我正在尝试学习C ++和x86之间的关系，并且一直在查看某些C ++代码的相应汇编指令。引起我注意的是，下一条指令似乎有多个JMP。这不仅仅是在浪费空间和时钟周期吗？
源代码非常简单，只是来自教科书的简单游戏。
// Lost Fortune
// A personalized adventure!
#include <iostream>
#include <string>
using std::cout;
using std::cin;
using std::endl;
using std::string;
int main()
{
const int GOLD_PIECES = 900;
int adventurers, killed, survivors;
string leader;
// Get info.
cout << "Welcome fo Lost Fortune\n\n";
cout << "Please enter the following for your personalized adventure\n";
cout << "Enter a number: ";
cin >> adventurers;
cout << "Enter a number, smaller than the first: ";
cin >> killed;
survivors = adventurers - killed;
cout << "Enter your last name: ";
cin >> leader;
// Tell story.
cout << "\nA brave group of " << adventurers << " set out on a quest ";
cout << "-- in search of the lost treasure of the Ancient Dwarves. ";
cout << "The group was led by that legendary rogue, " << leader << ".\n";
cout << "\n Along the way, a bang of marauding ogres ambushed the party. ";
cout << "All fought bravely under the command of " << leader;
cout << ", and the ogres were defeated, but at a cost. ";
cout << "Of the adventurers, " << killed << " were vanquished, ";
cout << "leaving just " << survivors << " in the group.\n";
cout << "\nThe party was about to give up all hope. ";
cout << "But while laying the deceased to rest, ";
cout << "they stumbled upon the buried fortune. ";
cout << "So the adventurers split " << GOLD_PIECES << " gold pieces. ";
cout << leader << " held on to the extra " << (GOLD_PIECES % survivors);
cout << " pieces to keep things fair of course.\n";
return 0;
}
组装的摘录如下。请注意，有几个JMP转到下一条指令，地址为00002053,00002076等。如果有帮助，这些JMP的操作码为E9 00 00 00 00.
00002035 mov eax, dword [ebp+var_64] ; CODE XREF=_main+148
00002038 lea ecx, dword [eax-0x1f91+0x3c60] ; "Welcome fo Lost Fortune\\n\\n"
0000203e mov edx, esp
00002040 mov dword [edx+4], ecx
00002043 mov ecx, dword [eax-0x1f91+__ZNSt3__14coutE_400c] ; __ZNSt3__14coutE_400c
00002049 mov dword [edx], ecx
0000204b call imp___symbol_stub___ZNSt3__1lsINS_11char_traitsIcEEEERNS_13basic_ostreamIcT_EES6_PKc ; std::__1::basic_ostream<char, std::__1::char_traits<char> >& std::__1::operator<< <std::__1::char_traits<char> >(std::__1::basic_ostream<char, std::__1::char_traits<char> >&, char const*)
00002050 mov dword [ebp+var_74], eax
00002053 jmp loc_2058
loc_2058:
00002058 mov eax, dword [ebp+var_64] ; CODE XREF=_main+211
0000205b lea ecx, dword [eax-0x1f91+0x3c7a] ; "Please enter the following for your personalized adventure\\n"
00002061 mov edx, esp
00002063 mov dword [edx+4], ecx
00002066 mov ecx, dword [eax-0x1f91+__ZNSt3__14coutE_400c] ; __ZNSt3__14coutE_400c
0000206c mov dword [edx], ecx
0000206e call imp___symbol_stub___ZNSt3__1lsINS_11char_traitsIcEEEERNS_13basic_ostreamIcT_EES6_PKc ; std::__1::basic_ostream<char, std::__1::char_traits<char> >& std::__1::operator<< <std::__1::char_traits<char> >(std::__1::basic_ostream<char, std::__1::char_traits<char> >&, char const*)
