3 ms·

> Template instantiations are duplicated even for types where the instantiations could have identical code. compilers and linkers had identical code folding op

> Template instantiations are duplicated even for types where the instantiations could have identical code. compilers and linkers had identical code folding optimizations for a looooong time (20+ years for msvc, a decade or so for GNU gold (--icf option) and GCC (-fipa-icf))

saagarjha 7y ago
I was going to reply with a similar comment, but I'm unable to actually get this to happen when I tried it now: https://godbolt.org/z/4VkEQc https://godbolt.org/z/4VkEQc. Perhaps the optimization needs more work?
- jcelerier 7y ago
  No, it's just that the assembly output of godbolt happens before these optimization phases. On my machine I get no unsigned instantiation, only the signed one : $ objdump -D a.out | c++filt | grep foo 563: e8 21 01 00 00 callq 689 <bool foo<int>(int)> 56c: e8 18 01 00 00 callq 689 <bool foo<int>(int)> 0000000000000689 <bool foo<int>(int)>: and the main : 0000000000000560 <main>: 560: 53 push %rbx 561: 31 ff xor %edi,%edi 563: e8 21 01 00 00 callq 689 <bool foo<int>(int)> 568: 31 ff xor %edi,%edi 56a: 89 c3 mov %eax,%ebx 56c: e8 18 01 00 00 callq 689 <bool foo<int>(int)> 571: 0f b6 d3 movzbl %bl,%edx 574: 48 8d 3d b1 01 00 00 lea 0x1b1(%rip),%rdi # 72c <_IO_stdin_used+0x4> 57b: 0f b6 f0 movzbl %al,%esi 57e: 31 c0 xor %eax,%eax 580: e8 cb ff ff ff callq 550 <printf@plt> 585: 31 c0 xor %eax,%eax 587: 5b pop %rbx 588: c3 retq 589: 00 00 add %al,(%rax) 58b: 00 00 add %al,(%rax) 58d: 00 00 add %al,(%rax)
cwzwarich 7y ago
> compilers and linkers had identical code folding optimizations for a looooong time (20+ years for msvc, a decade or so for GNU gold (--icf option) and GCC (-fipa-icf)) If you look at the resulting binaries, they don't work as well as you might think.

saagarjha 7y ago

I was going to reply with a similar comment, but I'm unable to actually get this to happen when I tried it now: https://godbolt.org/z/4VkEQc https://godbolt.org/z/4VkEQc. Perhaps the optimization needs more work?

jcelerier 7y ago
No, it's just that the assembly output of godbolt happens before these optimization phases. On my machine I get no unsigned instantiation, only the signed one : $ objdump -D a.out | c++filt | grep foo 563: e8 21 01 00 00 callq 689 <bool foo<int>(int)> 56c: e8 18 01 00 00 callq 689 <bool foo<int>(int)> 0000000000000689 <bool foo<int>(int)>: and the main : 0000000000000560 <main>: 560: 53 push %rbx 561: 31 ff xor %edi,%edi 563: e8 21 01 00 00 callq 689 <bool foo<int>(int)> 568: 31 ff xor %edi,%edi 56a: 89 c3 mov %eax,%ebx 56c: e8 18 01 00 00 callq 689 <bool foo<int>(int)> 571: 0f b6 d3 movzbl %bl,%edx 574: 48 8d 3d b1 01 00 00 lea 0x1b1(%rip),%rdi # 72c <_IO_stdin_used+0x4> 57b: 0f b6 f0 movzbl %al,%esi 57e: 31 c0 xor %eax,%eax 580: e8 cb ff ff ff callq 550 <printf@plt> 585: 31 c0 xor %eax,%eax 587: 5b pop %rbx 588: c3 retq 589: 00 00 add %al,(%rax) 58b: 00 00 add %al,(%rax) 58d: 00 00 add %al,(%rax)

cwzwarich 7y ago

> compilers and linkers had identical code folding optimizations for a looooong time (20+ years for msvc, a decade or so for GNU gold (--icf option) and GCC (-fipa-icf)) If you look at the resulting binaries, they don't work as well as you might think.