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c++ inline测试

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总结

  • 测试cpo与tag_invoke时, 反汇编发现有些理应inline的函数, 并没有真的inline, 于是打算测试下inline的机制
  • gcc文档: https://gcc.gnu.org/onlinedocs/gcc/Inline.html#Inline
  • 总的来说, inline只是标记, 让编译器可以的话进行内联, 但是具体是否内联, 还是要看编译器决定
  • 我们就没有办法自己定了吗?
  • 一般编译时, 会通过-O0 -O1 -O3 之类的编译选项来控制
  • 通过gcc -Q --help=optimizers -O3命令 我们可以看到不同的编译选项下的开关
  • 当然, 也是可以在编译时强制指定编译选项
  • 也可以用inline void foo (const char) __attribute__((always_inline));格式来强制内联

编译选项

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# gcc -Q --help=optimizers -O3|grep -i inline
-ffold-simple-inlines                 [available in C++, ObjC++]
-finline                              [enabled]
-finline-atomics                      [enabled]
-finline-functions                    [enabled]
-finline-functions-called-once        [enabled]
-finline-small-functions              [enabled]
-flive-patching                       -flive-patching=inline-clone
-flive-patching=[inline-only-static|inline-clone]     [default]

# gcc -Q --help=optimizers -O0|grep -i inline
-ffold-simple-inlines                 [available in C++, ObjC++]
-finline                              [disabled]
-finline-atomics                      [enabled]
-finline-functions                    [disabled]
-finline-functions-called-once        [disabled]
-finline-small-functions              [disabled]
-flive-patching                       -flive-patching=inline-clone
-flive-patching=[inline-only-static|inline-clone]     [default]

测试代码

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#include <cstdio>

#if true
#define print_i printf("%d", i);
#else
#define print_i
#endif
#define BODY() \
  int i = 0;   \
  i += 10;     \
  print_i

void no_inline_func() { BODY() }
inline void inline_func() { BODY() }
class Holder {
  public:
  void no_inline_class_func() { BODY() }
  inline void inline_class_func() { BODY() }
  template <class T>
  void template_no_inline_class_func() {
    BODY()
  }
  template <class T>
  inline void template_inline_class_func() {
    BODY()
  }
};
int main(int argc, char **argv) {
  no_inline_func();
  inline_func();
  Holder h{};
  h.no_inline_class_func();
  h.inline_class_func();
  h.template_no_inline_class_func<int>();
  h.template_inline_class_func<int>();

  return 0;
}

反汇编

-O3选项下, 基本开启了全部内联选项, 和代码裁减

  • 关闭print, BODY()的内容都可以裁掉了, i没有被用到, 属于无用分支
  • 基本被裁减干净了
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0000000000001040 <main>:
    1040:	f3 0f 1e fa          	endbr64 
    1044:	48 83 ec 08          	sub    $0x8,%rsp
    1048:	e8 13 01 00 00       	call   1160 <_Z14no_inline_funcv>
    104d:	31 c0                	xor    %eax,%eax
    104f:	48 83 c4 08          	add    $0x8,%rsp
    1053:	c3                   	ret    
    1054:	66 2e 0f 1f 84 00 00 	cs nopw 0x0(%rax,%rax,1)
    105b:	00 00 00 
    105e:	66 90                	xchg   %ax,%ax
    
0000000000001160 <_Z14no_inline_funcv>:
    1160:	f3 0f 1e fa          	endbr64 
    1164:	c3                   	ret