00002073 mov dword [ebp+var_78], eax
00002076 jmp loc_207b
loc_207b:
0000207b mov eax, dword [ebp+var_64] ; CODE XREF=_main+246
0000207e lea ecx, dword [eax-0x1f91+0x3cb6] ; "Enter a number: "
00002084 mov edx, esp
00002086 mov dword [edx+4], ecx
00002089 mov ecx, dword [eax-0x1f91+__ZNSt3__14coutE_400c] ; __ZNSt3__14coutE_400c
0000208f mov dword [edx], ecx
00002091 call imp___symbol_stub___ZNSt3__1lsINS_11char_traitsIcEEEERNS_13basic_ostreamIcT_EES6_PKc ; std::__1::basic_ostream<char, std::__1::char_traits<char> >& std::__1::operator<< <std::__1::char_traits<char> >(std::__1::basic_ostream<char, std::__1::char_traits<char> >&, char const*)
00002096 mov dword [ebp+var_7C], eax
00002099 jmp loc_209e
loc_209e:
0000209e mov eax, esp ; CODE XREF=_main+281
000020a0 lea ecx, dword [ebp+var_44]
000020a3 mov dword [eax+4], ecx
000020a6 mov ecx, dword [ebp+var_64]
000020a9 mov edx, dword [ecx-0x1f91+__ZNSt3__13cinE_4008] ; __ZNSt3__13cinE_4008
000020af mov dword [eax], edx
000020b1 call imp___symbol_stub___ZNSt3__113basic_istreamIcNS_11char_traitsIcEEErsERi ; std::__1::basic_istream<char, std::__1::char_traits<char> >::operator>>(int&)
000020b6 mov dword [ebp+var_80], eax
000020b9 jmp loc_20be
为什么编译器会这样做？非常感谢任何帮助：）
答案 0 :(得分：5)
在没有优化的情况下构建时，编译器的优先级是生成易于调试的代码，并尽可能地匹配原始源代码，而不是快速运行的代码。
在优化时（例如使用-O3），编译器的工作就是快速编写代码，并且不会像你观察到的那样使用JMP做愚蠢的事情。< / p>
优化的代码可能运行得更快，但调试起来非常困难。因此，如果您打算调试代码以查找错误，那么未经优化的版本几乎总是更可取。
答案 1 :(得分：0)
不需要那些跳转，它是特定于编译器的。 Visual Studio生成的汇编代码片段，作为调试版本。
library(dplyr)
getById <- function(id) {
# create a connection and close it on exit
conn <- DBI::dbConnect(
drv = RMySQL::MySQL(),
dbname = "shinydemo",
host = "shiny-demo.csa7qlmguqrf.us-east-1.rds.amazonaws.com",
username = "guest",
password = "guest"
)
on.exit(DBI::dbDisconnect(conn))
# get a specific row based on ID
conn %>% tbl("City") %>% filter(ID == id) %>% collect()
}
parallel::mclapply(1:10, getById, mc.cores = 12)
调试期间看到的汇编代码：
; Line 19
push OFFSET ??_C@_0BK@OBIFJKE@Welcome?5fo?5Lost?5Fortune?6?6?$AA@
mov eax, DWORD PTR __imp_?cout@std@@3V?$basic_ostream@DU?$char_traits@D@std@@@1@A
push eax
call ??$?6U?$char_traits@D@std@@@std@@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@0@AAV10@PBD@Z ; std::operator<<<std::char_traits<char> >
add esp, 8
; Line 20
push OFFSET ??_C@_0DM@EEOOGOCI@Please?5enter?5the?5following?5for?5y@
mov eax, DWORD PTR __imp_?cout@std@@3V?$basic_ostream@DU?$char_traits@D@std@@@1@A
push eax
call ??$?6U?$char_traits@D@std@@@std@@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@0@AAV10@PBD@Z ; std::operator<<<std::char_traits<char> >
add esp, 8
; Line 22
push OFFSET ??_C@_0BB@ONCLGIPK@Enter?5a?5number?3?5?$AA@
mov eax, DWORD PTR __imp_?cout@std@@3V?$basic_ostream@DU?$char_traits@D@std@@@1@A
push eax
call ??$?6U?$char_traits@D@std@@@std@@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@0@AAV10@PBD@Z ; std::operator<<<std::char_traits<char> >
add esp, 8
; Line 23
答案 2 :(得分：-1)
它们可能是为了帮助您在调试过程中单步执行，因为它们对应于源级别的每个语句。虽然，如果我是gcc，我宁愿插入int 3并稍后担心其他事情。