3 ms·

At the level that LLVM's LTO operates, no information about classes or objects is left, so LLVM itself can't really devirtualize C++ methods in most cases

nwallin 2y ago
You appear to be correct. Clang does not devirtualize in LTO, but GCC does. Personally I consider this very strange. $ cat animal.h cat.cpp main.cpp // animal.h #pragma once class animal { public: virtual ~animal() {} virtual void speak() = 0; }; animal& get_mystery_animal(); // cat.cpp #include "animal.h" #include <cstdio> class cat final : public animal { public: ~cat() override{} void speak() override{ puts("meow"); } }; static cat garfield{}; animal& get_mystery_animal() { return garfield; } // main.cpp #include "animal.h" int main() { animal& a = get_mystery_animal(); a.speak(); } $ make clean && CXX=clang++ make -j && objdump --disassemble=main -C lto_test rm -f *.o lto_test clang++ -c -flto -O3 -g cat.cpp -o cat.o clang++ -c -flto -O3 -g main.cpp -o main.o clang++ -flto -O3 -g cat.o main.o -o lto_test lto_test: file format elf64-x86-64 Disassembly of section .init: Disassembly of section .plt: Disassembly of section .plt.got: Disassembly of section .text: 00000000000011b0 <main>: 11b0: 50 push %rax 11b1: 48 8b 05 58 2e 00 00 mov 0x2e58(%rip),%rax # 4010 <garfield> 11b8: 48 8d 3d 51 2e 00 00 lea 0x2e51(%rip),%rdi # 4010 <garfield> 11bf: ff 50 10 call *0x10(%rax) 11c2: 31 c0 xor %eax,%eax 11c4: 59 pop %rcx 11c5: c3 ret Disassembly of section .fini: $ make clean && CXX=g++ make -j && objdump --disassemble=main -C lto_test|sed -e 's,^, ,' rm -f *.o lto_test g++ -c -flto -O3 -g cat.cpp -o cat.o g++ -c -flto -O3 -g main.cpp -o main.o g++ -flto -O3 -g cat.o main.o -o lto_test lto_test: file format elf64-x86-64 Disassembly of section .init: Disassembly of section .plt: Disassembly of section .plt.got: Disassembly of section .text: 0000000000001090 <main>: 1090: 48 83 ec 08 sub $0x8,%rsp 1094: 48 8d 3d 75 2f 00 00 lea 0x2f75(%rip),%rdi # 4010 <garfield> 109b: e8 50 01 00 00 call 11f0 <cat::speak()> 10a0: 31 c0 xor %eax,%eax 10a2: 48 83 c4 08 add $0x8,%rsp 10a6: c3 ret Disassembly of section .fini:
- ranger_danger 2y ago
  What if you add -fwhole-program-vtables on clang?
- JonChesterfield 2y ago
  I think this is a bug. There's dedicated metadata that's supposed to end up on the indirect call to list the possible targets and when that list of possible targets is this short it should be turning into a switch over concrete targets. Don't have time to dig into the IR now but it might be worth posting to the github llvm issues.

nwallin 2y ago

You appear to be correct. Clang does not devirtualize in LTO, but GCC does. Personally I consider this very strange. $ cat animal.h cat.cpp main.cpp // animal.h #pragma once class animal { public: virtual ~animal() {} virtual void speak() = 0; }; animal& get_mystery_animal(); // cat.cpp #include "animal.h" #include <cstdio> class cat final : public animal { public: ~cat() override{} void speak() override{ puts("meow"); } }; static cat garfield{}; animal& get_mystery_animal() { return garfield; } // main.cpp #include "animal.h" int main() { animal& a = get_mystery_animal(); a.speak(); } $ make clean && CXX=clang++ make -j && objdump --disassemble=main -C lto_test rm -f *.o lto_test clang++ -c -flto -O3 -g cat.cpp -o cat.o clang++ -c -flto -O3 -g main.cpp -o main.o clang++ -flto -O3 -g cat.o main.o -o lto_test lto_test: file format elf64-x86-64 Disassembly of section .init: Disassembly of section .plt: Disassembly of section .plt.got: Disassembly of section .text: 00000000000011b0 <main>: 11b0: 50 push %rax 11b1: 48 8b 05 58 2e 00 00 mov 0x2e58(%rip),%rax # 4010 <garfield> 11b8: 48 8d 3d 51 2e 00 00 lea 0x2e51(%rip),%rdi # 4010 <garfield> 11bf: ff 50 10 call *0x10(%rax) 11c2: 31 c0 xor %eax,%eax 11c4: 59 pop %rcx 11c5: c3 ret Disassembly of section .fini: $ make clean && CXX=g++ make -j && objdump --disassemble=main -C lto_test|sed -e 's,^, ,' rm -f *.o lto_test g++ -c -flto -O3 -g cat.cpp -o cat.o g++ -c -flto -O3 -g main.cpp -o main.o g++ -flto -O3 -g cat.o main.o -o lto_test lto_test: file format elf64-x86-64 Disassembly of section .init: Disassembly of section .plt: Disassembly of section .plt.got: Disassembly of section .text: 0000000000001090 <main>: 1090: 48 83 ec 08 sub $0x8,%rsp 1094: 48 8d 3d 75 2f 00 00 lea 0x2f75(%rip),%rdi # 4010 <garfield> 109b: e8 50 01 00 00 call 11f0 <cat::speak()> 10a0: 31 c0 xor %eax,%eax 10a2: 48 83 c4 08 add $0x8,%rsp 10a6: c3 ret Disassembly of section .fini:

ranger_danger 2y ago
What if you add -fwhole-program-vtables on clang?
JonChesterfield 2y ago
I think this is a bug. There's dedicated metadata that's supposed to end up on the indirect call to list the possible targets and when that list of possible targets is this short it should be turning into a switch over concrete targets. Don't have time to dig into the IR now but it might be worth posting to the github llvm issues.