I have a very simple c++ program
int main()
{
volatile int index = 0;
for (int i = 0; i < 1000000000; i++) {
for(int j = 0; j < 50; j++)
{
index++;
}
}
}
I compiled the code in release and debug mode and checked the assembly codes.
// Release mode assembly
main PROC ; COMDAT
; 9 : volatile int index = 0;
mov DWORD PTR index$[rsp], 0
; 10 : for (int i = 0; i < 1000000000; i++) {
mov edx, 1000000000 ; 3b9aca00H
npad 3
$LL4@main:
; 11 : for(int j = 0; j < 50; j++)
mov eax, 50 ; 00000032H
npad 11
$LL7@main:
; 12 : {
; 13 : index++;
mov ecx, DWORD PTR index$[rsp]
inc ecx
mov DWORD PTR index$[rsp], ecx
sub rax, 1
jne SHORT $LL7@main
; 10 : for (int i = 0; i < 1000000000; i++) {
sub rdx, 1
jne SHORT $LL4@main
; 14 : }
; 15 : }
; 16 : }
ret 0
main ENDP
_TEXT ENDS
END
// Debug mode assembly
main PROC ; COMDAT
; 8 : {
$LN9:
push rbp
push rdi
sub rsp, 328 ; 00000148H
lea rbp, QWORD PTR [rsp+32]
lea rcx, OFFSET FLAT:__38206C9F_Source@cpp
call __CheckForDebuggerJustMyCode
; 9 : volatile int index = 0;
mov DWORD PTR index$[rbp], 0
; 10 : for (int i = 0; i < 1000000000; i++) {
mov DWORD PTR i$1[rbp], 0
jmp SHORT $LN4@main
$LN2@main:
mov eax, DWORD PTR i$1[rbp]
inc eax
mov DWORD PTR i$1[rbp], eax
$LN4@main:
cmp DWORD PTR i$1[rbp], 1000000000 ; 3b9aca00H
jge SHORT $LN3@main
; 11 : for(int j = 0; j < 50; j++)
mov DWORD PTR j$2[rbp], 0
jmp SHORT $LN7@main
$LN5@main:
mov eax, DWORD PTR j$2[rbp]
inc eax
mov DWORD PTR j$2[rbp], eax
$LN7@main:
cmp DWORD PTR j$2[rbp], 50 ; 00000032H
jge SHORT $LN6@main
; 12 : {
; 13 : index++;
mov eax, DWORD PTR index$[rbp]
inc eax
mov DWORD PTR index$[rbp], eax
; 14 : }
jmp SHORT $LN5@main
$LN6@main:
; 15 : }
jmp SHORT $LN2@main
$LN3@main:
; 16 : }
xor eax, eax
lea rsp, QWORD PTR [rbp+296]
pop rdi
pop rbp
ret 0
main ENDP
_TEXT ENDS
END
Based on the assemblies variables i and j are just in registers in release mode while in debug mode they are in memory and any change to them is written back instantly.
So I think release mode should be faster than debug mode but it was actually twice slower. I checked the program outside the visual studio by using stopwatch, release mode took 1 minute to finish while debug mode took only 30 seconds.
I'm really curious why it is slower!
