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Hi,

On Tuesday, February 16, 2016 at 3:19:21 AM UTC-6, Wiktor S. wrote:
> >> This was a one-line problem in assembly code:
> >> 
> >>     rep/movsl (%esi),(%edi)
> >> 
> >> Newest as.exe generates wrong binary code on the above line. After
> >> changing to "rep movsl" with space it worked.
> >> 
> >> I don't know if rep/movsl syntax is correct or not, but if it is
> >> not, AS should fail instead of silently generating bad code.
> > 
> > Do you mean that GCC itself is generating [sic] "rep/movsl" or is it
> > some inline or handwritten (external) assembly file?
> 
> It is in handwritten assembly.
> 
> The dissasembled (wrong) instruction is
> 
>     rep add %eax,%esi

I don't know if this is truly a bug or not. I also don't know if the
BinUtils maintainers will fix it. I also don't know what Juan plans
to do (apparently BNU2251BR4.ZIP is already on mirrors, uploaded
yesterday). I would not necessarily rush to delete BNU226B.ZIP just yet.

Yes, I see d_copy.s (circa 1998) from Q1SOURCE.ZIP has this line.
That's obviously not your fault, only the original author's, but
I suspect that BinUtils maintainers will just tell you to change
your code locally.

While I'm not sure exactly, I've never noticed anybody using "rep/..."
in their sources (with a slash). Normally you only see tabs or blanks,
but I guess other variations exist as well. I did see "rep ; ret"
a few years back, but I wasn't 100% comfortable using it (and don't
know if it really helped speed much).

That's my point here, mostly. "rep" (and other prefixes) are sometimes
used in weird ways. I think extra prefixes are usually just ignored.
And sometimes new instructions will use a formerly-invalid combo
since nobody was using it for anything anyways. Not sure if
"as --march=..." will help you here or not.

For instance, I was already vaguely aware that "pause" [SSE2?] was
really "rep nop". And a quick check shows that it didn't stop there.
Apparently "lzcnt" [SSE4.2?] is really "rep bsr".

So, for testing, here's a simple FASM example:

N.B. FASM dude is very savvy, so if his accepts something, I assume
he has good reason. He supports (AFAIK) all modern instructions,
including AVX and probably the others (MOVBE, etc). EDIT: Even
NASM 2.11.08 doesn't complain.

=== blah.asm begins ===
use32
format coff

nop
rep add eax,ebx
rep xor eax,ebx
rep aam 16
nop

rep ret
rep nop
nop

bsr eax,[ebx]
lzcnt eax,[ebx]
nop

ret
=== blah.asm ends ===

And here's what (DJGPP) Objdump 2.25.1 says:

=== objdump.out begins ===
blah.o:     file format coff-go32

Disassembly of section .flat:

00000000 <.flat>:
   0:	90                   	nop
   1:	f3 01 d8             	repz add eax,ebx
   4:	f3 31 d8             	repz xor eax,ebx
   7:	f3 d4 10             	repz aam 0x10
   a:	90                   	nop
   b:	f3 c3                	repz ret 
   d:	f3 90                	pause  
   f:	90                   	nop
  10:	0f bd 03             	bsr    eax,DWORD PTR [ebx]
  13:	f3 0f bd 03          	lzcnt  eax,DWORD PTR [ebx]
  17:	90                   	nop
  18:	c3                   	ret    
=== objdump.out ends ===

Now, of course you can argue that "rep/movsl" isn't anywhere close to
"rep add", so that's not intentional and presumably bad. I don't
disagree, just saying, it's probably hard to catch every possible
error like this.