How ARMCC optimizes the C++ codes

Question

I have a question when I compile my code with ARMCC. The following is my code(code just for test).

void COptimizerAppUi::DoRealWorksByRef(COptimizerAppUi& aObject)
{
if( NULL == &aObject )
    {
    RDebug::Printf("This is for testing reference,add:%P", &aObject); 
    aObject.HandleStatusPaneSizeChange();
    }
}

Compiling it for my release version with –asm –interleave, I have the following output. There is only one ASM statement produced. All of the code in the body of the function is missing.

COptimizerAppUi::DoRealWorksByRef(COptimizerAppUi&)
;;;216    
;;;217    void COptimizerAppUi::DoRealWorksByRef(COptimizerAppUi& aObject)
000254  4770              BX       lr
;;;218        {
;;;219        if( NULL == &aObject )
;;;220            {
;;;221            RDebug::Printf("This is for testing reference,add:%P", &aObject); 
;;;222            aObject.HandleStatusPaneSizeChange();
;;;223            }
;;;224        }
;;;225

And then, I compiled it adding –O0, then I get the following output. Here all of the code expected has their own ASM instructions.

                  _ZN15COptimizerAppUi16DoRealWorksByRefERS_ PROC ; 
COptimizerAppUi::DoRealWorksByRef(COptimizerAppUi&)
;;;216    
;;;217    void COptimizerAppUi::DoRealWorksByRef(COptimizerAppUi& aObject)
000328  b570              PUSH     {r4-r6,lr}
;;;218        {
00032a  0005              MOVS     r5,r0
00032c  000c              MOVS     r4,r1
;;;219        if( NULL == &aObject )
00032e  2c00              CMP      r4,#0
000330  d108              BNE      |L1.836|
;;;220            {
;;;221            RDebug::Printf("This is for testing reference,add:%P", &aObject); 
000332  0021              MOVS     r1,r4
000334  a01a              ADR      r0,|L1.928|
000336  f7fffffe          BL       _ZN6RDebug6PrintfEPKcz ; RDebug::Printf(const char*, ...)
;;;222            aObject.HandleStatusPaneSizeChange();
00033a  6820              LDR      r0,[r4,#0]
00033c  3080              ADDS     r0,r0,#0x80
00033e  6a81              LDR      r1,[r0,#0x28]
000340  0020              MOVS     r0,r4
000342  4788              BLX      r1
                  |L1.836|
;;;223            }
;;;224        }
000344  bd70              POP      {r4-r6,pc}
;;;225    
                          ENDP

So according to the compare of the two outputs, I know the body of the COptimizerAppUi::DoRealWorksByRef was removed by the compiler. Yes, I know there are –O0, –O1, –O2, –O3 to controls the behavior of optimizing. But I searched so many materials and didn’t get how the compiler optimizes the code. So do you have any rules about the compiler optimization? Any comments/detailed info are welcome.

Thanks in advance.

By the way, in my environment: C:\armcc

ARM C/C++ Compiler, RVCT4.0 [Build 902]

score 5 · Answer 1 · answered Feb 03 '12 at 07:53

The C++ standard dictates that getting a reference to NULL invokes undefined behavior. So the compiler is leveraging that knowledge to optimize away your if check entirely--that is, it's illegal to have a reference to NULL, so your if statement should never be true in a well-formed program. Or in other words, if you have a reference to NULL, your code is invalid, so the program is free to go haywire.

score 3 · Answer 2 · edited May 23 '17 at 12:03

3

Please read the answer to this question:

Is null reference possible?

References are not pointers. References are not allowed to be NULL. Your condition NULL == &aObject will always evaluate to false. The compiler knows this, so it knows that your function will never do anything. When you turn optimizations on, it uses this knowledge to shorten your function.

edited May 23 '17 at 12:03

Community

1
1

answered Feb 03 '12 at 07:49

David Grayson

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score 0 · Answer 3 · answered Feb 06 '12 at 02:27

Thanks for your answers firstly. Another test I made, code is showing below. The ShowLength function can really output something with optimization enabling. Both G++ and CL output.

#include <iostream>
#include <vector>

using namespace std;

void ShowLength( vector<int>& vec )
{
    if( 0 == &vec )
    {
        cout << "vec::lenght = 0" << endl;
    }
}

int main(int argc, char* arv[] )
{   
    vector<int> *p = 0;
    vector<int> &ref = *p;
    ShowLength( ref );

    return 0;
}

How ARMCC optimizes the C++ codes

3 Answers3