在C ++中强制执行语句顺序 [英] Enforcing statement order in C++
问题描述
假设我有一些语句，我想在
中执行固定顺序。我想使用g ++优化级别2，所以一些
语句可以重新排序。
考虑下面的例子。
using Clock = std :: chrono :: high_resolution_clock;
auto t1 = Clock :: now（）; // Statement 1
foo（）; //语句2
auto t2 = Clock :: now（）; //语句3
auto elapsedTime = t2 - t1;
在此示例中，语句1-3在
中执行很重要，订购。但是，编译器认为语句2是不是
独立于1和3并执行代码如下？
using Clock = std :: chrono :: high_resolution_clock;
foo（）; //语句2
auto t1 = Clock :: now（）; //语句1
auto t2 = Clock :: now（）; //语句3
auto elapsedTime = t2 - t1;
摘要 p>
似乎没有可靠的方法来防止重新排序，但只要链接时/全程序优化未启用，在单独的位置定位被调用的函数编译单元似乎是一个相当好的赌注。 （至少使用GCC，尽管逻辑意味着这可能与其他编译器一样。）这是以函数调用为代价的 - 内联代码是在同一个编译单元中定义的并且可以重新排序。
原始答案：
GCC会对-O2优化下的通话进行重新排序：
#include< chrono>
static int foo（int x）//'static'或不在这里不影响排序。
{
return x * 2;
}
int fred（int x）
{
auto t1 = std :: chrono :: high_resolution_clock :: now（）;
int y = foo（x）;
auto t2 = std :: chrono :: high_resolution_clock :: now（）;
return y;
}
GCC 5.3.0：
$ b b
g ++ -S --std = c + + 11 -O0 fred.cpp ：
_ZL3fooi：
pushq％rbp
movq％rsp，％rbp
movl％ecx，16（％rbp）
movl 16（％rbp） ，％eax
addl％eax，％eax
popq％rbp
ret
_Z4fredi：
pushq％rbp
movq％rsp，％rbp
subq $ 64，％rsp
movl％ecx，16（％rbp）
call _ZNSt6chrono3_V212system_clock3nowEv
movq％rax，-16（％rbp）
movl 16（％rbp） ，％ecx
call _ZL3fooi
movl％eax，-4（％rbp）
call _ZNSt6chrono3_V212system_clock3nowEv
movq％rax，-32（％rbp）
movl -4 （％rbp），％eax
addq $ 64，％rsp
popq％rbp
ret
但：
g ++ -S --std = c ++ 11 -O2 fred.cpp ：
_Z4fredi：
pushq％rbx
subq $ 32，％rsp
movl％ecx，％ebx
call _ZNSt6chrono3_V212system_clock3nowEv
call _ZNSt6chrono3_V212system_clock3nowEv
leal（％rbx，％rbx），％eax
addq $ 32，％rsp
popq％rbx
ret
现在，使用foo（）作为extern函数：
#include< chrono>
int foo（int x）;
int fred（int x）
{
auto t1 = std :: chrono :: high_resolution_clock :: now（）;
int y = foo（x）;
auto t2 = std :: chrono :: high_resolution_clock :: now（）;
return y;
}
g ++ -S --std = c ++ 11 -O2 fred.cpp ：
_Z4fredi：
pushq％rbx $ b b subq $ 32，％rsp
movl％ecx，％ebx
call _ZNSt6chrono3_V212system_clock3nowEv
movl％ebx，％ecx
call _Z3fooi
movl％eax，％ebx
call _ZNSt6chrono3_V212system_clock3nowEv
movl％ebx，％eax
addq $ 32，％rsp
popq％rbx
ret
但是，如果使用-flto（链接时优化）链接：
0000000100401710< main>：
100401710：53 push％rbx
100401711：48 83 ec 20 sub $ 0x20，％rsp
100401715：89 cb mov％ecx，％ebx
100401717：e8 e4 ff ff ff callq 100401700< _ _ main>
10040171c：e8 bf f9 ff ff callq 1004010e0< _ZNSt6chrono3_V212system_clock3nowEv>
100401721：e8 ba f9 ff ff callq 1004010e0< _ZNSt6chrono3_V212system_clock3nowEv>
100401726：8d 04 1b lea（％rbx，％rbx，1），％eax
100401729：48 83 c4 20 add $ 0x20，％rsp
10040172d：5b pop％rbx
10040172e：c3 retq
Suppose I have a number of statements that I want to execute in a fixed order. I want to use g++ with optimization level 2, so some statements could be reordered. What tools does one have to enforce a certain ordering of statements?
