Squ*_*all 43 c optimization linker inline
在该文件中file1.c,调用在文件中实现的函数file2.c.当我链接file1.o,并file2.o为可执行文件,如果该功能file2是非常小的,将连接器自动检测功能小,内联呼吁？
Mic*_*urr 68
除了Jame McNellis提到的对链接时间代码生成(LTCG)的支持之外,GCC工具链还支持链接时间优化.从版本4.5开始,GCC支持-flto启用链接时间优化(LTO)的交换机,这是一种完整的程序优化形式,它允许内联函数来自单独的目标文件(以及编译器在编译所有内容时可以进行的任何其他优化)对象文件就像它们来自单个C源文件一样).
这是一个简单的例子:
test.c:
void print_int(int x);
int main(){
print_int(1);
print_int(42);
print_int(-1);
return 0;
}
print_int.c:
#include <stdio.h>
void print_int( int x)
{
printf( "the int is %d\n", x);
}
首先使用GCC4.5.x编译它们 - 来自GCC docs使用的示例-O2,但为了在我的简单测试中获得可见结果,我必须使用-O3:
C:\temp>gcc --version
gcc (GCC) 4.5.2
# compile with preparation for LTO
C:\temp>gcc -c -O3 -flto test.c
C:\temp>gcc -c -O3 -flto print_int.c
# link without LTO
C:\temp>gcc -o test-nolto.exe print_int.o test.o
为了获得LTO的效果,您甚至应该在链接阶段使用优化选项 - 链接器实际上调用编译器来编译编译器在上面的第一步中放入目标文件的中间代码片段.如果您在此阶段也未传递优化选项,则编译器将不会执行您要查找的内联.
# link using LTO
C:\temp>gcc -o test-lto.exe -flto -O3 print_int.o test.o
在没有链接时间优化的情况下反汇编版本.请注意,调用print_int()函数:
C:\temp>gdb test-nolto.exe
GNU gdb (GDB) 7.2
(gdb) start
Temporary breakpoint 1 at 0x401373
Starting program: C:\temp/test-nolto.exe
[New Thread 3324.0xdc0]
Temporary breakpoint 1, 0x00401373 in main ()
(gdb) disassem
Dump of assembler code for function main:
0x00401370 <+0>: push %ebp
0x00401371 <+1>: mov %esp,%ebp
=> 0x00401373 <+3>: and $0xfffffff0,%esp
0x00401376 <+6>: sub $0x10,%esp
0x00401379 <+9>: call 0x4018ca <__main>
0x0040137e <+14>: movl $0x1,(%esp)
0x00401385 <+21>: call 0x401350 <print_int>
0x0040138a <+26>: movl $0x2a,(%esp)
0x00401391 <+33>: call 0x401350 <print_int>
0x00401396 <+38>: movl $0xffffffff,(%esp)
0x0040139d <+45>: call 0x401350 <print_int>
0x004013a2 <+50>: xor %eax,%eax
0x004013a4 <+52>: leave
0x004013a5 <+53>: ret
使用链接时间优化来反汇编版本.请注意,调用printf()是直接进行的:
C:\temp>gdb test-lto.exe
GNU gdb (GDB) 7.2
(gdb) start
Temporary breakpoint 1 at 0x401373
Starting program: C:\temp/test-lto.exe
[New Thread 1768.0x126c]
Temporary breakpoint 1, 0x00401373 in main ()
(gdb) disassem
Dump of assembler code for function main:
0x00401370 <+0>: push %ebp
0x00401371 <+1>: mov %esp,%ebp
=> 0x00401373 <+3>: and $0xfffffff0,%esp
0x00401376 <+6>: sub $0x10,%esp
0x00401379 <+9>: call 0x4018da <__main>
0x0040137e <+14>: movl $0x1,0x4(%esp)
0x00401386 <+22>: movl $0x403064,(%esp)
0x0040138d <+29>: call 0x401acc <printf>
0x00401392 <+34>: movl $0x2a,0x4(%esp)
0x0040139a <+42>: movl $0x403064,(%esp)
0x004013a1 <+49>: call 0x401acc <printf>
0x004013a6 <+54>: movl $0xffffffff,0x4(%esp)
0x004013ae <+62>: movl $0x403064,(%esp)
0x004013b5 <+69>: call 0x401acc <printf>
0x004013ba <+74>: xor %eax,%eax
0x004013bc <+76>: leave
0x004013bd <+77>: ret
End of assembler dump.