  • 开启print 也基本都内联了
  • 除了no_inline_func外, 全部原地展开了
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00000000000011e0 <_Z14no_inline_funcv>:
    11e0:	f3 0f 1e fa          	endbr64 
    11e4:	ba 0a 00 00 00       	mov    $0xa,%edx
    11e9:	48 8d 35 14 0e 00 00 	lea    0xe14(%rip),%rsi        # 2004 <_IO_stdin_used+0x4>
    11f0:	bf 01 00 00 00       	mov    $0x1,%edi
    11f5:	31 c0                	xor    %eax,%eax
    11f7:	e9 54 fe ff ff       	jmp    1050 <__printf_chk@plt>
0000000000001060 <main>:
1060:	f3 0f 1e fa          	endbr64
1064:	53                   	push   %rbx
1065:	48 8d 1d 98 0f 00 00 	lea    0xf98(%rip),%rbx        # 2004 <_IO_stdin_used+0x4>
106c:	e8 6f 01 00 00       	call   11e0 <_Z14no_inline_funcv>
1071:	48 89 de             	mov    %rbx,%rsi
1074:	ba 0a 00 00 00       	mov    $0xa,%edx
1079:	31 c0                	xor    %eax,%eax
107b:	bf 01 00 00 00       	mov    $0x1,%edi
1080:	e8 cb ff ff ff       	call   1050 <__printf_chk@plt>
1085:	48 89 de             	mov    %rbx,%rsi
1088:	ba 0a 00 00 00       	mov    $0xa,%edx
108d:	31 c0                	xor    %eax,%eax
108f:	bf 01 00 00 00       	mov    $0x1,%edi
1094:	e8 b7 ff ff ff       	call   1050 <__printf_chk@plt>
1099:	48 89 de             	mov    %rbx,%rsi
109c:	ba 0a 00 00 00       	mov    $0xa,%edx
10a1:	31 c0                	xor    %eax,%eax
10a3:	bf 01 00 00 00       	mov    $0x1,%edi
10a8:	e8 a3 ff ff ff       	call   1050 <__printf_chk@plt>
10ad:	48 89 de             	mov    %rbx,%rsi
10b0:	ba 0a 00 00 00       	mov    $0xa,%edx
10b5:	31 c0                	xor    %eax,%eax
10b7:	bf 01 00 00 00       	mov    $0x1,%edi
10bc:	e8 8f ff ff ff       	call   1050 <__printf_chk@plt>
10c1:	48 89 de             	mov    %rbx,%rsi
10c4:	ba 0a 00 00 00       	mov    $0xa,%edx
10c9:	31 c0                	xor    %eax,%eax
10cb:	bf 01 00 00 00       	mov    $0x1,%edi
10d0:	e8 7b ff ff ff       	call   1050 <__printf_chk@plt>
10d5:	31 c0                	xor    %eax,%eax
10d7:	5b                   	pop    %rbx
10d8:	c3                   	ret    
10d9:	0f 1f 80 00 00 00 00 	nopl   0x0(%rax)

-O0选项下

  • 基本关掉了所有内联选项
  • 无论是否是有需要的分支, 全部编译到目标中了
  • 这里对于没有内联的函数, 只保留了一个, 其他的都相似, 省略掉了
  • 开启打印
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000000000000128a <_ZN6Holder17inline_class_funcEv>:
    128a:	f3 0f 1e fa          	endbr64 
    128e:	55                   	push   %rbp
    128f:	48 89 e5             	mov    %rsp,%rbp
    1292:	48 83 ec 20          	sub    $0x20,%rsp
    1296:	48 89 7d e8          	mov    %rdi,-0x18(%rbp)
    129a:	c7 45 fc 00 00 00 00 	movl   $0x0,-0x4(%rbp)
    12a1:	83 45 fc 0a          	addl   $0xa,-0x4(%rbp)
    12a5:	8b 45 fc             	mov    -0x4(%rbp),%eax
    12a8:	89 c6                	mov    %eax,%esi
    12aa:	48 8d 05 53 0d 00 00 	lea    0xd53(%rip),%rax        # 2004 <_IO_stdin_used+0x4>
    12b1:	48 89 c7             	mov    %rax,%rdi
    12b4:	b8 00 00 00 00       	mov    $0x0,%eax
    12b9:	e8 b2 fd ff ff       	call   1070 <printf@plt>
    12be:	90                   	nop
    12bf:	c9                   	leave  
    12c0:	c3                   	ret    
    12c1:	90                   	nop
    
