user957121 user957121 - 1 year ago 56
C Question

Is inline assembly language slower than native C++ code?

I tried to compare the performance of inline assembly language and C++ code, so I wrote a function that add two arrays of size 2000 for 100000 times. Here's the code:

#define TIMES 100000
void calcuC(int *x,int *y,int length)
for(int i = 0; i < TIMES; i++)
for(int j = 0; j < length; j++)
x[j] += y[j];

void calcuAsm(int *x,int *y,int lengthOfArray)
mov edi,TIMES
mov esi,0
mov ecx,lengthOfArray
mov edx,x
push edx
mov eax,DWORD PTR [edx + esi*4]
mov edx,y
mov ebx,DWORD PTR [edx + esi*4]
add eax,ebx
pop edx
mov [edx + esi*4],eax
inc esi
loop label
dec edi
cmp edi,0
jnz start


int main() {
bool errorOccured = false;
int *xC,*xAsm,*yC,*yAsm;
xC = new int[2000];
xAsm = new int[2000];
yC = new int[2000];
yAsm = new int[2000];
for(int i = 0; i < 2000; i++)
xC[i] = 0;
xAsm[i] = 0;
yC[i] = i;
yAsm[i] = i;
time_t start = clock();

// calcuAsm(xAsm,yAsm,2000);
// for(int i = 0; i < 2000; i++)
// {
// if(xC[i] != xAsm[i])
// {
// cout<<"xC["<<i<<"]="<<xC[i]<<" "<<"xAsm["<<i<<"]="<<xAsm[i]<<endl;
// errorOccured = true;
// break;
// }
// }
// if(errorOccured)
// cout<<"Error occurs!"<<endl;
// else
// cout<<"Works fine!"<<endl;

time_t end = clock();

// cout<<"time = "<<(float)(end - start) / CLOCKS_PER_SEC<<"\n";

cout<<"time = "<<end - start<<endl;
return 0;

Then I run the program five times to get the cycles of processor, which could be seen as time. Each time I call one of the function mentioned above only.

And here comes the result.

Function of assembly version:

Debug Release
732 668
733 680
659 672
667 675
684 694
Average: 677

Function of C++ version:

Debug Release
1068 168
999 166
1072 231
1002 166
1114 183
Average: 182

The C++ code in release mode is almost 3.7 times faster than the assembly code. Why?

I guess that the assembly code I wrote is not as effective as those generated by GCC. It's hard for a common programmer like me to wrote code faster than its opponent generated by a compiler.Does that mean I should not trust the performance of assembly language written by my hands, focus on C++ and forget about assembly language?

Answer Source

Yes, most times.

First of all you start from wrong assumption that a low-level language (assembly in this case) will always produce faster code than high-level language (C++ and C in this case). It's not true. Is C code always faster than Java code? No because there is another variable: programmer. The way you write code and knowledge of architecture details greatly influence performance (as you saw in this case).

You can always produce an example where handmade assembly code is better than compiled code but usually it's a fictional example or a single routine not a true program of 500.000+ lines of C++ code). I think compilers will produce better assembly code 95% times (moreover we don't have to forget that an assembler is a compiler too and it may do few optimizations) and sometimes and only some rare times you may need to write assembly code for few, short, highly used, performance critical routines or when you have to access features your favorite high-level language does not expose. Do you want a touch of this complexity? Read this awesome answer here on SO.

Why this?

First of all because compilers can do optimizations that we can't even imagine (see this short list) and they will do them in seconds (when we may need days).

When you code in assembly you have to make well-defined functions with a well-defined call interface. However they can take in account whole-program optimization and inter-procedural optimization such as register allocation, constant propagation, common subexpression elimination, instruction scheduling and other complex, not obvious optimizations (Polytope model, for example). On RISC architecture guys stopped worrying about this many years ago (instruction scheduling, for example, is very hard to tune by hand) and modern CISC CPUs have very long pipelines too.

For some complex microcontrollers even system libraries are written in C instead of assembly because their compilers produce a better (and easy to maintain) final code.

Compilers sometimes can automatically use some MMX/SIMDx instructions by themselves, and if you don't use them you simply can't compare (other answers already reviewed your assembly code very well). Just for loops this is a short list of loop optimizations of what is commonly checked for by a compiler (do you think you could do it by yourself when your schedule has been decided for a C# program?) If you write something in assembly, I think you have to consider at least some simple optimizations. The school-book example for arrays is to unroll the cycle (its size is known at compile time). Do it and run your test again.

These days it's also really uncommon to need to use assembly language for another reason: the plethora of different CPUs. Do you want to support them all? Each has a specific microarchitecture and some specific instruction sets. They have different number of functional units and assembly instructions should be arranged to keep them all busy. If you write in C you may use PGO but in assembly you will then need a great knowledge of that specific architecture (and rethink and redo everything for another architecture). For small tasks the compiler usually does it better, and for complex tasks usually the work isn't repaid (and compiler may do better anyway).

If you sit down and you take a look at your code probably you'll see that you'll gain more to redesign your algorithm than to translate to assembly (read this great post here on SO), there are high-level optimizations (and hints to compiler) you can effectively apply before you need to resort to assembly language.

All this said, even when you can produce a 5~10 times faster assembly code, you should ask your customers if they prefer to pay one week of your time or to buy a 50$ faster CPU. Extreme optimization more often than not (and especially in LOB applications) is simply not required from most of us.