[Solved No bug] Relative jump out of range with 'loop' asm

[Savapo]

Hello,

Could you check distance of 127 bytes for 'loop' in asm, please ?

Thank you.

Code: Select all

ProcedureDLL.L ASM_TEST_LOOP()
  
  EnableASM
  
  MOV ECX,100
  
  label:
  
  NOP
  NOP
  NOP
  NOP
  NOP
  NOP
  NOP
  NOP
  NOP
  NOP
  
  NOP                        ; <- WITHOUT WORK FINE FOR 10 BYTES
  
  LOOP l_asm_test_loop_label
  
  MOV EAX,ECX
  
  ProcedureReturn

  DisableASM
  
EndProcedure

Debug ASM_TEST_LOOP() ; 0

[wilbert]

Why would you want to restrict yourself to 127 bytes ?
The loop instruction is very slow on most processors.
It's much faster to use the dec / jnz combination that also doesn't limit you to 127 bytes.

Code: Select all

  DEC ECX
  JNZ LABEL

[luis]

I've changed

LOOP l_asm_test_loop_label

to

LOOP ll_asm_test_loop_label

because I'm using 5.31 and a "l" has been added for "l"ocal.

Anyway, I confirm the problem you have encountered, but it's there only if you compile the code for a debug build.
In the debug build code is added behind the scene (it could interfere with your byte counting) so maybe that's the cause.
I'm just guessing, didn't check.


But if you write your code this way:

Code: Select all

ProcedureDLL.L ASM_TEST_LOOP()
 
  ;EnableASM
 
  !MOV ECX,100
 
  label:
 
  !NOP
  !NOP
  !NOP
  !NOP
  !NOP
  !NOP
  !NOP
  !NOP
  !NOP
  !NOP
 
  !NOP                        ; <- NOW IT WORKS
 
  !LOOP ll_asm_test_loop_label
 
  !MOV EAX,ECX
 
  ProcedureReturn

  ;DisableASM
 
EndProcedure

Debug ASM_TEST_LOOP() ; 0

...you can bypass the problem entirely since PB it's not looking at that as PB code anymore, in fact you can't even put a breakpoint on those lines anymore while with EnableASM you can.


EDIT:

I've checked it:

Release build

Code: Select all

401080 : B9 64 00 00 00                                         mov ecx, 00000064h
401085 : 90                                                     nop 
401086 : 90                                                     nop 
401087 : 90                                                     nop 
401088 : 90                                                     nop 
401089 : 90                                                     nop 
40108A : 90                                                     nop 
40108B : 90                                                     nop 
40108C : 90                                                     nop 
40108D : 90                                                     nop 
40108E : 90                                                     nop 
40108F : E2 F4                                                  loop 00401085h
401091 : 89 C8                                                  mov eax, ecx

Debug build

Code: Select all

4010CB : 68 15 00 00 00                                         push 00000015h
4010D0 : E8 00 01 00 00                                         call 004011D5h
4010D5 : B9 64 00 00 00                                         mov ecx, 00000064h
4010DA : 68 19 00 00 00                                         push 00000019h
4010DF : E8 F1 00 00 00                                         call 004011D5h
4010E4 : 90                                                     nop 
4010E5 : 68 1A 00 00 00                                         push 0000001Ah
4010EA : E8 E6 00 00 00                                         call 004011D5h
4010EF : 90                                                     nop 
4010F0 : 68 1B 00 00 00                                         push 0000001Bh
4010F5 : E8 DB 00 00 00                                         call 004011D5h
4010FA : 90                                                     nop 
4010FB : 68 1C 00 00 00                                         push 0000001Ch
401100 : E8 D0 00 00 00                                         call 004011D5h
401105 : 90                                                     nop 
401106 : 68 1D 00 00 00                                         push 0000001Dh
40110B : E8 C5 00 00 00                                         call 004011D5h
401110 : 90                                                     nop 
401111 : 68 1E 00 00 00                                         push 0000001Eh
401116 : E8 BA 00 00 00                                         call 004011D5h
40111B : 90                                                     nop 
40111C : 68 1F 00 00 00                                         push 0000001Fh
401121 : E8 AF 00 00 00                                         call 004011D5h
401126 : 90                                                     nop 
401127 : 68 20 00 00 00                                         push 00000020h
40112C : E8 A4 00 00 00                                         call 004011D5h
401131 : 90                                                     nop 
401132 : 68 21 00 00 00                                         push 00000021h
401137 : E8 99 00 00 00                                         call 004011D5h
40113C : 90                                                     nop 
40113D : 68 22 00 00 00                                         push 00000022h
401142 : E8 8E 00 00 00                                         call 004011D5h
401147 : 90                                                     nop 
401148 : 68 26 00 00 00                                         push 00000026h
40114D : E8 83 00 00 00                                         call 004011D5h
401152 : E2 86                                                  loop 004010DAh
401154 : 68 28 00 00 00                                         push 00000028h
401159 : E8 77 00 00 00                                         call 004011D5h
40115E : 89 C8                                                  mov eax, ecx

For each of your NOP you have 10 bytes used by the debugger, that's why your byte counting is off and why not using the debugger or bypassing the compiler with the "!" makes all work as expected.

Also you can just wrap your code with DisableDebugger/EnableDebugger which also seems to stop the generation of the extra code.

But it's not a bug.

[Savapo]

Thank you very much for your help and for your explanations.

I did not know that the debugger added more additional lines.