Lazarus

Free Pascal => General => Topic started by: Kays on May 16, 2018, 11:13:10 am

Title: asm: j is jo, but only sometimes
Post by: Kays on May 16, 2018, 11:13:10 am

Hi,

why does my attached program translate an instruction into something totally different? I use FPC 3.0.4.

Code: ASM  [Select][+][-]

1. $ objdump -d --start-address=0x4001d8 --stop-address=0x4001e5 -M intel --insn-width=4 sizeOfDemo
2.  
3. sizeOfDemo:     file format elf64-x86-64
4.  
5.  
6. Disassembly of section .text:
7.  
8. 00000000004001d8 <P$SIZEOFDEMO_$$_SUM$array_of_SMALLINT$$INT64+0x18>:
9.   4001d8:       66 8b 14 4e     mov    dx,WORD PTR [rsi+rcx*2]
10.   4001dc:       48 01 d0        add    rax,rdx
11.   4001df:       70 04           jo     4001e5 <P$SIZEOFDEMO_$$_SUM$array_of_SMALLINT$$INT64+0x25>
12.   4001e1:       e2 f5           loop   4001d8 <P$SIZEOFDEMO_$$_SUM$array_of_SMALLINT$$INT64+0x18>
13.   4001e3:       70 03           jo     4001e8 <P$SIZEOFDEMO_$$_SUM$array_of_SMALLINT$$INT64+0x28>

The second jo-instruction clearly reads in the source code as j @sum_done (unconditional)

Title: Re: asm: j is jo, but only sometimes
Post by: marcov on May 16, 2018, 11:25:37 am

I don't know the "j" mnemonic, but best is to file a bug. Maybe test with the normal opcode ("jmp") first.

Title: Re: asm: j is jo, but only sometimes
Post by: Kays on May 16, 2018, 11:35:48 am

Quote from: marcov on May 16, 2018, 11:25:37 am

I don't know the "j" mnemonic, but best is to file a bug. Maybe test with the normal opcode ("jmp") first.

Bingo.