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VC++ destructors are evil!

fredizzimo · 2000-11-08T20:35:34

Consider the following code: #include <iostream.h> class Scalar { public: int integer; Scalar(int i) {integer=i;} ~Scalar(){} const Scalar operator+(const Scalar& r)const {return Scalar(integer+r.integer);} void operator=(const Scalar& r) {integer=r.integer;} }; int main(int argc, char* argv[]) { Scalar a(5); Scalar b(7); Scalar c(0); c=a+b; cout << c.integer << "\n"; return 0; } Do you see any reason why the + operator won''t go inline, when compiled with /GX, /EHs, or /EHa (use exception handling)? Take a look at the disassembly output. 00401000 sub_401000 proc near ; CODE XREF: start+AF.p 00401000 00401000 var_1C = dword ptr -1Ch 00401000 var_18 = dword ptr -18h 00401000 var_14 = dword ptr -14h 00401000 var_10 = byte ptr -10h 00401000 var_C = dword ptr -0Ch 00401000 var_4 = dword ptr -4 00401000 00401000 push 0FFFFFFFFh ;* 00401002 push offset loc_407CB8 ;* 00401007 mov eax, large fs:0 ;set up for exception handling 0040100D push eax ;* 0040100E mov large fs:0, esp ;* 00401015 sub esp, 10h ;reserve stack for variables 00401018 mov [esp+1Ch+var_18], 5 ;a 00401020 xor eax, eax 00401022 mov [esp+1Ch+var_1C], 7 ;b 0040102A mov [esp+1Ch+var_4], eax ;some sort of object counter? 0040102E mov [esp+1Ch+var_14], eax ;c 00401032 lea eax, [esp+1Ch+var_1C] ;b 00401036 lea ecx, [esp+1Ch+var_10] ;the temporary created by a+b 0040103A push eax ;b 0040103B push ecx ;temp 0040103C lea ecx, [esp+24h+var_18] ;a 00401040 mov byte ptr [esp+24h+var_4], 2 ;two objects to the counter 00401045 call sub_401090 ;operator+ 0040104A mov eax, [eax] ;eax=the value of temp+0=integer 0040104C push offset unk_409040 00401051 push eax 00401052 mov ecx, offset dword_40B9C8 00401057 mov [esp+24h+var_14], eax ;c 0040105B call ??6ostream@@QAEAAV0@H@Z ; ostream::operator<<(int) 00401060 mov ecx, eax 00401062 call ??6ostream@@QAEAAV0@PBD@Z ; ostream::operator<<(char const *) 00401067 mov ecx, [esp+1Ch+var_C] ;* 0040106B xor eax, eax ;end exception handling 0040106D mov large fs:0, ecx ;* 00401074 add esp, 1Ch ;return stack 00401077 retn 00401077 sub_401000 endp As you can see the compiler makes a function call and produces stack undwinding code. This is not so good when making for example an optimized vector class. Now lets remove the destructor and recompile: 00401000 sub_401000 proc near ; CODE XREF: start+AF.p 00401000 push offset unk_409040 00401005 push 0Ch ;5+7=0Ch 00401007 mov ecx, offset dword_40B9C8 0040100C call ??6ostream@@QAEAAV0@H@Z ; ostream::operator<<(int) 00401011 mov ecx, eax 00401013 call ??6ostream@@QAEAAV0@PBD@Z ; ostream::operator<<(char const *) 00401018 xor eax, eax 0040101A retn 0040101A sub_401000 endp Now that is more like the code I want to see. Also take a look at the topic "C++ Operator Overloading, and why it blows chunks", to see the actual differencies in speed. Well, looking at the msdn it states that a function won''t go inline when, "the function returns an unwindable object by value, when compiled with /GX, /EHs, or /EHa." Now what is an unwindable object? Well looking deeper in msdn, you can read that, when an exception is thrown, the destructors for all automatically created objects are called. So an unwindable object must be an object with a destructor. I am not an expert on how the compiler generates exception code, but I can''t see any reasons why an inline function will differ from a normal one. Do anyone else have an idea?

General and Gameplay Programming Programming

Started by fredizzimo November 08, 2000 02:18 PM

9 comments, last by fredizzimo 24 years, 2 months ago

fredizzimo

Author

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November 08, 2000 08:35 PM

quote: How do you make an asm dump of your code?

I am using a program called IDA - The interactive disassembler. Look at http://www.datarescue.com . It is much easier to use than the dumps you can make from vc++, but it is not free

If you want to make an assembly dump from vc++, just chose project settings and listing files.

VC++ destructors are evil!

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VC++ destructors are evil!

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