编译器会(或可以吗？)基于模板参数的 switch 语句？
例如，请参见以下函数，其中模板参数 funcStep1 用于选择适当的函数。这个 switch 语句是否会被删除，因为它的参数在编译时是已知的？我试图从汇编代码(下面给出)中学习它，但我还没有阅读汇编的经验，所以这对我来说不是一个真正可行的任务。
这个问题现在对我来说很重要，因为新引入的 if constexpr 提供了一个保证在编译时评估的替代方案。
template<int funcStep1>
double Mdp::expectedCost(int sdx0, int x0, int x1, int adx0, int adx1) const
{
switch (funcStep1)
{
case 1:
case 3:
return expectedCost_exact(sdx0, x0, x1, adx0, adx1);
case 2:
case 4:
return expectedCost_approx(sdx0, x0, x1, adx0, adx1);
default:
throw string("Unknown value (Mdp::expectedCost.cc)");
}
}
// Define all the template types we need
template double Mdp::expectedCost<1>(int, int, int, int, int) const;
template double Mdp::expectedCost<2>(int, int, int, int, int) const;
template double Mdp::expectedCost<3>(int, int, int, int, int) const;
template double Mdp::expectedCost<4>(int, int, int, int, int) const;
当使用 gcc -O2 -ffunction-sections 编译上述函数时，您可以在此处找到“objdump -D”的输出:
1expectedCost.o: file format elf64-x86-64
Disassembly of section .group:
0000000000000000 <.group>:
0: 01 00 add %eax,(%rax)
2: 00 00 add %al,(%rax)
4: 08 00 or %al,(%rax)
6: 00 00 add %al,(%rax)
8: 09 00 or %eax,(%rax)
...
Disassembly of section .group:
0000000000000000 <.group>:
0: 01 00 add %eax,(%rax)
2: 00 00 add %al,(%rax)
4: 0a 00 or (%rax),%al
6: 00 00 add %al,(%rax)
8: 0b 00 or (%rax),%eax
...
Disassembly of section .group:
0000000000000000 <.group>:
0: 01 00 add %eax,(%rax)
2: 00 00 add %al,(%rax)
4: 0c 00 or $0x0,%al
6: 00 00 add %al,(%rax)
8: 0d .byte 0xd
9: 00 00 add %al,(%rax)
...
Disassembly of section .group:
0000000000000000 <.group>:
0: 01 00 add %eax,(%rax)
2: 00 00 add %al,(%rax)
4: 0e (bad)
5: 00 00 add %al,(%rax)
7: 00 0f add %cl,(%rdi)
9: 00 00 add %al,(%rax)
...
Disassembly of section .bss:
0000000000000000 <_ZStL8__ioinit>:
...
Disassembly of section .text._ZNK3Mdp12expectedCostILi1EEEdiiiii:
0000000000000000 <_ZNK3Mdp12expectedCostILi1EEEdiiiii>:
0: e9 00 00 00 00 jmpq 5 <_ZNK3Mdp12expectedCostILi1EEEdiiiii+0x5>
Disassembly of section .text._ZNK3Mdp12expectedCostILi2EEEdiiiii:
0000000000000000 <_ZNK3Mdp12expectedCostILi2EEEdiiiii>:
0: e9 00 00 00 00 jmpq 5 <_ZNK3Mdp12expectedCostILi2EEEdiiiii+0x5>
Disassembly of section .text._ZNK3Mdp12expectedCostILi3EEEdiiiii:
0000000000000000 <_ZNK3Mdp12expectedCostILi3EEEdiiiii>:
0: e9 00 00 00 00 jmpq 5 <_ZNK3Mdp12expectedCostILi3EEEdiiiii+0x5>
Disassembly of section .text._ZNK3Mdp12expectedCostILi4EEEdiiiii:
0000000000000000 <_ZNK3Mdp12expectedCostILi4EEEdiiiii>:
0: e9 00 00 00 00 jmpq 5 <_ZNK3Mdp12expectedCostILi4EEEdiiiii+0x5>
Disassembly of section .text.startup._GLOBAL__sub_I_expectedCost.cc:
0000000000000000 <_GLOBAL__sub_I_expectedCost.cc>:
0: 48 8d 3d 00 00 00 00 lea 0x0(%rip),%rdi # 7 <_GLOBAL__sub_I_expectedCost.cc+0x7>
7: 48 83 ec 08 sub $0x8,%rsp
b: e8 00 00 00 00 callq 10 <_GLOBAL__sub_I_expectedCost.cc+0x10>
10: 48 8b 3d 00 00 00 00 mov 0x0(%rip),%rdi # 17 <_GLOBAL__sub_I_expectedCost.cc+0x17>
17: 48 8d 15 00 00 00 00 lea 0x0(%rip),%rdx # 1e <_GLOBAL__sub_I_expectedCost.cc+0x1e>
1e: 48 8d 35 00 00 00 00 lea 0x0(%rip),%rsi # 25 <_GLOBAL__sub_I_expectedCost.cc+0x25>
25: 48 83 c4 08 add $0x8,%rsp
29: e9 00 00 00 00 jmpq 2e <_GLOBAL__sub_I_expectedCost.cc+0x2e>
Disassembly of section .init_array:
0000000000000000 <.init_array>:
...
