevailing definition of this class (e.g., the
prevailing definition is in a shared library)

Implementation summaries:
1. In llvm/lib/LTO/LTO.cpp, `IsVisibleToRegularObj` is updated to look
at the global resolution's `IsPrevailing` bit for ThinLTO and
regularLTO.
2. In llvm/tools/llvm-lto2/llvm-lto2.cpp, 
- three command line options are added so `llvm-lto2` can override
`Conf.HasWholeProgramVisibility`, `Conf.ValidateAllVtablesHaveTypeInfos`
and `Conf.AllVtablesHaveTypeInfos`.
    
The test case is reduced from a small C++ program (main.cc, lib.cc/h
pasted below in [1]). To reproduce the program failure without this
patch, compile lib.cc into a shared library, and provide it to a ThinLTO
build of main.cc (commands are pasted in [2]).

[1]

* lib.h

```
#include <cstdio>

class Derived {
 public:
  void dispatch();
  virtual void print();
  virtual void sum();
};

void Derived::dispatch() {
  static_cast<Derived*>(this)->print();
  static_cast<Derived*>(this)->sum();
}

void Derived::sum() {
  printf("Derived::sum\n");
}

__attribute__((noinline)) void* create(int i);
__attribute__((noinline)) void* getPtr(int i);

```

* lib.cc

```
#include "lib.h"
#include <cstdio>
#include <iostream>

class Derived2 : public Derived {
public:
  void print() override {
    printf("DerivedSharedLib\n");
  }
  void sum() override {
    printf("DerivedSharedLib::sum\n");
  }
};


void Derived::print() {
  printf("Derived\n");
}

__attribute__((noinline)) void* create(int i) {
  if (i & 1)
    return new Derived2();
  return new Derived();
}

```

* main.cc

```
cat main.cc
#include "lib.h"

class DerivedN : public Derived {
 public:
};

__attribute__((noinline)) void* getPtr(int x) {
  return new DerivedN();
}


int main() {
  Derived*b = static_cast<Derived*>(create(201));
  b->dispatch();
  delete b;

  Derived* a = static_cast<Derived*>(getPtr(202));
  a->dispatch();
  delete a;
  return 0;
}

```

[2]
```
# compile lib.o in a shared library.
$ ./bin/clang++ -O2 -fPIC -c lib.cc -o lib.o
$ ./bin/clang++ -shared -o libdata.so lib.o

# Provide the shared library in `-ldata`
$ ./bin/clang++ -v -g -ldata --save-temps -fno-discard-value-names -Wl,-mllvm,-print-before=wholeprogramdevirt -Wl,-mllvm,-wholeprogramdevirt-check=trap -Rpass=wholeprogramdevirt -Wl,--lto-whole-program-visibility -Wl,--lto-validate-all-vtables-have-type-infos -mllvm -disable-icp=true -Wl,-mllvm,-disable-icp=false -flto=thin -fwhole-program-vtables  -fno-split-lto-unit -fuse-ld=lld   main.cc  -L . -o main >/tmp/wholeprogramdevirt.ir 2>&1

# Run the program hits a segmentation fault with  `-Wl,-mllvm,-wholeprogramdevirt-check=trap`

$ LD_LIBRARY_PATH=. ./main
DerivedSharedLib
Trace/breakpoint trap (core dumped)

```