(...)
main:
.LFB9320:
.cfi_startproc
movl $100, %eax
ret
.cfi_endproc
(...)
#include <boost/variant.hpp>
struct Foo
{
int get() { return 100; }
};
struct Bar
{
int get() { return 999; }
};
using Variant = boost::variant<Foo, Bar>;
int run(Variant v)
{
return boost::apply_visitor([](auto& x){return x.get();}, v);
}
int main()
{
Foo f;
return run(f);
}
然而,完整的汇编输出文件包含的内容远比上述摘录多得多,我认为它看起来从未被调用。 有没有办法告诉GCC/clang去除所有这些“噪音”,并只在程序运行时输出实际被调用的内容?
完整的汇编输出文件:
.file "main1.cpp"
.section .rodata.str1.8,"aMS",@progbits,1
.align 8
.LC0:
.string "/opt/boost/include/boost/variant/detail/forced_return.hpp"
.section .rodata.str1.1,"aMS",@progbits,1
.LC1:
.string "false"
.section .text.unlikely._ZN5boost6detail7variant13forced_returnIvEET_v,"axG",@progbits,_ZN5boost6detail7variant13forced_returnIvEET_v,comdat
.LCOLDB2:
.section .text._ZN5boost6detail7variant13forced_returnIvEET_v,"axG",@progbits,_ZN5boost6detail7variant13forced_returnIvEET_v,comdat
.LHOTB2:
.p2align 4,,15
.weak _ZN5boost6detail7variant13forced_returnIvEET_v
.type _ZN5boost6detail7variant13forced_returnIvEET_v, @function
_ZN5boost6detail7variant13forced_returnIvEET_v:
.LFB1197:
.cfi_startproc
subq $8, %rsp
.cfi_def_cfa_offset 16
movl $_ZZN5boost6detail7variant13forced_returnIvEET_vE19__PRETTY_FUNCTION__, %ecx
movl $49, %edx
movl $.LC0, %esi
movl $.LC1, %edi
call __assert_fail
.cfi_endproc
.LFE1197:
.size _ZN5boost6detail7variant13forced_returnIvEET_v, .-_ZN5boost6detail7variant13forced_returnIvEET_v
.section .text.unlikely._ZN5boost6detail7variant13forced_returnIvEET_v,"axG",@progbits,_ZN5boost6detail7variant13forced_returnIvEET_v,comdat
.LCOLDE2:
.section .text._ZN5boost6detail7variant13forced_returnIvEET_v,"axG",@progbits,_ZN5boost6detail7variant13forced_returnIvEET_v,comdat
.LHOTE2:
.section .text.unlikely._ZN5boost6detail7variant13forced_returnIiEET_v,"axG",@progbits,_ZN5boost6detail7variant13forced_returnIiEET_v,comdat
.LCOLDB3:
.section .text._ZN5boost6detail7variant13forced_returnIiEET_v,"axG",@progbits,_ZN5boost6detail7variant13forced_returnIiEET_v,comdat
.LHOTB3:
.p2align 4,,15
.weak _ZN5boost6detail7variant13forced_returnIiEET_v
.type _ZN5boost6detail7variant13forced_returnIiEET_v, @function
_ZN5boost6detail7variant13forced_returnIiEET_v:
.LFB9757:
.cfi_startproc
subq $8, %rsp
.cfi_def_cfa_offset 16
movl $_ZZN5boost6detail7variant13forced_returnIiEET_vE19__PRETTY_FUNCTION__, %ecx
movl $39, %edx
movl $.LC0, %esi
movl $.LC1, %edi
call __assert_fail
.cfi_endproc
.LFE9757:
.size _ZN5boost6detail7variant13forced_returnIiEET_v, .-_ZN5boost6detail7variant13forced_returnIiEET_v
.section .text.unlikely._ZN5boost6detail7variant13forced_returnIiEET_v,"axG",@progbits,_ZN5boost6detail7variant13forced_returnIiEET_v,comdat
