According to IA32 SDM, rep should be used with ins, outs, movs, lods, and stos,
while repz/repnz should be used with cmps and scas. Both assembler accepts
both:
[EMAIL PROTECTED] rep]$ cat good.s
rep insb
rep outsb
rep movsb
rep lodsb
rep stosb
repz cmpsb
repz scasb
repnz cmpsb
repnz scasb
[EMAIL PROTECTED] rep]$ cat bad.s
repz insb
repz outsb
repz movsb
repz lodsb
repz stosb
repnz insb
repnz outsb
repnz movsb
repnz lodsb
repnz stosb
rep cmpsb
rep scasb
[EMAIL PROTECTED] rep]$ make
./as -o good.o good.s
./as -o bad.o bad.s
./objdump -d good.o
good.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <.text>:
0: f3 6c repz insb (%dx),%es:(%rdi)
2: f3 6e repz outsb %ds:(%rsi),(%dx)
4: f3 a4 repz movsb %ds:(%rsi),%es:(%rdi)
6: f3 ac repz lods %ds:(%rsi),%al
8: f3 aa repz stos %al,%es:(%rdi)
a: f3 a6 repz cmpsb %es:(%rdi),%ds:(%rsi)
c: f3 ae repz scas %es:(%rdi),%al
e: f2 a6 repnz cmpsb %es:(%rdi),%ds:(%rsi)
10: f2 ae repnz scas %es:(%rdi),%al
./objdump -d bad.o
bad.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <.text>:
0: f3 6c repz insb (%dx),%es:(%rdi)
2: f3 6e repz outsb %ds:(%rsi),(%dx)
4: f3 a4 repz movsb %ds:(%rsi),%es:(%rdi)
6: f3 ac repz lods %ds:(%rsi),%al
8: f3 aa repz stos %al,%es:(%rdi)
a: f2 6c repnz insb (%dx),%es:(%rdi)
c: f2 6e repnz outsb %ds:(%rsi),(%dx)
e: f2 a4 repnz movsb %ds:(%rsi),%es:(%rdi)
10: f2 ac repnz lods %ds:(%rsi),%al
12: f2 aa repnz stos %al,%es:(%rdi)
14: f3 a6 repz cmpsb %es:(%rdi),%ds:(%rsi)
16: f3 ae repz scas %es:(%rdi),%al
--
Summary: rep and repz are misused
Product: binutils
Version: unspecified
Status: NEW
Severity: normal
Priority: P2
Component: gas
AssignedTo: unassigned at sources dot redhat dot com
ReportedBy: hjl at lucon dot org
CC: bug-binutils at gnu dot org
GCC build triplet: i686-pc-linux-gnu
GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu
http://sourceware.org/bugzilla/show_bug.cgi?id=2428
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