details:
release mode machine code
; Listing generated by Microsoft (R) Optimizing Compiler Version 19.30.30706.0
include listing.inc
INCLUDELIB OLDNAMES
EXTRN __security_check_cookie:PROC
PUBLIC main
; Function compile flags: /Ogtpy
; COMDAT main
_TEXT SEGMENT
index$ = 8
main PROC ; COMDAT
; File D:\Visual Studio Projects\MultiThread_Test\Source.cpp
; Line 9
00000 c7 44 24 08 00
00 00 00 mov DWORD PTR index$[rsp], 0
; Line 10
00008 ba 00 ca 9a 3b mov edx, 1000000000 ; 3b9aca00H
0000d 0f 1f 00 npad 3
$LL4@main:
; Line 11
00010 b8 32 00 00 00 mov eax, 50 ; 00000032H
00015 66 66 66 0f 1f
84 00 00 00 00
00 npad 11
$LL7@main:
; Line 13
00020 8b 4c 24 08 mov ecx, DWORD PTR index$[rsp]
00024 ff c1 inc ecx
00026 89 4c 24 08 mov DWORD PTR index$[rsp], ecx
0002a 48 83 e8 01 sub rax, 1
0002e 75 f0 jne SHORT $LL7@main
; Line 10
00030 48 83 ea 01 sub rdx, 1
00034 75 da jne SHORT $LL4@main
; Line 16
00036 c3 ret 0
main ENDP
_TEXT ENDS
END
Debug mode machine codes
; Listing generated by Microsoft (R) Optimizing Compiler Version 19.30.30706.0
include listing.inc
INCLUDELIB MSVCRTD
INCLUDELIB OLDNAMES
msvcjmc SEGMENT
__38206C9F_Source@cpp DB 01H
msvcjmc ENDS
PUBLIC main
PUBLIC __JustMyCode_Default
EXTRN _RTC_InitBase:PROC
EXTRN _RTC_Shutdown:PROC
EXTRN __CheckForDebuggerJustMyCode:PROC
; COMDAT pdata
pdata SEGMENT
$pdata$main DD imagerel $LN9
DD imagerel $LN9+107
DD imagerel $unwind$main
pdata ENDS
; COMDAT rtc$TMZ
rtc$TMZ SEGMENT
_RTC_Shutdown.rtc$TMZ DQ FLAT:_RTC_Shutdown
rtc$TMZ ENDS
; COMDAT rtc$IMZ
rtc$IMZ SEGMENT
_RTC_InitBase.rtc$IMZ DQ FLAT:_RTC_InitBase
rtc$IMZ ENDS
; COMDAT voltbl
voltbl SEGMENT
_volmd DB 01bH
DB 053H
DB 058H
voltbl ENDS
; COMDAT xdata
xdata SEGMENT
$unwind$main DD 025050f01H
DD 010a230fH
DD 070030029H
DD 05002H
xdata ENDS
; Function compile flags: /Odt
; COMDAT __JustMyCode_Default
_TEXT SEGMENT
__JustMyCode_Default PROC ; COMDAT
00000 c2 00 00 ret 0
__JustMyCode_Default ENDP
_TEXT ENDS
; Function compile flags: /Odtp /RTCsu /ZI
; COMDAT main
_TEXT SEGMENT
index$ = 4
i$1 = 36
j$2 = 68
main PROC ; COMDAT
; File D:\Visual Studio Projects\MultiThread_Test\Source.cpp
; Line 8
$LN9:
00000 40 55 push rbp
00002 57 push rdi
00003 48 81 ec 48 01
00 00 sub rsp, 328 ; 00000148H
0000a 48 8d 6c 24 20 lea rbp, QWORD PTR [rsp+32]
0000f 48 8d 0d 00 00
00 00 lea rcx, OFFSET FLAT:__38206C9F_Source@cpp
00016 e8 00 00 00 00 call __CheckForDebuggerJustMyCode
; Line 9
0001b c7 45 04 00 00
00 00 mov DWORD PTR index$[rbp], 0
; Line 10
00022 c7 45 24 00 00
00 00 mov DWORD PTR i$1[rbp], 0
00029 eb 08 jmp SHORT $LN4@main
$LN2@main:
0002b 8b 45 24 mov eax, DWORD PTR i$1[rbp]
0002e ff c0 inc eax
00030 89 45 24 mov DWORD PTR i$1[rbp], eax
$LN4@main:
00033 81 7d 24 00 ca
9a 3b cmp DWORD PTR i$1[rbp], 1000000000 ; 3b9aca00H
0003a 7d 23 jge SHORT $LN3@main
; Line 11
0003c c7 45 44 00 00
00 00 mov DWORD PTR j$2[rbp], 0
00043 eb 08 jmp SHORT $LN7@main
$LN5@main:
00045 8b 45 44 mov eax, DWORD PTR j$2[rbp]
00048 ff c0 inc eax
0004a 89 45 44 mov DWORD PTR j$2[rbp], eax
$LN7@main:
0004d 83 7d 44 32 cmp DWORD PTR j$2[rbp], 50 ; 00000032H
00051 7d 0a jge SHORT $LN6@main
; Line 13
00053 8b 45 04 mov eax, DWORD PTR index$[rbp]
00056 ff c0 inc eax
00058 89 45 04 mov DWORD PTR index$[rbp], eax
; Line 14
0005b eb e8 jmp SHORT $LN5@main
$LN6@main:
; Line 15
0005d eb cc jmp SHORT $LN2@main
$LN3@main:
; Line 16
0005f 33 c0 xor eax, eax
00061 48 8d a5 28 01
00 00 lea rsp, QWORD PTR [rbp+296]
00068 5f pop rdi
00069 5d pop rbp
0006a c3 ret 0
main ENDP
_TEXT ENDS
END