.LCOLDE3:
.section .text._ZN5boost6detail7variant13forced_returnIiEET_v,"axG",@progbits,_ZN5boost6detail7variant13forced_returnIiEET_v,comdat
.LHOTE3:
.section .text.unlikely,"ax",@progbits
.LCOLDB4:
.text
.LHOTB4:
.p2align 4,,15
.globl _Z3runN5boost7variantI3FooJ3BarEEE
.type _Z3runN5boost7variantI3FooJ3BarEEE, @function
_Z3runN5boost7variantI3FooJ3BarEEE:
.LFB9310:
.cfi_startproc
subq $8, %rsp
.cfi_def_cfa_offset 16
movl (%rdi), %eax
cltd
xorl %edx, %eax
cmpl $19, %eax
ja .L7
jmp *.L9(,%rax,8)
.section .rodata
.align 8
.align 4
.L9:
.quad .L30
.quad .L10
.quad .L7
.quad .L7
.quad .L7
.quad .L7
.quad .L7
.quad .L7
.quad .L7
.quad .L7
.quad .L7
.quad .L7
.quad .L7
.quad .L7
.quad .L7
.quad .L7
.quad .L7
.quad .L7
.quad .L7
.quad .L7
.text
.p2align 4,,10
.p2align 3
.L7:
call _ZN5boost6detail7variant13forced_returnIiEET_v
.p2align 4,,10
.p2align 3
.L30:
movl $100, %eax
.L8:
addq $8, %rsp
.cfi_remember_state
.cfi_def_cfa_offset 8
ret
.p2align 4,,10
.p2align 3
.L10:
.cfi_restore_state
movl $999, %eax
jmp .L8
.cfi_endproc
.LFE9310:
.size _Z3runN5boost7variantI3FooJ3BarEEE, .-_Z3runN5boost7variantI3FooJ3BarEEE
.section .text.unlikely
.LCOLDE4:
.text
.LHOTE4:
.globl _Z3runN5boost7variantI3FooI3BarEEE
.set _Z3runN5boost7variantI3FooI3BarEEE,_Z3runN5boost7variantI3FooJ3BarEEE
.section .text.unlikely
.LCOLDB5:
.section .text.startup,"ax",@progbits
.LHOTB5:
.p2align 4,,15
.globl main
.type main, @function
main:
.LFB9320:
.cfi_startproc
movl $100, %eax
ret
.cfi_endproc
.LFE9320:
.size main, .-main
.section .text.unlikely
.LCOLDE5:
.section .text.startup
.LHOTE5:
.section .rodata
.align 32
.type _ZZN5boost6detail7variant13forced_returnIvEET_vE19__PRETTY_FUNCTION__, @object
.size _ZZN5boost6detail7variant13forced_returnIvEET_vE19__PRETTY_FUNCTION__, 58
_ZZN5boost6detail7variant13forced_returnIvEET_vE19__PRETTY_FUNCTION__:
.string "T boost::detail::variant::forced_return() [with T = void]"
.align 32
.type _ZZN5boost6detail7variant13forced_returnIiEET_vE19__PRETTY_FUNCTION__, @object
.size _ZZN5boost6detail7variant13forced_returnIiEET_vE19__PRETTY_FUNCTION__, 57
_ZZN5boost6detail7variant13forced_returnIiEET_vE19__PRETTY_FUNCTION__:
.string "T boost::detail::variant::forced_return() [with T = int]"
.ident "GCC: (Ubuntu 5.3.0-3ubuntu1~14.04) 5.3.0 20151204"
.section .note.GNU-stack,"",@progbits
.LCOLDE3:
和.LHOTE3:
几乎是纯粹的噪音。我认为它们不会影响目标文件,甚至不会影响符号表或其他元数据。(而且是的,剥离它们已经是一个解决的问题:godbolt.org背后的脚本在GitHub上是开源的)。我也推荐使用http://gcc.godbolt.org/(使用`-O3 -Wall -Wextra -march=...选项)来查看代码。但要记住,如果你只想查看汇编代码,请省略
main()`并使用编译时常量调用它,这样你就可以只查看处理函数参数的代码。 - Peter Cordesg++ -g -O3 -std=c++14 -c test.cc -o test.o && objdump -dS test.o
- Leandros