Re: Immediate arguments optimizer fails

On Thu, 20 Mar 2008 05:21:48 -0700, H. Peter Anvin wrote
> Peter Johnson wrote:
> > On Wed, 19 Mar 2008, H. Peter Anvin wrote:
> >> anonymous coward wrote:
> >>>>>      5 0000000A 68FFFFFFFF        
     push 0xffffFFFF        ;
doesn't fit into sbyte -> dword
> >>>>  Broken.  The sbyte form is
functionally identical.
> >>> Wrong.
> >>>
> >>> The user did request FFFF_FFFF, which is
0000_0000_FFFF_FFFF.
> >>> Which is different from sign-extending FF
to FFFF_FFFF_FFFF_FFFF.
> >>>
> >> Talking about 32-bit mode here; for 64-bit
mode, you're of course correct.
> > 
> > Actually, you can't encode push
0000_0000_FFFF_FFFF in 64-bit mode.  Push 
> > imm32 in 64-bit mode sign-extends the imm32.  So
6AFF and 68FFFFFFFF are 
> > exactly equivalent in 64-bit mode (they both push
FFFF_FFFF_FFFF_FFFF).
> >
> 
> Well, "push 0FFFFFFFFh" should give a warning
in 64-bit mode, since 
> it will push something other than what the user
requested.  It could 
> still be downgraded to a 2-byte sequence (6A FF) since
the behaviour 
> is identical to the equally flawed 5-byte sequence (68
FF FF FF FF).
> 
> However, in 32-bit mode, the two-byte sequence 6A FF
will push 
> FFFFFFFF on the stack, so "push 0FFFFFFFFh"
should silently be 
> compressed to a two-byte sequence.

Yes, exactly.  Likewise push 0FFFFFFFFFFFFFFFFh in 64-bit
mode should be
encoded as two bytes, but warn and 2-byte encode in 32-bit
mode.

Yasm already correctly warns for the push 0FFFFFFFFh case in
64-bit mode, but
I've not yet optimized its encoding to 2-byte.  I have a
patch (for Yasm) for
all other optimization cases but this one ready to commit. 
It's about 10
added lines of code .

-Peter

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Peter Johnson wrote:
>>
>> However, in 32-bit mode, the two-byte sequence 6A
FF will push 
>> FFFFFFFF on the stack, so "push
0FFFFFFFFh" should silently be 
>> compressed to a two-byte sequence.
> 
> Yes, exactly.  Likewise push 0FFFFFFFFFFFFFFFFh in
64-bit mode should be
> encoded as two bytes, but warn and 2-byte encode in
32-bit mode.
> 
> Yasm already correctly warns for the push 0FFFFFFFFh
case in 64-bit mode, but
> I've not yet optimized its encoding to 2-byte.  I have
a patch (for Yasm) for
> all other optimization cases but this one ready to
commit.  It's about 10
> added lines of code .
> 

Well, I have a patch to make NASM do the right thing, except
it warns in 
a few instances it shouldn't.

It's becoming clear to me that the lack of a formal
automatic test suite 
is one of the things that is slowing us down most.  It makes
it hard to 
see if a change broke something, which of course results in
hesitancy to 
get rid of crap code.

	-hpa

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