00000000000011a7 <main>:
    11a7:	f3 0f 1e fa          	endbr64 
    11ab:	55                   	push   %rbp
    11ac:	48 89 e5             	mov    %rsp,%rbp
    11af:	48 83 ec 20          	sub    $0x20,%rsp
    11b3:	89 7d ec             	mov    %edi,-0x14(%rbp)
    11b6:	48 89 75 e0          	mov    %rsi,-0x20(%rbp)
    11ba:	64 48 8b 04 25 28 00 	mov    %fs:0x28,%rax
    11c1:	00 00 
    11c3:	48 89 45 f8          	mov    %rax,-0x8(%rbp)
    11c7:	31 c0                	xor    %eax,%eax
    11c9:	e8 a6 ff ff ff       	call   1174 <_Z14no_inline_funcv>
    11ce:	e8 4b 00 00 00       	call   121e <_Z11inline_funcv>
    11d3:	48 8d 45 f7          	lea    -0x9(%rbp),%rax
    11d7:	48 89 c7             	mov    %rax,%rdi
    11da:	e8 73 00 00 00       	call   1252 <_ZN6Holder20no_inline_class_funcEv>
    11df:	48 8d 45 f7          	lea    -0x9(%rbp),%rax
    11e3:	48 89 c7             	mov    %rax,%rdi
    11e6:	e8 9f 00 00 00       	call   128a <_ZN6Holder17inline_class_funcEv>
    11eb:	48 8d 45 f7          	lea    -0x9(%rbp),%rax
    11ef:	48 89 c7             	mov    %rax,%rdi
    11f2:	e8 cb 00 00 00       	call   12c2 <_ZN6Holder29template_no_inline_class_funcIiEEvv>
    11f7:	48 8d 45 f7          	lea    -0x9(%rbp),%rax
    11fb:	48 89 c7             	mov    %rax,%rdi
    11fe:	e8 f7 00 00 00       	call   12fa <_ZN6Holder26template_inline_class_funcIiEEvv>
    1203:	b8 00 00 00 00       	mov    $0x0,%eax
    1208:	48 8b 55 f8          	mov    -0x8(%rbp),%rdx
    120c:	64 48 2b 14 25 28 00 	sub    %fs:0x28,%rdx
    1213:	00 00 
    1215:	74 05                	je     121c <main+0x75>
    1217:	e8 44 fe ff ff       	call   1060 <__stack_chk_fail@plt>
    121c:	c9                   	leave  
    121d:	c3                   	ret
  • 关闭打印
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0000000000001206 <_ZN6Holder17inline_class_funcEv>:
    1206:	f3 0f 1e fa          	endbr64 
    120a:	55                   	push   %rbp
    120b:	48 89 e5             	mov    %rsp,%rbp
    120e:	48 89 7d e8          	mov    %rdi,-0x18(%rbp)
    1212:	c7 45 fc 00 00 00 00 	movl   $0x0,-0x4(%rbp)
    1219:	83 45 fc 0a          	addl   $0xa,-0x4(%rbp)
    121d:	90                   	nop
    121e:	5d                   	pop    %rbp
    121f:	c3                   	ret    

000000000000115f <main>:
    115f:	f3 0f 1e fa          	endbr64 
    1163:	55                   	push   %rbp
    1164:	48 89 e5             	mov    %rsp,%rbp
    1167:	48 83 ec 20          	sub    $0x20,%rsp
    116b:	89 7d ec             	mov    %edi,-0x14(%rbp)
    116e:	48 89 75 e0          	mov    %rsi,-0x20(%rbp)
    1172:	64 48 8b 04 25 28 00 	mov    %fs:0x28,%rax
    1179:	00 00 
    117b:	48 89 45 f8          	mov    %rax,-0x8(%rbp)
    117f:	31 c0                	xor    %eax,%eax
    1181:	e8 c3 ff ff ff       	call   1149 <_Z14no_inline_funcv>
    1186:	e8 4b 00 00 00       	call   11d6 <_Z11inline_funcv>
    118b:	48 8d 45 f7          	lea    -0x9(%rbp),%rax
    118f:	48 89 c7             	mov    %rax,%rdi
    1192:	e8 55 00 00 00       	call   11ec <_ZN6Holder20no_inline_class_funcEv>
    1197:	48 8d 45 f7          	lea    -0x9(%rbp),%rax
    119b:	48 89 c7             	mov    %rax,%rdi
    119e:	e8 63 00 00 00       	call   1206 <_ZN6Holder17inline_class_funcEv>
    11a3:	48 8d 45 f7          	lea    -0x9(%rbp),%rax
    11a7:	48 89 c7             	mov    %rax,%rdi
    11aa:	e8 71 00 00 00       	call   1220 <_ZN6Holder29template_no_inline_class_funcIiEEvv>
    11af:	48 8d 45 f7          	lea    -0x9(%rbp),%rax
    11b3:	48 89 c7             	mov    %rax,%rdi
    11b6:	e8 7f 00 00 00       	call   123a <_ZN6Holder26template_inline_class_funcIiEEvv>
    11bb:	b8 00 00 00 00       	mov    $0x0,%eax
    11c0:	48 8b 55 f8          	mov    -0x8(%rbp),%rdx
    11c4:	64 48 2b 14 25 28 00 	sub    %fs:0x28,%rdx
    11cb:	00 00 
    11cd:	74 05                	je     11d4 <main+0x75>
    11cf:	e8 7c fe ff ff       	call   1050 <__stack_chk_fail@plt>
    11d4:	c9                   	leave  
    11d5:	c3                   	ret