这是与MSVC相同的实验(首先是LTCG):
C:\temp>cl -c /GL /Zi /Ox test.c
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 16.00.40219.01 for 80x86
Copyright (C) Microsoft Corporation. All rights reserved.
test.c
C:\temp>cl -c /GL /Zi /Ox print_int.c
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 16.00.40219.01 for 80x86
Copyright (C) Microsoft Corporation. All rights reserved.
print_int.c
C:\temp>link /LTCG test.obj print_int.obj /out:test-ltcg.exe /debug
Microsoft (R) Incremental Linker Version 10.00.40219.01
Copyright (C) Microsoft Corporation. All rights reserved.
Generating code
Finished generating code
C:\temp>"\Program Files (x86)\Debugging Tools for Windows (x86)"\cdb test-ltcg.exe
Microsoft (R) Windows Debugger Version 6.12.0002.633 X86
Copyright (c) Microsoft Corporation. All rights reserved.
CommandLine: test-ltcg.exe
// ...
0:000> u main
*** WARNING: Unable to verify checksum for test-ltcg.exe
test_ltcg!main:
00cd1c20 6a01 push 1
00cd1c22 68d05dcd00 push offset test_ltcg!__decimal_point_length+0x10 (00cd5dd0)
00cd1c27 e8e3f3feff call test_ltcg!printf (00cc100f)
00cd1c2c 6a2a push 2Ah
00cd1c2e 68d05dcd00 push offset test_ltcg!__decimal_point_length+0x10 (00cd5dd0)
00cd1c33 e8d7f3feff call test_ltcg!printf (00cc100f)
00cd1c38 6aff push 0FFFFFFFFh
00cd1c3a 68d05dcd00 push offset test_ltcg!__decimal_point_length+0x10 (00cd5dd0)
00cd1c3f e8cbf3feff call test_ltcg!printf (00cc100f)
00cd1c44 83c418 add esp,18h
00cd1c47 33c0 xor eax,eax
00cd1c49 c3 ret
0:000>
现在没有LTCG.请注意,对于MSVC,您必须编译.c文件,而不是/GL为了防止链接器执行LTCG - 否则链接器会检测到/GL指定的,并且它会强制/LTCG选择(嘿,这就是您第一次想要的那个) with /GL):
C:\temp>cl -c /Zi /Ox test.c
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 16.00.40219.01 for 80x86
Copyright (C) Microsoft Corporation. All rights reserved.
test.c
C:\temp>cl -c /Zi /Ox print_int.c
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 16.00.40219.01 for 80x86
Copyright (C) Microsoft Corporation. All rights reserved.
print_int.c
C:\temp>link test.obj print_int.obj /out:test-noltcg.exe /debug
Microsoft (R) Incremental Linker Version 10.00.40219.01
Copyright (C) Microsoft Corporation. All rights reserved.
C:\temp>"\Program Files (x86)\Debugging Tools for Windows (x86)"\cdb test-noltcg.exe
Microsoft (R) Windows Debugger Version 6.12.0002.633 X86
Copyright (c) Microsoft Corporation. All rights reserved.
CommandLine: test-noltcg.exe
// ...
0:000> u main
test_noltcg!main:
00c41020 6a01 push 1
00c41022 e8e3ffffff call test_noltcg!ILT+5(_print_int) (00c4100a)
00c41027 6a2a push 2Ah
00c41029 e8dcffffff call test_noltcg!ILT+5(_print_int) (00c4100a)
00c4102e 6aff push 0FFFFFFFFh
00c41030 e8d5ffffff call test_noltcg!ILT+5(_print_int) (00c4100a)
00c41035 83c40c add esp,0Ch
00c41038 33c0 xor eax,eax
00c4103a c3 ret
0:000>
微软的链接器在LTCG 中支持的一件事(GCC不支持)(据我所知)是Profile Guided Optimization(PGO).该技术允许Microsoft的链接器根据从以前的程序运行中收集的分析数据进行优化.这允许链接器执行诸如将"热"函数收集到相同的存储器页面上的事情,并且很少将代码序列用于其他存储器页面以减少程序的工作集.
编辑(2011年8月28日):GCC支持配置文件引导优化使用的选项-fprofile-generate和-fprofile-use,但我完全不了解它们.
感谢Konrad Rudolph指出这一点.
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