Consider the following example.
using Clock = std::chrono::high_resolution_clock;
auto t1 = Clock::now(); // Statement 1
foo(); // Statement 2
auto t2 = Clock::now(); // Statement 3
auto elapsedTime = t2 - t1;
In this example it is important that the statements 1-3 are executed in the given order. However, can't the compiler think statement 2 is independent of 1 and 3 and execute the code as follows?
using Clock=std::chrono::high_resolution_clock;
foo(); // Statement 2
auto t1 = Clock::now(); // Statement 1
auto t2 = Clock::now(); // Statement 3
auto elapsedTime = t2 - t1;
Summary:
There seems to be no guaranteed way to prevent reordering, but as long as link-time/full-program optimisation is not enabled, locating the called function in a separate compilation unit seems a fairly good bet. (At least with GCC, although logic would suggest that this is likely with other compilers too.) This comes at the cost of the function call - inlined code is by definition in the same compilation unit and open to reordering.
Original answer:
GCC reorders the calls under -O2 optimisation:
#include <chrono>
static int foo(int x) // 'static' or not here doesn't affect ordering.
{
return x*2;
}
int fred(int x)
{
auto t1 = std::chrono::high_resolution_clock::now();
int y = foo(x);
auto t2 = std::chrono::high_resolution_clock::now();
return y;
}
GCC 5.3.0:
g++ -S --std=c++11 -O0 fred.cpp :
_ZL3fooi:
pushq %rbp
movq %rsp, %rbp
movl %ecx, 16(%rbp)
movl 16(%rbp), %eax
addl %eax, %eax
popq %rbp
ret
_Z4fredi:
pushq %rbp
movq %rsp, %rbp
subq $64, %rsp
movl %ecx, 16(%rbp)
call _ZNSt6chrono3_V212system_clock3nowEv
movq %rax, -16(%rbp)
movl 16(%rbp), %ecx
call _ZL3fooi
movl %eax, -4(%rbp)
call _ZNSt6chrono3_V212system_clock3nowEv
movq %rax, -32(%rbp)
movl -4(%rbp), %eax
addq $64, %rsp
popq %rbp
ret
But:
g++ -S --std=c++11 -O2 fred.cpp :
_Z4fredi:
pushq %rbx
subq $32, %rsp
movl %ecx, %ebx
call _ZNSt6chrono3_V212system_clock3nowEv
call _ZNSt6chrono3_V212system_clock3nowEv
leal (%rbx,%rbx), %eax
addq $32, %rsp
popq %rbx
ret
Now, with foo() as an extern function:
#include <chrono>
int foo(int x);
int fred(int x)
{
auto t1 = std::chrono::high_resolution_clock::now();
int y = foo(x);
auto t2 = std::chrono::high_resolution_clock::now();
return y;
}
g++ -S --std=c++11 -O2 fred.cpp :
_Z4fredi:
pushq %rbx
subq $32, %rsp
movl %ecx, %ebx
call _ZNSt6chrono3_V212system_clock3nowEv
movl %ebx, %ecx
call _Z3fooi
movl %eax, %ebx
call _ZNSt6chrono3_V212system_clock3nowEv
movl %ebx, %eax
addq $32, %rsp
popq %rbx
ret
BUT, if this is linked with -flto (link-time optimisation):
0000000100401710 <main>:
100401710: 53 push %rbx
100401711: 48 83 ec 20 sub $0x20,%rsp
100401715: 89 cb mov %ecx,%ebx
100401717: e8 e4 ff ff ff callq 100401700 <__main>
10040171c: e8 bf f9 ff ff callq 1004010e0 <_ZNSt6chrono3_V212system_clock3nowEv>
100401721: e8 ba f9 ff ff callq 1004010e0 <_ZNSt6chrono3_V212system_clock3nowEv>
100401726: 8d 04 1b lea (%rbx,%rbx,1),%eax
100401729: 48 83 c4 20 add $0x20,%rsp
10040172d: 5b pop %rbx
10040172e: c3 retq
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