Disassembly of section .comment:
0000000000000000 <.comment>:
0: 00 47 43 add %al,0x43(%rdi)
3: 43 3a 20 rex.XB cmp (%r8),%spl
6: 28 55 62 sub %dl,0x62(%rbp)
9: 75 6e jne 79 <_ZStL8__ioinit+0x79>
b: 74 75 je 82 <_ZStL8__ioinit+0x82>
d: 20 37 and %dh,(%rdi)
f: 2e 33 2e xor %cs:(%rsi),%ebp
12: 30 2d 32 37 75 62 xor %ch,0x62753732(%rip) # 6275374a <_ZStL8__ioinit+0x6275374a>
18: 75 6e jne 88 <_ZStL8__ioinit+0x88>
1a: 74 75 je 91 <_ZStL8__ioinit+0x91>
1c: 31 7e 31 xor %edi,0x31(%rsi)
1f: 38 2e cmp %ch,(%rsi)
21: 30 34 29 xor %dh,(%rcx,%rbp,1)
24: 20 37 and %dh,(%rdi)
26: 2e 33 2e xor %cs:(%rsi),%ebp
29: 30 00 xor %al,(%rax)
Disassembly of section .eh_frame:
0000000000000000 <.eh_frame>:
0: 14 00 adc $0x0,%al
2: 00 00 add %al,(%rax)
4: 00 00 add %al,(%rax)
6: 00 00 add %al,(%rax)
8: 01 7a 52 add %edi,0x52(%rdx)
b: 00 01 add %al,(%rcx)
d: 78 10 js 1f <.eh_frame+0x1f>
f: 01 1b add %ebx,(%rbx)
11: 0c 07 or $0x7,%al
13: 08 90 01 00 00 10 or %dl,0x10000001(%rax)
19: 00 00 add %al,(%rax)
1b: 00 1c 00 add %bl,(%rax,%rax,1)
1e: 00 00 add %al,(%rax)
20: 00 00 add %al,(%rax)
22: 00 00 add %al,(%rax)
24: 05 00 00 00 00 add $0x0,%eax
29: 00 00 add %al,(%rax)
2b: 00 10 add %dl,(%rax)
2d: 00 00 add %al,(%rax)
2f: 00 30 add %dh,(%rax)
31: 00 00 add %al,(%rax)
33: 00 00 add %al,(%rax)
35: 00 00 add %al,(%rax)
37: 00 05 00 00 00 00 add %al,0x0(%rip) # 3d <.eh_frame+0x3d>
3d: 00 00 add %al,(%rax)
3f: 00 10 add %dl,(%rax)
41: 00 00 add %al,(%rax)
43: 00 44 00 00 add %al,0x0(%rax,%rax,1)
47: 00 00 add %al,(%rax)
49: 00 00 add %al,(%rax)
4b: 00 05 00 00 00 00 add %al,0x0(%rip) # 51 <.eh_frame+0x51>
51: 00 00 add %al,(%rax)
53: 00 10 add %dl,(%rax)
55: 00 00 add %al,(%rax)
57: 00 58 00 add %bl,0x0(%rax)
5a: 00 00 add %al,(%rax)
5c: 00 00 add %al,(%rax)
5e: 00 00 add %al,(%rax)
60: 05 00 00 00 00 add $0x0,%eax
65: 00 00 add %al,(%rax)
67: 00 14 00 add %dl,(%rax,%rax,1)
6a: 00 00 add %al,(%rax)
6c: 6c insb (%dx),%es:(%rdi)
6d: 00 00 add %al,(%rax)
6f: 00 00 add %al,(%rax)
71: 00 00 add %al,(%rax)
73: 00 2e add %ch,(%rsi)
75: 00 00 add %al,(%rax)
77: 00 00 add %al,(%rax)
79: 4b 0e rex.WXB (bad)
7b: 10 5e 0e adc %bl,0xe(%rsi)
7e: 08 00 or %al,(%rax)
最佳答案
是的，这是优化过的。有一些东西可以使阅读汇编更容易，例如名称分解(例如:_ZNK3Mdp12expectedCostILi1EEEdiiiii 是 double Mdp::expectedCost<1>(int, int, int, int, int) const 的分解形式)、剥离注释和文本(并使用 Intel 语法):
double expectedCost<1>(int, int, int, int, int): # @double expectedCost<1>(int, int, int, int, int)
jmp expectedCost_exact(int, int, int, int, int) # TAILCALL
double expectedCost<2>(int, int, int, int, int): # @double expectedCost<2>(int, int, int, int, int)
jmp expectedCost_approx(int, int, int, int, int) # TAILCALL
double expectedCost<3>(int, int, int, int, int): # @double expectedCost<3>(int, int, int, int, int)
jmp expectedCost_exact(int, int, int, int, int) # TAILCALL
double expectedCost<4>(int, int, int, int, int): # @double expectedCost<4>(int, int, int, int, int)
jmp expectedCost_approx(int, int, int, int, int) # TAILCALL
上述站点为您简化了整个过程。
在这种情况下，我没有提供 expectedCost_approx 的定义所以编译器只是跳转。但无论如何，编译器绝对足够聪明，可以意识到每个模板函数在开关中都有一个常量值。
关于c++ - 打开模板参数 : does gcc remove the switch?，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/55726326/