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[ThinLTO] Copy UnnamedAddr when spliting module.

Authored by zequanwu on Apr 14 2021, 10:42 AM.

Description

[ThinLTO] Copy UnnamedAddr when spliting module.

The unnamedaddr property of a function is lost when using
-fwhole-program-vtables and thinlto which causes size increase under linker's
safe icf mode.

The size increase of chrome on Linux when switching from all icf to safe icf
drops from 5 MB to 3 MB after this change, and from 6 MB to 4 MB on Windows.

There is a repro:

# a.h
struct A {
  virtual int f();
  virtual int g();
};

# a.cpp
#include "a.h"
int A::f() { return 10; }
int A::g() { return 10; }

# main.cpp
#include "a.h"

int g(A* a) {
  return a->f();
}

int main(int argv, char** args) {
  A a;
  return g(&a);
}

$ clang++ -O2 -ffunction-sections -flto=thin -fwhole-program-vtables -fsplit-lto-unit -c main.cpp -o main.o  && clang++ -Wl,--icf=safe -fuse-ld=lld  -flto=thin main.o -o a.out && llvm-readobj -t a.out | grep -A 1 -e _ZN1A1fEv -e _ZN1A1gEv
    Name: _ZN1A1fEv (480)
    Value: 0x201830
--
    Name: _ZN1A1gEv (490)
    Value: 0x201840

Differential Revision: https://reviews.llvm.org/D100498

Details